Biology

Exercise

Q. 1 Multiple Choice Questions

1. Griffith worked on ............. 10. Place the following event of translation

a. Bacteriophage b. Drosophila in the correct sequence

c. Frog eggs c. Streptococci i. Binding of met-tRNA to the start

2. The molecular knives of DNA are codon.

………….. ii. Covalent bonding between two

a. Ligases b. Polymerases amino acids.

c. Endonucleases d. Transcriptase iii. Binding of second tRNA.

3. Translation occurs in the ............... iv. Joining of small and large ribosome

a. Nucleus b. Cytoplasm subunits.

c. Nucleolus d. Lysosomes A. iii, iv, i, ii B. i, iv, iii, ii

4. The enzyme required for transcription is C. iv, iii, ii, i D. ii, iii, iv, i
..................
a. DNA polymerase Q. 2 Very Short Answer Questions:
b. RNA polymerase 1. What is the function of an RNA primer
c. Restriction enzyme during protein synthesis?
d. RNAase 2. Why the genetic code is considered as
commaless?
5. Transcription is the transfer of genetic 3. What is genome?
information from .............. 4. Which enzyme does remove supercoils
a. DNA to RNA from replicating DNA?
b. tRNA to mRNA 5. Why are Okazaki fragments formed
c. DNA to mRNA on lagging strand only?
d. mRNA to tRNA 6. When does DNA replication take
place?
6. Which of the following is NOT part of 7. Define term- codon and codogen.
8. What is degeneracy of genetic code?
protein synthesis? 9. Which are the nucleosomal 'core'
histones?
a. Replication b. Translation

c. Transcription d. All of these

7. In the RNA molecule, which nitrogen

base is found in place of thymine? Q. 3 Short Answer Questions:
1. Write short note on DNA packaging
a. Guanine b. Cytosine in eukaryotic cell.
2. Enlist the characteristics of genetic
c. Thymine d. Uracil code.
3. Write a note on applications of DNA
8. How many codons are needed to specify finger printing.
three amino acid? 4. Explain the role of lactose in ‘Lac
a. 3 b. 6 Operon’.
c. 9 d. 12
Q. 4 Short Answer Questions:
9. Which out of the following is NOT an 1. Write a note on Human genome
example of inducible operon? project (HGP).
a. Lactose operon
b. Histidine operon
c. Arabinose operon
d. Tryptophan operon

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2. Describe the structure of ‘Operon’. Q. 5 Long Answer Questions:
3. In the figure belowA, B and C are three 1. Explain the process of DNA replication.
2. Describe the process of transcription in
types of ____________________. protein synthesis.
3. Describe the process of translation in
Ribosome Amino acid protein synthesis.
4. Describe the ‘Lac-operon’.
Uracil 5. Justify the statements. If the answer is
false, change the underlined word(s) to
A BC make the statement true.
i. The DNA molecule is double
4. Identify the labeled structures on the stranded and the RNA molecule is
following diagram of translation. single stranded.
ii. The process of translation occurs at
C the ribosome.
iii. The job of mRNA is to pick up
CCG GCC ACU CCC amino acids and transport them to
the ribosomes.
A GGG iv. Transcription must occur before
translation may occur.
UGA 6. Guess (i) the possible locations of DNA
on the collected evidence from a crime
B scene and (ii) the possible sources of
DNA.
Part A is the ________________________.
Part B is the ________________________. Evidence Possible Sources of
Part C is the ________________________. location of DNA
DNA on the
5. Match the entries in column I with evidence
those of column II and choose the
correct answer.

Column I Column II

A. Alkali treatment i. Separation of DNA e.g. Eyeglasses e.g. Ear e.g. Sweat,
fragments on gel slab
pieces Skin
B. Southern blotting ii. Split DNA
fragments into single Bottle, Can, Sides, ------------
strands
Glass mouthpiece ----
C. Electrophoresis iii. DNA transferred to
nitrocellulose sheet ---------------- Handle Sweat,
skin, blood

Used cigarette Cigarette butt -------------

D. PCR iv. X-ray photography Bite mark ------------- saliva

E. Autoradiography v. Produce fragments ------------- Surface area Hair,
of different sizes semen,
sweat,
F. DNA treated with vi. DNA amplification urine
REN

Project : Collect information about B and Z forms of DNA. Sketch the diagrams and write
the differences between these two forms.

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5 Origin and Evolution of life

Can you recall? Observe and Discuss

1. What is evolution? 1. Redi’s Experiment
2. Where was first life formed? 2. Louis Pasteur’s Experiment
3. Why are many species of plants and
Broth is boiled
animals getting (becoming) extinct? No No Curved neck is
4. The Lamarck’s theory of inheritance of removed
flies maggots
acquired characters.
5. What is Speciation?

5.1 Origin of life : (Protobiogenesis) Flies maggots
The living matter shows attributes or
Flies Broth remains free
characters like responsiveness, growth, maggots of microorganisms
metabolism, energy transformations and
reproduction. Microorganisms

As far as origin of life is considered, it has grow in broth
remained an enigma for intellectuals at all
times. Despite of advancements in various 1. 2.
fields like biochemistry, astrobioloy, geography,
molecular biology, etc. scientists are unable to d. Theory of biogenesis :
ascertain the truth. Various theories and According to this theory, living organisms
hypotheses have been proposed to find the
probable answer to this question. are always produced from pre-existing living
forms, by process called reproduction.
a. Theory of special creation :
It is the oldest theory and is based on Theory of biogenesis however could not
explain origin of first life on earth but could
religious belief without any scientific proof. It explain only the continuity of life.
states that all living organisms are created by a
super-natural power. 5.2 Chemical Evolution of Life (Self assembly
theory of origin of life ) :
b. Cosmozoic theory/Theory of Panspermia: According to this theory, life originated on
This theory advocates that life did not arise
earth by combinations of several chemicals
on the planet Earth. It may have descended to through constant chemical reactions over a
the earth from other planets in the form of long period of time. This theory is also called
spores or micro-organisms, called cosmozoa/ self assembly theory of origin of life or
panspermia. Recently, NASA has reported biochemical origin of life.
fossils of bacteria-like organisms on a piece of
Martian rock recovered from Antarctica. This theory was first formulated by Haeckel
but later developed by the Russian scientist
c. Theory of spontaneous generation Alexander I. Oparin (1924) and British biologist
(Abiogenesis) : J. B. S. Haldane (1929). The process of
According to this theory, life originated chemical evolution can be divided into
following steps :
from non-living material spontaneously. This
theory was disproved by Louis Pasteur.

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a. Origin of Earth and Primitive rays, radiations, lightening and volcanic
atmosphere: activities, the early molecules of hydrocarbons,
The origin of universe was explained by ammonia, methane and water underwent
reactions like condensation, polymerisation,
the Big-Bang theory of Georges Lemaitre oxidation and reduction. These reactions
(1931). According to this theory the Universe resulted in formation of simple molecules like
originated about 20 billion years ago by a monosaccharides, amino acids, purines,
single huge titanic explosion. As the universe pyrimidines, fatty acids, glycerol, etc. All these
expanded, the temperature decreased and simple organic molecules accumulated at the
various galaxies of solid objects were formed. bottom of water bodies. Haldane described it
Milky Way is one such galaxy of which our as the ‘‘hot dilute soup’’ or ‘‘primitive broth’’.
solar system is one small part. Earth is one of It did not show any degradation due to absence
the planets of solar system and originated about of free oxygen and enzymes.
4.6 billion year ago. When formed, it was a
rotating cloud of hot gases and cosmic dust d. Formation of complex organic molecules:
called Nebula. The condensation and cooling The primitive broth was neutral and free
resulted in stratification with heavier elements
like nickel and iron passing to the core and from oxygen. Polymerisation took place and
lighter ones like helium, hydrogen, nitrogen, simple organic molecules aggregated to form
oxygen, carbon, etc. remaining on the surface. new complex organic molecules like
They formed the atmosphere of the earth. The polysaccharides, fats, proteins, nucleosides
primitive atmosphere of the earth was quite and nucleotides. Polymerisation of amino acids
different from the present one and it was of a formed protoproteins which later formed
reducing type, devoid of free oxygen. proteins. Formation of protein molecules is
considered as landmark in the origin of life.
b. Formation of ammonia, water and Proteins (enzymes) accelerated the rate of other
methane: chemical reactions.
Primitive atmosphere was very hot. As it
e. Formation of Nucleic acids :
slowly cooled, the lighter elements started to Nucleotides may have been formed by the
react with each other. The early atmosphere
was rich in hydrogen, carbon, nitrogen and reaction between phosphoric acid, sugar and
sulphur of which hydrogen being more active, nitrogenous bases (purines and pyrimidines).
it reacted with other elements to form chemicals Number of nucleotides join together to form
on earth like CH4, NH3, H2O and H2S. nucleic acids (RNA, DNA). Nucleic acids
acquired self-replicating ability which is a
c. Formation of simple organic molecules : fundamental property of living form.
As temperature of the earth decreased,
f. Formation of Protobionts or Procells :
steam condensed into water that resulted in Nucleic acids along with inorganic and
heavy rain fall and the earth gradually cooled.
Rain water got accumulated on the land to organic molecules formed the first form of life
form rivers, streams, lakes, seas and oceans. called protobionts. Protobionts are the
The atmosphere then did not contain ozone prebiotic chemical aggregates having some
layer and thus ultra-violet radiations reached properties of living system.
the surface of earth directly. Under the influence
of available energy sources such as ultra-violet

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Protobionts are formed due to coacervation Urey provided the first experimental evidence
i.e. aggregation of organic molecules. Oparin in support of chemical evolution theory of
(1924) called them coacervates and Sidney Oparin.
Fox called protenoids or microspheres.
They designed a glass-apparatus called
Coacervates spark-discharge apparatus.

Fig. 5.1 : Coacervates The apparatus (Fig. 5.2) was first sterilized
and evacuated. Methane, ammonia and
Coacervates are colloidal aggregations of hydrogen gases were pumped in the proportion
hydrophobic proteins and lipids (lipoid of 1:2:2 into the glass chamber. A tube carrying
bubbles). Coacervates grew in size by taking water vapour was also connected to the
up material from surrounding aqueous medium. chamber. Lightning effect was mimicked by
As they grew, they became thermodynamically electric discharge carbon arc spark in the
unstable and split into smaller units, comparable chamber. Process of evaporation and
to daughter cells of budding organisms. precipitation was also simulated by the use of
Microspheres are protenoids formed from heating mantle and condenser respectively.
colloidal hydrophilic complexes surrounded by
water molecules. These bodies may have outer The mixture of CH4, NH3, H2 was exposed
double-membrane, like primitive cell. Diffusion continuously to electric discharge for several
and osmosis may have occurred across the days causing the gases to interact, after which
membrane. They were more stable than these were condensed. The liquid collected in
coacervates. Coacervates and microspheres the U-tube turned brown. Chemical analysis of
were non-living colloidal aggregations of lipids this liquid reported the presence of simple
and proteinoids respectively. They had some organic compounds. (urea, amino acids, lactic
basic properties of living cells, such as growth acid, etc). This experiment strongly supports
and division. These colloidal aggregations that the simple molecules present in the earth’s
turned into first primitive living system called early atmosphere combined to form the organic
eobionts or protocell. building blocks of life.
g. Formation of first cell :
To CH4 Gasses Electrodes
When RNA or DNA system developed vaccum
within protocells, they look like bacteria or pump NH3 Carbon
viruses. They regulated various metabolic H3O Spark
activities. First cell was anaerobic, heterotrophic H2 Water
and obtained energy by chemoheterotrophic
processes. Condenser out
Water
Urey and Miller’s Experiment :
Stanley L.Miller and his teacher Harold C. in

Water droplets

Boiling water Water containing
organic compounds

Liquid in water trap

Fig. 5.2 : Urey and Miller’s Experiment

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RNA World Hypothesis : Besides, biomolecules like Acetyl-Co-A
Oparin Haldane theory and Miller Urey have a nucleotide in their molecular structure.
Major evidence is existence of ribosome (the
experiment gives us an understanding that protein assembly unit) in the cell. In ribosomes,
pathway of origin of life on earth goes from translation process is catalysed by RNA. (Refer
non-living to living. ‘Like begets like’ chapter- Molecular Basis of Inheritance).
necessitates presence of stable genetic material
and cellular machinery to carry out routine These molecules might have undergone
activities essential for survival. repeated replication and mutation forming
varieties of RNA molecules with varying sizes
We are also aware that certain proteins and catalytic properties. Eventually they might
which we call enzymes, catalyse the chemical have developed their own protein coats and
reactions in the cell. It was in 1980 that Sidney machinery to survive the assembly of primitive
Altman and Thomas Cech independently found cell. In due course, a double stranded stable
out that RNAs can also act as biocatalysts. structure, the DNA, might have been formed
These catalytic RNAs are called as Ribozymes. and thus continued the ongoing journey which
For this discovery, Altman and Cech earned resulted in rich biodiversity on earth.
Nobel Prize in chemistry in 1989.
5.3 Organic Evolution :
This discovery provided important support Evolution (Latin word, e = from; volvere =
for RNA World hypothesis. The hypothesis
suggests that early life must have been based to roll) means the act of unrolling or unfolding
exclusively on nucleic acids, most probably of nature. It brings about orderly changes from
RNA. It was first proposed by Carl Woese, one form to another. These changes result in
Francis Crick and Leslie Orgel in 1960, long descendants becoming different from the
before discovery of ribozymes. ancestors.

Fact is that RNA is found abundantly in all Organic evolution can be defined as slow,
living cells, it is structurally related to DNA gradual, continuous and irreversible changes
and chains of RNA can evolve or undergo through which the present day complex forms
mutations, replicate and catalyse reactions, all of the life developed (or evolved) from their
support this hypothesis. simple pre-existing forms.

RNA self- RNA forms RNA catalyses According to Charles Darwin, evolution
replicates from inorganic protein synthesis is ‘descent with modification’. You have
(via ribozymes) already studied the Lamarck’s theory of
sources inheritance of acquired characters in 10th std.
According to this theory, the traits are acquired
Membrane formation due to internal force, changes in environment,
changes internal chemistry new needs and the use and disuse of organs.
allowing new functionality After several generations, it gives rise to new
species.
DNA becomes RNA codes Proteins catalyse
master template both DNA and cellular activities This theory was disproved by a German
biologist August Weismann, who cut the tails
protein of many rats for several generations but could
not find any change in size of tail even after
Fig. 5.3 : RNA World Hypothesis

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21 generations. He concluded that variations potential of humans is greater than others. This
produced in somatic cells (somatoplasm) are resulted in multiplication of human population
not inherited while variations produced in germ without corresponding increase in the food
cells (germplasm) are inherited to next supply. This increase lead to competition and
generation and he proposed the theory of struggle for existence of human species.
Germplasm.
He was also influenced by plant/animal
5.4 Darwinism : breeders who by using artificial selection,
altered the characteristic of cultivated plant
Before darwinism, several theories were and domestic animals.
proposed to explain the process organic
evolution. Few of them are explained below : Darwinism is based on five main postulates:

Lamarck (1809) published theory of origin 1. Overproduction (Prodigality of nature)
of acquired characters. Which was then ruled - It is the natural tendency to produce more
out. number of progeny in geometric ratio, for
perpetuation of the species. He observed
Fig. 5.4 : Charles Darwin (1809 - 1882) prodigality potential many species of
Darwnism (theory of origin of species by plants and animals e.g. Salmon fish
Natural Selection). produces about 28 lakh eggs in a single
Charles Darwin as a naturalist travelled season. In a span of 750 years single pair
extensively the world over from 27th December of elephants would produce 19,000,000
1831 to October 1836 and returned to England. elephants. However the size of given
Based on his collections of living and fossil species in a given area remains relatively
material he published a book ‘‘The origin of constant because of fluctuations that occur
species by Natural Selection’’ in 1859. Before seasonally.
publishing his book, he was very much
influenced by C. Lyell (Geologist) who pointed 2. Struggle for existence - Tendency of over
out that the natural forces that existed in the production leads to the struggle for
past are same as those existing at present. existence between the members of
Darwin observed that the natural forces resulted population for limited supply of food or to
in the changes in the earth’s crust along South overcome adverse environmental
America. He observed variety of strange plants conditions or for a space or to escape from
and animal in Galapagos islands and other enemies etc.
nearby islands in the Archipealago in terms of
variations between the tortoises and finches. 3. Organic variations - The variations speak
Similar observations were also made by for all kinds of differences that occur in
Wallace. He was also influenced by R. Malthus, morphology, physiology, nutrition, habit
an Economist, who worked on human behavioural patterns etc. Darwin
population mentioning that the reproductive recognized these variations as raw material
for evolution. Variations were observed
among members of the same species and
even in different species.

4. Natural selection - Organic variations can
serve as evidence that for the some
organisms have better adapted to survive
under existing environmental conditions
than the others. In the struggle for existence
organisms with favourable variations are

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selected by the nature while those with e. According to natural selection new species
unfavourable variations perish. According are formed by gradual accumulation of
to Darwin, the principle by which useful useful variations. If it is so, then their
variations are preserved by nature, is called should be intermediate forms. But in most
‘Natural Selection’. H. Spencer named cases intermediate form were not
this process as ‘survival of fittest’. recognised. Moreover, Darwinism also
could not explain existence of neutral
5. Origin of new species (speciation) - As flowers and the sterility of hybrids.
favourable variations are transmitted from
generation to generation, successive 5.5 Mutation Theory :
generations become better adapted to
environment. Gradually these adaptation This theory was proposed by Hugo de Vries
with few new modification become fixed (1901), after the rediscovery of Mendel’s work
in the life cycle and finally giving rise to a (1900). He proposed this theory based on his
new species. observations on seven generations of the plant-
evening primrose (Oenothera Lamarckiana).
Evidences Darwinism include - (i) He found that though most of the offsprings
Evolution of long-necked Giraffe to pluck and resembled their parents in many characters,
eat more leaves from tall trees and woody some of the offsprings show the appearance of
climbers. This adaptation became fixed in the sudden or spontaneous variation clearly
life for survival. The Giraffe borne tall could different from the phenotypic expression of the
survive in famine heat areas. This adaptation parent. These sudden variations were called
was transmitted to their offspring. This is how, mutations or discontinuous variations. The
present long-necked Giraffe came to existence. variant offsprings produced variants and not
(ii) Black colour peppered moths evolved normal plants i.e. these changes were
gradually as new species. (iii) DDT resistance inheritable. He also observed that some variants
in mosquitoes-intensive DDT spraying also produced more variations. He noted that
destroyed all types of mosquitoes. However these sudden changes are inheritable, and
some mosquitoes developed resistance to DDT proposed the Mutation theory.
and survived the on slaught of DDT spray.
Such resistant mosquitoes survived and The main features of mutation theory are :
reproduce giving rise to more resistant
offspring. • Mutations are large, sudden and
discontinuous variations in a population.
Drawbacks and Objections to Darwnism -
• These changes are inheritable.
a. He considered minute fluctuating variation
as principal factors which are not heritable • Mutations provide the raw material for
and not part of evolution. organic evolution. ‘

b. He also did not distinguish somatic and • Mutation may be useful or harmful. Useful
germinal variation and considered all mutations are selected by nature.
variations are heritable.
• Accumulation of these mutations over a
c. He did not explain the ‘arrival of the period of time leads to the origin and
fittest’. d. He also did not explain the cause, establishment of new species.
origin and inheritance of variations and of
vestigial organs, nor he could explain the • Harmful mutation may persist or get
extinction of species. eliminated by nature.

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Objections to Mutation Theory : • All individuals of the some species

i. The large and discontinuous variation constitute a population. The populations
observed by Hugo de Vries were actually occur in small groups of ‘interbreeding

due to chromosomal aberrations were as populations’. Such small interbreeding

gene mutations usually bring about minor group of a population is referred as

changes. ‘Mendelian population’.

ii. Rate of mutation is very slow as compared • The total genetic information encoded in
to the requirement of evolution. sum total of genes in a Mendelian
population is called gene pool. Simply,
iii. Chromosomal aberrations have little gene pool means the total number of genes
significance in evolution as they are quite of all individuals in a population. The
unstable. gametes produced by individual furnish a
pool of genes of next generation from which
Always Remember the genes will be selected.

1. According to Darwin variations are small • The migration of population effectively
and directional where as mutations are alter the gene pool. The gene pool also
large, sudden, random and direction less. changes due to replacement of one
generation by another in the Mendelian
2. Darwin believed that the gradual population. Thus any change in the gene
inheritable variations over a long period pool affects population.
of time, lead to Speciation (formation of
new species) while de Vries believed that • Genes are arranged linearly on the
mutations are the cause of speciation. chromosome having definite positions.
According to Mendel, every gene that
3. A single step large mutation is called influences a trait has two alleles. The
saltation. proportion of an allele in the gene pool, to
the total number of alleles at a given locus,
5.6 Modern Synthetic Theory of Evolution : is called gene frequency.
• It is the result of true synthesis of all
• Modern synthetic theory comprises five
biological discipline. Studies pertaining to main factors that are broadly divided into
genetical, ecological, anatomical, three main concepts like - i. genetic
geographical, palaeontological etc. were variations caused due to various aspects
persued to explain mechanism of evolution. of mutation, recombination and migration.
Also due importance was given to both ii. natural selection and iii. isolation, for
mutations and natural selection. explaining the evolution of species.

• R. Fischer, J. B. S. Haldane, T. Dobzhansky a. Genetic variations : The change in gene
J. Huxley, E. Mayr, Simpson, Stebbins, and gene frequencies, is known as genetic
Fisher, Sewall Wright, Medel, T. H. Morgan variation. Genetic variations are caused by
etc. are the main contributors of modern following factors :
theory of evolution.
i. Gene Mutation : Sudden permanent
• Stebbins in his book discussed five key heritable change is called mutation.
factors such as gene mutations, mutations Mutation can occur in the gene, in the
in the chromosome structure and number, chromosome and in chromosome number.
genetic recombinations natural selection Mutation that occurs within the single gene,
and reproductive isolation, contributed in
the evolution of new species.

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is called point mutation or gene mutation. b. Duplication : Genes are repeated or
This leads to the change in the phenotype of doubled in number on chromosome.
the organism, causing what is called
variation. c. Inversion : A particular segment of
ii. Genetic recombination : In sexually chromosome is broken and gets
reproducing organisms, during gamete reattached to the same chromosome in
formation, exchange of genetic material an inverted position due to 1800 twist.
occurs between non-sister chromatids of There is no loss or gain of gene
homologous chromosomes. This is called complement of the chromosome.
crossing over. It produces new genetic
combinations which result in variation. d. Translocation : Transfer
Fertilization between opposite mating (transposition) of a part of chromosome
gametes leads to various recombinations or a set of genes to a non-homologous
resulting into the phenotypic variations chromosome is called translocation. It
causing change in the frequencies of alleles. is effected naturally by the transposons
iii. Gene flow : Gene flow is movement of present in the cell.
genes into or out of a population. Gene
movement may be in the form of migration • Deletion ABCDE FGH ABCE FGH
of organism, or gametes (dispersal of
pollens) or segments of DNA loss of a chromosomal segment
(transformation). Gene flow also alters
gene frequency causing evolutionary • Duplication A B C D E F G H ABCBCDE FGH
changes.
repeat a segment
iv. Genetic drift : Any random fluctuation
(alteration) in allele frequency, occurring in • Inversion ABCDE FGH ADCBE FGH
the natural population by pure chance, is MNOCDE FGH
called genetic drift. For example, when the reverses a segment
size of a population is severely reduced due • Translocation A B C D E F G H
to natural disasters like earthquakes, floods,
fires, etc. cause elimination of particular move segment from one chromosome to another
alleles from a population. Smaller
populations have greater chances for Fig. 5.5 : Chromosomal aberrations
genetic drift. It will result in the change in
the gene frequency. Genetic drift is also an b. Natural selection :
important factor for evolutionary change. According to Darwin, natural selection is

v. Chromosomal aberrations : The the main driving force behind the evolution.
structural, morphological change in This holds that genetic variations rise within
chromosome due to rearrangement, is the population. The ‘fittest’ will be at the
called chromosomal aberrations. It changes selective advantage and will be more likely to
the genes arrangement (order or sequence) produce offsprings than the rest, as the ‘fit’
that results in the variation. Chromosomal continues to enjoy greater survival and
aberrations occur due to - reproductivity, new species will eventually
a. Deletion : Loss of genes from evolve.
chromosome.
Alternatively, natural selection is the
process by which better adapted organisms
grow and produce more number of offsprings
in the population.

It brings about evolutionary changes by
favouring differential reproduction of genes
that bring about changes in gene frequency
from one generation to next generation.

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Selection against harmful mutations leads c. Isolation :
to a mutation balance in which allele frequency Isolation is the separation of the population
of harmful recessives remain constant
generation after generation. of a particular species into smaller units which
prevents interbreeding between them. Some
Fig. 5.6 : Natural selection (Biston betularia barrier which prevents gene flow or exchange
and Biston carbonaria) of genes between isolated populations, is called
isolating mechanism.
Natural selection encourages those genes
or traits that assure highest degree of adaptive Number of isolating mechanisms are
efficiency between population and its operated in nature and therefore divergence
environment. Industrial melanism is one of and speciation may occur. The isolating
the best example for natural selection. In Great mechanisms are of two types namely,
Britain, before industrilisation (1845) grey geographical isolation and reproductive
white winged moths (Biston betularia) were isolation.
more in number than black-winged moth
(Biston carbonaria). I. Geographical Isolation :
It is also called as physical isolation. It
These moths are nocturnal and during day
time they rest on tree trunk. White-winged occurs when an original population is divided
moth can camouflaged (hide in the background) into two or more groups by geographical
well with the lichen covered trees that helped barriers such as river, ocean, mountain, glacier
them to escape from the predatory birds. on etc. These barriers prevent interbreeding
other hand, the black-winged moth resting on between isolated groups.
lichen covered tree trunks were easy victims
for the predatory birds and their number was The separated groups are exposed to
reduced. During industrial revolution, large different kinds of environmental factors and
number of industries came up in Great Britain. they acquired new traits by mutations. The
The industries released black sooty smoke that separated populations develop distinct gene
covered and killed the lichens growing on tree pool and they do not interbreed. Thus, new
and turn the tree black due to pollution. species have been formed by geographical
isolation. E.g. Darwin’s Finches.
This change become an advantage to the
black-winged moth that camoflaged well with II. Reproductive Isolation :
the black tree trunks and their number increased Reproductive isolations occurs due to
while the white-winged moth become victims
to predatory birds due to which their number change in genetic material, gene pool and
reduced. Thus natural selection has resulted in structure of genital organs. It prevents
the establishment of a phenotypic traits in interbreeding between population.
changing the environmental conditions.
Types of Isolating Mechanisms :

A. Pre-mating or pre-zygotic isolating
mechanism : This mechanism prevent
fertilization and zygote formation.

i. Habitat isolation or (Ecological isolation)
: Members of a population living in the
same geographic region but occupy
separate habitats so that potential mates do
not meet.

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ii. Seasonal or temporal isolation : Members Mutations are already described earlier in this
of a population living in the same chapter. Gene mutations produce new alleles
geographic region but are sexually mature which are added to gene pool.
at different years or different times of the Gene recombination - These are variation
year. produce due to coming together of alleles
during sexual reproduction. Gene
iii. Ethological isolation : Due to specific recombinations occur due to random union of
mating behaviour the members of gametes, anaphasic separation of chromosomes
population do not mate. and crossing over.
Gene flow - It is the transfer of gene during
iv. Mechanical Isolation : Members of two interbreeding of populations that are genetically
population have difference in the structure different. As explained earlier in this chapter
of reproductive organs. gene flow is due to emigration and imigration.
Its brings about changes in the allele frequency.
B. Post-mating or Post-zygotic barriers : Genetic drift - Any alternation in allete
i. Gamete mortality - Gametes have a frequency in the natural population by chance,
is called genetic drift. Concept of genetic drift
limited life span. Due to one or the other was first given Sewall wright, hence, called as
reasons, if union of the two gametes does Sewall wright effect. For example, elimination
not occur in the given time, it results in the of a particular allele from a population due to
gamete mortality. events like accidental death prior to mating of
ii. Zygote mortality - Here, egg is fertilized an organism. Genetic drifts are random or
but zygote dies due to one or the other directionless. The effect of genetic drift is more
reasons. significant in small population than in large
iii. Hybrid sterility - Hybrids develop to population. Due to genetic drift, some alleles of
maturity but become sterile due to failure of a population are lost or reduced by chance and
proper gametogenesis (meiosis). e.g. Mule some others may be increased. Some time, a
is an intergeneric hybrid which is sterile. few individuals become isolated from the large
population and they produce new population in
Can you tell? new geographical area. The allele frequency of
new population become different. The original
1. What is variation? drifted population (i.e. colonizing ancestor/
2. What is mutation? pioneer) becomes ‘founders’ and the effect is
3. What is gene frequency? called founder effect.
4. Why variations are occurs in population?
A bottle neck effect is seen when much of a
5.7 Mechanism of organic evolution : population is killed due to a natural disaster
One has to give the importance to the and only a few remaining individuals are left to
begin a new population.
population while considering the mechanism of Natural selection - It is a process by which
evolution. It is the population that evolves and better adapted individuals with useful
not its individual members. Individual’s role is variations are selected by nature and leave
to pass its genetic variation to its offspring. greater or more number of progenies
(Differential reproduction).
The following are the basic processes
which bring about evolution viz. Mutations,
gene recombination, gene flow (migration),
genetic drift, natural selection, isolation and
speciation.
Mutations - These are permanent heritable
changes in the genetic material of an organism.

103

Type of Natural selection : Number of IndividualsSelection against an extremes
a. Stabilizing selection : (Balancing selection)
1. Here more individuals of a population Number of Individuals Population
Number of Individuals after selection
acquired a mean character value.
2. It tends to favour the intermediate forms Original
population
and eliminate both the phenotypic extreme.
For e.g. More number of infants with Time
intermediate weight survive better as
compare to those who are over-weight or Fig. 5.8 : Directional selection
under-weight.
3. It reduces variations. c. Disruptive Natural selection :
4. It does not lead to evolutionary change but 1. Here more number of individuals acquire
tend to maintain phenotypic stability within
population, therefore, it is described as peripheral character value at both ends of
stabilizing selection. the distribution curve.
5. Genetically stabilizing selection represents 2. Nature select extreme phenotypes and
a situation where a population is adapted to eliminate intermediate. Hence two peaks
its environment. are formed in distribution of traits.
3. This kind of selection is rare.
Selection against both extremes 4. It ensures the effect on the entire genepool
of a population, considering all mating
Population types or systems.
after selection 5. Example - It was observed in the different
beak size of African seed cracker finches.
Original The birds have different size of beak and
population they feed on seeds. The avilable seeds were
of two kinds small and large sized seeds.
Time Large beak sized birds feeds on large seeds
while small beak sized birds feed on small
Fig. 5.7 : Stabilising selection seeds and their number was increased.
Intermediate beak sized birds are unable to
b. Directional selection : feed on either type of seeds so their
1. In this type, more individuals acquired population was decreased gradually and
then eliminated by natural selection.
value other than the mean character value.
2. Naural selection acts to eliminate one of Selection against the intermediates

the extremes of the phenotypic range and Population
favour the other. e.g. systematic elimination after selection
of homozygous recessives.
3. Directional selection operates for many Original
generations, it results in an evolutionary population
trend within a population and shifting a
peak in one direction. Time
4. e.g. Industrial melanism, DDT resitant
mosquito etc. Fig. 5.9 : Disruptive selection

104

Isolation - It is separation of a single There are few factors such as gene
migration (gene flow), genetic drift, mutation,
interbreeding population into subunits. genetic recombinations, natural selection, non-
random mating, etc. which affect or change the
Isolation restricts gene flow between descrete Hardy-Weinberg equilibrilium,

(non continuous population due to different If these factors do not occur in the
population, then population is genetically
barriers like geographical barriers. (This part stable or non evolving population.
is already explained in detail earlier in this
chapter) Can you tell?
In which conditions the gene frequency
Speciation - The sub units of single of a population will remain constant?
interbreeding population due to the
geographical barriers like river, mountains, 5.9 Adaptive Radiation :
desert, sea etc. become isolated in such a way The process of evolution which results in
that their interbreeding is prevented. This will
finally lead to origin of new species (i.e. transformation of original species to many
speciation). (Discussed in detail ahead in this different varieties, is called, adaptive radiation.
chapter).
Darwin’s Finches is one of the best example
5.8 Hardy-Weinberg’s principle : of adaptive radiation. During his visit to
Galpagos Islands. Charls Darwin also noticed
It is also known as Hardy-Weinberg’s a variety of small birds. These birds are called
equilibrium law. The law states that ‘at Darwin’s finches.
equilibrium point both the gene (allele)
frequency and geneotypic frequency remain Fig. 5.10 : Darwin’s finches
constant from generation to generation’. It Darwin concluded that the American main
occurs only in the diploid, sexually reproducing, land species of bird was the original one from
large, free interbreeding population in which which they migrated to the different islands of
mating is random and no selection or other Galpagos. They adapted to the different
factors are present for changing the allele environmental conditions of these islands.
frequency. e.g. A single locus has two alleles (A From original seed eating features many other
and a). The frequencies of these allele are p forms with altered beaks evolved into
and q respectively. The allele frequency for any insectivorous features.
locus is always one. i.e. P + Q = 1. The genotypic Another example of adaptive radiation is
frequencies of both the alleles are represented Australian Marsupials. In Australia, there are
by (p + q)2 = 1. The binomial expansion of this many marsupial mammals who evolved from
is P2 + 2pq + q2 = 1 i.e. AA=P2, aa=q2 and for common ancestor.
2Aa= 2pq.

Hence P2 + 2pq + q2 = 1 This is a binomial
expansion of (p + q)2.

This can be explained by punnet square as
follow.

Hybrid A a

A (p) a (q)

Hybrid A (p) AA (p2) Aa (pq)
Aa a (q) Aa (pq) aa (q2)

Like allele frequency, the genotypic
frequencies together are also equal to 1.

105

Can you tell? 2. Moulds : These are the hardened
encasements formed in the outer parts
1. What is carbon dating and how does it of organic remains which later decayed
work ? leaving cavities. Body parts of plants or
animals later decays but the impression
2. What is fossils ? still remains and becomes permanent.
3. What is Homologous organs ? For example Foot – prints are formed
4. What is Embryology ? in this manner.

5.10 Evidences of organic evolution : 3. Cast : They are hardened pieces of
The theory of organic evolution states that mineral matter deposited in the cavities
of moulds.
the present day complex organisms have
originated from earlier simpler forms of life. 4. Compressions : Internal structure is
absent but a thin carbon film indicates
The process of evolution is supported by the outline of external features.
evidences provided by various branches of
biology such as : Palaeontology, comparative Significance of Palaeontology :
anatomy, embryology and molecular biology.
1. It is useful in reconstruction of phylogeny.
A. Palaeontology : The study of ancient life 2. It helps in studying various forms and
with help of fossils is called palaeontology.
Fossils are the dead remains of plants structures of extinct animals.
and animals that lived in past in various 3. It provides record of missing link
geological layers.
between two groups of organsims.
• The study of fossils provides the most 4. It helps in the study of habits of extinct
convincing and direct evidence of
evolution. organisms.
5. Palaeontology provides the following
• Fossils are formed in sedimentary rocks,
amber (yellowish fossils resin), ice, peat types of evidences.
bogs etc. Connecting link (missing link) :

• During fossilization, the primitive forms These are fossil forms transitional or
of organisms occupy the older, lower intermediate between two groups of organisms.
layers and the advanced forms occupy It shows some characters to both the groups.
the upper, more recent layers of the Thus it indicate the evolutionary line Seymouria
earth. (between amphibians and reptiles). e.g.
Archaeopteryx (between reptiles and birds).
Types of fossils :
Archaeopteryx lithographica :
1. Actual remains : These are most It is fossilized crow size toothed bird found
common type of fossils. The plants,
animals and human bodies got from jurassic rocks in Germany. It is known as
embedded in permafrost of arctic or missing link between reptiles and birds because
alpine snow remain preserved in the it shows characters of both.
actual state, e.g. Wooly Mammoth Reptilian characters :
in Siberia. Amber or hardened resin
contains preserved bodies of many 1. Presence of long tail, claws and scales
insects and arthropods. on the body.

2. Single headed ribs.
3. Abdominal ribs are present which look

like ribs of crocodile.
4. Jaws with homodont teeth.

106

5. Sternum without keel. Do you know ?
6. Bones are solid (nonpneumatic).
First fossil of Archaeopteryx was found in
7. Hind limbs had four clawed digits. Jurassic rocks of Bavaria. It was discovered
in 1861 and preserved in British museum by
Beak Andreas Wanger.

Scales on the body The second specimen found in 1877,
known as Archaeornis, is kept in the Berlin
Teeth museum.

Clawed Wings Feathers B. Morphology : Morphology deals with
Digits study of external structures while, anatomy
deals with study of internal structures. From
Fig. 5.11 : Archaeopteryx comparative study of morphology and anatomy
we can understand the evolutionary aspects
Avian characters : in the form of homologous, analogous and
1. Feathery exoskeleton. vestigeal organs.
2. Forelimbs are modified into wings.
3. Jaws are modified into beak. a. Homologous organs : Homologous organs
4. Skull bone is completely fused. are those organs, which are structurally
5. Large rounded cranium. similar but perform different functions.
6. Cranium with large orbits and a single For example :
condyle. 1. Forelimbs of vertebrates such as
7. Limb bones are bird like. lizzard, bird, bat, horse, whale and man,
8. Hind limbs with four toes first toe is
opposible. Humerus

Thus from the above study it is very clear Radius
that birds evolved from reptiles. Huxley justified Ulna
this by calling birds as glorified reptiles. Carpals

Activity : Bat Metacarpals
Whale Bird Man Phalanges
Horse

Complete the following chart. Lizard A

Animals Connecting link Leaf
1. Balanoglossus
between Tendril
....................................

2. .................. annelida and arthropoda

3. Lung fishes .............................. Thorn

4. ................... reptiles and mammals Stem

5. Seymouria .............................

6. Ichthyostegia fishes and amphibia Bougainvillea Cucurbita

B

Fig. 5.12 : Homologous organs

107

all of them have humerus, radius-ulna, Fore wing
carpals, metacarpals and phalanges
in their forelimbs. Forelimbs of these Humerus Radius Ulna
vertebrates are structurally similar but Feathers
perform different functions. Hind wing

2. Vertebrate heart and brain. Insect Bird

3. In plants, thorns of Bougainvillea Fig. 5.13 : Analogous organs
and tendrils of cucurbita represent
homology. c. Vestigeal organs : (Rudimentary organs)
Vestigeal organs are imperfectly developed
The structural similarities between the and non-functional, degenerate structures
homologous organs indicates that they have a which were functional in some related
common ancestory. and other animals or in ancestors. The
vestigeal organs are no longer required by
Differences in homologous organs are the organism but indicate the relationship
examples of divergent evolution or adaptive with those organisms were these organs are
radiation. fully developed.
Examples : Human beings show some
b. Analogous organs : Analogous organs vestigeal organs like -
are those which are structurally dissimilar 1. Presence of vestigeal nictititating
but functionally similar. These organs membranes.
have external superficial similarity due 2. Presence of wisdom teeth (third molars).
to similar functions but they are different 3. Coccyx (tail bone) : It is greatly reduced
anatomically. in man since the tail is of no use due to
erect posture.
For e.g. wings of butterfly (insects) and of
birds look superficially alike but they are Canine
no anatomically similar structures though 1) 2)
they perform similar functions.
Plica semilumaris Third Small
Other examples of analogous organs. molar intestine
3) (wisdom
1. Eye of the octopus (mollusca) and of Sacrum tooth)
mammals. They differ in their retinal
position, structure of lens and origin of 4)
different eye parts.
Colon
2. The flippers of penguins (birds) and
dolphins (mammals). Caecum

3. Sweet potato (root modification) and Coccyx (fused tail vertebrae) Vermiform appendix
potato (stem modification) store food
in form of starch. Fig. 5.14 : Vestigeal organs

Analogous organs leads to convergent
evolution i.e. different organisms shows same
superficial structural similarities due to similar
functions or habitat. These organs do not help
to trace the common ancestry. Thus analogous
organs do not have significant role in evolution.

108

4. Vermiform appendix and the caecum. It e.g. 14 different species of finches in
is functional in herbivorous mammals Galapagos islands and several marsupial
for digestion of cellulose. In man due species in the Australian continent.
to eating of cooked food it has lost its ii. Sympatric speciation: Formation of
function. species within single population without
geographical isolation. These are formed
Presence of these vestigeal organs provide due to reproductive isolation. e.g. Cichlid
evidence that man has (evolved) descended fishes in Lake Victoria. Mutations are
from simple primates. helpful in sympatric speciation.

Can you recall? b. Interspecific Speciation :
Hybridisation :
Give examples of embryological
evidences studied in lower classes. Two different species on crossing may give
rise to a new species. e.g. Mule is a hybrid
d. Molecular Evidences : produced by interbreeding between a male
1. Cell is the basic structural and functional donkey and a female horse. Hinny is offspring
of male horse and female donkey.
unit of life in all organisms.
2. Similarities in proteins and genetic material Can you recall?

performing a similar function among 1. What are fossils? why should we study
diverse organisms gives evidence of a fossils?
common ancestry.
3. Basic metabolic activities also occur in a 2. How do we find age of fossils ?
similar manner in all organisms. 3. Where do we find fossils ?
4. ATP is the energy source in all living
organisms. 5.12 Geological time scale :
5.11 Speciation :
The process of formation of a new The planet earth with its present biodiversity
species from the per-existing species is called was not so when it was born. Study of fossils
speciation. tells us that life forms were not the same millions
Species is a group of similar organisms of years ago (MYA). Geological time scale is
that can interbreed and produce a fertile used to understand the sequence of events that
offspring in nature. New species are formed by to place on the earth in different ages over
the following modes. a period of time. It is divided into six major
‘Eras’ Eras ended with major environmental
a. Intraspecific Speciation : changes on earth resulting into extinction and
emergence of new species. The eras are further
i. Allopatric speciation : Formation of divided into periods and epochs based on minor
a new species due to separation of a but landmark events in each era. Table 5.15
segment of population from the original shows the geological time scale at a glance.
population by distanced or a geographical
barrier cutting across the species range. The first life appeared on the earth some
e.g. creeping glaciers, development of 2000 million years ago. It took billions of years
mountains. Migration of individual also for this process to take place, from protenoids
causes allopatric speciation. The mode of to first cells the transition is still a mystery.
evolution here is called adaptive radiation Once formed the living forms diversified into
various groups. Life began in the sea water and

109

plants were the first living beings to adapt to but the variety of the lobefin fish, called
terrestrial life. Fishes evolved and diversified. coelacanth was caught in 1938 in South Africa.
The lobefin group of fishes too got diversified.
Some developed stout and strong fins and could Reptiles evolved from amphibians. They
go to land and come back to water. are the first true land vertebrates. They do not
have to go to water for reproduction. (Hint :
The coelecanth was considered a living think of amphibian and reptilian eggs). But
fossils. It was thought that lobefins are extinct, about 200 million years ago (mya) some reptiles

Table 5.15 : Geological time scale

Era Period Time Epoch Plant life Animal life
MYA
Quarternary Recent Angiosperms Age of mammals : Development
0.1-0.6 (Holocene) of modern man, birds, fishes and
insects. Development of human
0.01-2.0 0.6 - 2.0 Pleistocene Increase in herbs culture.
Extinction of great mammals.
Appearance of primitive man.

2-13 Pliocene Hard woody plants Emergence or origin of man.

conifers, grasslands Evolution of ruminants - horse,

Cenozoic bryophytes, monocots camel, elephant.

Tertiary 13-26 Miocene Abundance of deciduous Formation of first man like apes.
26-38 trees origin of grasses Adaptive radiation or spread of
2-6.5 mammals.
38-54 Oligocene Rise of monocots and Extinction of Archiac mammals,
54-65 flowering plants Appearance of apes and monkeys.
65-135 Turtles and crocodiles attained
Eocene Development of development.
135-165 angiosperms Diversification of placental
165-225 mammals and modern birds.
Palaeocene Modernisation of Arrival of early or first primates, rise
flowering plants of placental mammals.
Extinction of Dinasours and toothed
Cretaceous Decline of ferns - birds. Appearance of placental
sphenopsids (horsetails) mammals and first modern birds
Mesozoic and Gymnosperms
Ginkos, Gnetales. 1st Age of reptiles. Dinasaurs dominant,
Triassic Jurassic appearance of flowering Appearance of toothed birds
plants. (Archaeopteryx) Rise of marsupials.
Appearance and rise of dinosours.
Origin of angiospersms Extinction of primitive amphibians.
Dominance of lycopods, Diversification of reptiles. Rise of
ferns, conifers, cycads. oviparous mammals. Therapsids,
diversification of flies.
Dominance of
gymnosperms extinction
of seed ferns

110

permian 225-280 Origin of conifers Riseofmoderninsects, Disappearance
280-345 Decline of lycopods of trilobites, appearance of mammal
345-400 Abundance of ferns, like reptiles (Pelycosaurs). Decline of
400-440 cycads and conifers amphibians.
440-500
Carboniferous 500-590 Development of diverse Abundance of amphibians (age of
600-1600 pteridophytes, mosses amphibian).Appearance of reptiles
and gymnosperms. age and winged insects.
of ferns and coal forests,
different fungal groups

Palaeozoic Devoniam Appearance of first pro - Diversification of fishes. Evolution
gymnosperms. Formation of amphibians. Appearance of
of forests, wood decaying ammonites
fungi, chytrids and origin
of bryophytes.

Silurian Appearance of lycopods Appearance of first terrestrial
and ferns. Domiance of animals, wingless insects and jawed
algae, ascomycetean fish
fungi

Cambrian Ordovician Appearance of first Abundance of diversed invertebrates.
seedless vascular land Appearance of first vertebrates
plants, abundant algae jawless fishes, Appearance of corals,
giant cephalopods like Nautilus.

Rhynia like plants. All Abundance or age of trilobites,
types of marine algae Diversification of invertebrate phyla.

Proterozoic Tracheophyte ancestors, Primitive flat worms, annelids,
chlorophyte ancestors sponges, coelenterates, primitive
bacterial single-cellled metazoans, scanty fossils of
protista, blue green algae prokaryotes

Azoic Archaeozoic 1600- No fossil records Origin of life
3800 Simple unicellular forms like viruses,
bacteria and algae

3800- No life Absence of living being
4600 chemical evolution
Formation of earth,

* (MYA = Million Years Ago)

111

2 Ma:

230-66 Ma: First Hominins 4550 Ma :
Non-avian dinosaurs Formation of the Earth

Hominins
Mammals
c. 380 Ma: Land plants
First vertebrate land animals Animals
Multicellular life 4527
c. 530 Ma: Eukaryotes Formation Ma : Moon
Cambrian explosion Prokaryotes of the

c. 4000 Ma: End of the

750-635 Ma: Late Heavy Bombardment

Two Snowball Earths first life

c. 3200 Ma:
Earliest start
of photosyntheisis

c. 2300 Ma:
Atmosphere becomes oxygen-rich

first Snowball Earth

Fig. 5.16 : Geological time scale pie diagram

moved back to aquatic mode of life and gained to phenomenon like the continental drift i.e.
fish like form as seen in Ichthyosaurs. The continents moved from their original place.
giant reptiles like Dinosaurs once dominated As a consequence when south America joined
the earth but are now extinct. When was this? north America, ancestral forms of horse,
Around 65 million years ago! why it must have hippos, rabbits, etc. native to south america
happened? Can we give affirmative reason for were dominated by north American animals.
this extinction? At around the same time giant At the same time, marsupial diversified into
ferns were present on earth. However, they different habitats in Australia. These survived
also became extinct and got converted to fossil due to lack of competition.
fuels. How this must have happened?
From the fossil records we can trace
Decline of giant reptiles marked the complete evolutionary history of horse,
begining of dominance of mammals. These elephant, dog, etc. Human beings are the most
viviporous organisms were more intelligent. evolved animals on the earth.
They could avoid danger. Early mammals
were small shrew like organisms, but this Internet my friend
group diversified. Whales, dolphins, seals
and sea cow live in water, bats are the flying You may gather information out of curiosity
mammals, Kangaroo rats are fossorial, lemurs about geological events occurred in the past.
are arboreal. Major physical disturbances led

112

5.13 Human Evolution : Do you know ?

Use your brain power There is a difference of only 2.5 %
between DNA of chimpanzee and man while
Since your earlier school days you have between monkey and man it is 10 %.
been solving mysteries/puzzles labelled as
use your brain power. Did you ever wonder The major evolutionary trends in transition
why human brain has such a capacity? Why from ape to man are considered further. Special
and how we evolved along these lines? characteristics have been acquired by man in
What is the extent of similarity between are the course of evolution. Major changes that
humans, chimpanzees and monkeys ? took place in evolution of man include increase
in size and complexity of brain and enhanced
It has been traced that the human intelligence, increase in cranial capacity,
evolution appeared to have evolved from a bipedal locomotion, opposable thumb, erect
tree dwelling shrew like animal. This process posture, shortening of forelimbs and lengthening
began in Palaeocene epoch. During this period, of hind limbs, development of chin, broadening
dwindling forests forced arboreal mammals to of pelvic girdle, development of lumbar
adapt to life on land. This descent must have curvature, social and cultural development
been the driving force. In the following chart, it (articulated speech, art, development of tools,
can be seen that we are most closely related to etc).
gibbons, chimpanzees and gorillas.

Class : Mammalia

Marsupials Monotremes Eutherians
(Pouched mammals) (Egg laying mammals) (Placental mammals)

Sub order : Prosimii Order : Primate
Suborder : Anthropoidea

Lemurs Tarsier

New world monkeys Old world monkeys Hominoidea

Long non-prehensile tail Short non-prehensile tail Tailers

e.g. squirrel monkey, spider monkey e.g. Baboons, macaques, langur, etc. Great apes

Hyalobatidae Pongidae Hominidae
Arboreal apes, semi-arboreal apes, Gorilla, Erect posture
Gibbons Chimpanzee, Orangutan Human beings

Chart 5.17 : Classification of mammals
113

Table 5.18 : Human evolution

Heads Dryopi- Ramapi- Australopi- Homo Homo Neanderthal Homo-
thecus erectus man sapiens
thecus thecus habilis
Ape man
Appearance Ape like Man like Connecting Handy Advanced Modern
link man like prehistoric man
between man
Ape and
man

Site of Lake Shivalik Toung Olduvai Java and Neanderthal Africa
Gorge peking valley,
fossil record victoria Hills in in South Tanzania Germany
in Africa
of Africa, India and Africa,

Haritalynga, even in Ethiopia,

Himachal Kenya Tanzania

Pradesh

Period Miocene Miocene late Late middle of Late
epoch 20 –
25 mya and early pliocene pliocene pleistocene pleistocene

pliocene or early or early epoch 1.5 epoch

epoch 14 pleistocene pleistocene mya ago 100000 to

-12 mya epoch about 2.5 to 1.4 40000 yrs

4 – 1.8 mya mya ago

Skeltal Not taller Lower 5 feet in Heavy
features
than 4 feet, jaw, lighty height built short

jaws larger, built, prognathus prominant

prognathus dentition face, brow

face, chin more like massive ridges low

absent, modern jaws, Huge forehead,

lumbar man, teath, chin deep jaws,

curvature smaller absent, chin absent,

present. molars Bony eye outwardly

brow ridges curved thigh

present bones

Posture semi erect erect upright erect erect erect erect

Cranial 450 to 600 650 to 800 900 cc 1400 cc 1450 cc
capacity cc cc

Special close man with probably probably used hide, developed
features if similarity to ape brain distinct
any chimpanzee did not ate meat, burried races.
Developed
eat meat, omnivorous their dead, cave art
about
made might have constructed 18000 yrs
ago
tools from used fire flint tools

stones,

nicknamed

handy man

114

Cranial capacity of human begins increased Use your brain power
over a period of time and large size of frontal
lobe helped in development of high forehead. Even though the cranium of elephant
is larger than that of man, humans are
Increase in intelligence necessitated considered more intelligent than elephant.
physical development so that body and brain Why is it so ?
could be used effectively and productively.
Freedom of forelimbs from locomotory function Some of our ancestors and their
and opposable thumb led to better utilization evolutionary history is shown in the table.
of hands for holding objects effectively and
development of motor skills etc. The above table clearly shows the
gradual increase in cranial capacity, shape of
Bipedal locomotion, upright posture skull and dentition of the ancestral humans till
coupled with stereoscopic vision helped man to date.
move around safely on land.
Our journey of evolution still
Evolutionary history of man was traced continues.....
with the help of fossil remains found over a
period of time.

Fig. 5.19 : Human Evolution

Internet my friend Think about it

1. Recently a fossil park has been Human being is said to be most
established in Gadchiroli district evolved, intelligent living being. Yet we are
of Maharashtra state. Find more not self sufficient. Think of various aspects
information about Wadadham fossil for which we depend on other living beings
park. for our survival.

2. Find out information about caves in
India. One such place is in Madhya
Pradesh. It is at Bhimbetka rock shelter
in Raisen district. Here we can see cave
paintings by prehistoric humans.

115

Activity :
Collect the information about the organisams depicted in the following diagrams
and write on the same.
A. B.
C. D.

E.

116

Exercise

Q. 1 Multiple choice questions. 6. Identify the wrong statement regarding
evolution.
1. Who proposed that the first form of life a. Darwin’s variations are small and
could have come from per- existing non- directional.
living organic molecules? b. Mutations are random and non-
a. Alfred Wallace directional.
b. oparin and Haldane c. Adaptive radiations leads to
c. Charles Darwin divergent evolution.
d. Louis Pasteur d. Mutations are non- radamon and
directional.
2. The sequence of origin of life may be-
a. Organic materials- inorganic 7. Gene frequency in a population remain
materials – Eobiont- colloidal constant due to –
aggregates- cell. a. Mutation
b. Inorganic materials – organic b. Migration
materials – colloidal aggregates – c. Random mating
Eobiont- cell. d. Non- random mating
c. Organic materials- inorganic
materials – colloidal aggregates - 8. Which of the following characteristic is
cell not shown by the ape?
d. Inorganic materials- organic a. Prognathous face
materials – Eobiont- colloidal b. tail is present
aggregates - cell c. Chin is absent
d. forelimbs are longer than hind limbs
3. In Hardy - Weinberg equation , the
9. .................. can be considered as
frequency of homozygous recessive conncting link between between ape
and man.
individual is represented by :- a. Austalopithecus
b. Homo hablis
a. P2 b. pq c. q2 d. 2pq c. Homo erectus
d. Neanderthal man.
4. Select the analogous organs-
a. Forelimbs of whale and bat 10. The Cranial capacity of Nanderthal
b. Flippers of dolphins and penguin man was
c. Thorn and tendrils of bougainvillea a. 600 cc b. 940 cc
and cucurbita. b. 1400 cc d. 1600 cc
d. Vertebrates hearts or brains.
Q. 2 Very short answer question.
5. Archaeopteryx is known as missing
link because it is a fossil and share 1. Define the following terms-
characters of both- a. Gene pool b. gene frequency
a. Fishes and amphibians c. Organic evolution d. Population
b. Annelida and arthropoda. e. Speciation
c. Birds and reptiles
d. Chordates and nonchordates.

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2. What is adaptive radiation? Q. 5 Long answer questions.
3. If the variation occur in population by
1. Would you consider wings of butterfly
chance alone and not by natural selection and bat as homologous or analogous
and bring change in frequencies of an and why?
allele. What is it called?
2. What is adaptive radiation? Explain
4. State the Hardy – Weinberg eqnilibrium. with suitable example.

5. What is homologous organs? 3. By talking industrial melanism as one
example. Explain the concept of natural
6. What is vestigeal organ? selection.

7. What is the scientific name of modern 4. Describe the Urey and Millers
man? experiment.

8. What is coacervate? 5. What is Isolation? Describe the different
types of reproductive Isolatons.
9. Which period is known as “age of
Reptilia”? 6. What is Genetic variations? Explain the
different factors responsible for genetic
10. Name the ancestor of human which is variations.
described as man with ape brain.
Q. 6 Complere the chart.
Q. 3 Short answer question.
Era Dominating group of
1. Write a note on Genetic drift. 1. Cenozoic animal

2. Enlist the different factors that are ..................................
responsible for changing gene frequency.
2. .................... Reptiles
3. Draw a graph to show that natural
selection leads to disruptive change. 3. Palaeozic ..................................

4. Give the significance of fossils. 4. .................... Invertebrates

5. Write the objections to Mutation theory
of Hugo de vries.

6. What is disrruptive selection? Give
example.

Columu- I Column – II Project :
1. August Weismann a. Mutation theory
2. Hugo de vries Prepare a diagrammatic chart of the
3. Charl Darwin b. Germplasm theory chemical evolution of life.

4. Lamark c. Theory of acquired
characters
d. Theory of natural
selection

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6 Plant Water Relation

1. Can you recall? Curiosity Box :
1. Which are the various parts of plant body?
2. What are the functions of various parts of 1. What is hydrogen bond?
2. What are the meanings of specific heat,
plant body?
3. Which plant tissues are involved in heat of vaporization and heat of fusion?
3. What are adhesive and cohesive forces?
transport of water and minerals?
Water molecules have good adhesive and
Use your brain power cohesive forces of attraction. Due to high
surface tension and high adhesive and cohesive
You know that we need a water pump force, it can easily rise in the capillaries. It is
to lift water at top of the building. but, how therefore, a significant molecule that connects
does plants lift the water from soil upto physical world with biological processes.
canopy without any pump?
6.2 Water absorbing organ:
Plant obtains variety of substances like Root :
water, minerals, nutrients, food and gases
like O2 and CO2, from its surroundings. Root is the main organ of water and
Productivity in plants is mainly affected by the mineral absorption. In terestrial plants, plants
non-availibility of water. absorb water in the form of liquid from the soil
however, epiphytic plants like orchids absorb
Water is considered as ‘elixir of life’. water vapours from air with the help of epiphytic
Water constitutes almost 90 to 95% of most roots having special tissue called velamen.
plant cells and tissues. Water helps the cells Typical root is divisible into four different
to maintain turgidity and shape. It shows regions. In the zone of absorption, epidermal
following properties due to which it has great cells (epiblema cells) form unicellular hair like
biological importance. extensions called root hairs.

Maturation zone

6.1 Properties of water: Root hair zone
It is in the liquid form at room temperature Root hair

and is the best solvent for most of the solutes. Zone of elongation
It is inert inorganic compound with neutral pH
when in pure form. Due to this, water is best Meristematic region
transporting medium for dissolved minerals Root cap
and food molecules. It is best aqueous medium
for all biochemical reactions occurring in Fig. 6.1 a. : Root tip showing root hair zone
the cells. It is an essential raw material for
photosynthesis. Water has high specific heat, A root hair cell
high heat of vaporization and high heat of Mitochondria Cell membrane
fusion. Due to this, it acts as thermal buffer.
These various properties are due to hydrogen Cell wall
bonds between the water molecules.
Nucleus Vacuole Cytoplasm

Root epithelial

cells

Fig. 6.1 b. : Structure of root hair

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Structure of root hair : a. Imbibition:
Root hair is cytoplasmic extension Imbibition is swelling up of hydrophillic

(prolongation) of epiblema cell. Each root hair colloids due to adsorption of water. Substance
may be approximately 1 to 10mm long and that adsorbs water / liquid, is called as imbibant
tube like structure. It is colourless, unbranched, and water/ liquid, that gets imbibed is called
short-lived (ephemeral) and very delicate. It as imbibate. The root hair cell wall is made
has a large central vacuole surrounded by thin up of pectic compounds and cellulose which
film of cytoplasm, plasma membrane and thin are hydrophillic colloids. During Imbibition,
cell wall, which is two layered. Outer layer is water molecules get tightly adsorbed without
composed of pectin and inner layer is made up the formation of solution. Imbibition continues
of cellulose. Cell wall is freely permeable but till the equilibrium is reached. In other words,
plasma membrane is selectively permeable. water moves along the concentration gradient.

6.3 Water available to roots for absorption: Imbibition is significant in soaking of
Plants absorb water from the rhizosphere seeds, swelling up of dried raisins, kneading of
flour etc.
(the microenvironment surrounding the root).
Water present in the soil occurs as gravitational Use your brain power
(free) water, hygroscopic water, combined
water and capillary water. Water percolates Why do the wooden doors become
deep, due to the gravity, in the soil, is called very hard to close and open in rainy season?
‘gravitational water’. This is not available
to plants for absorption. Fine soil particles b. Diffusion: Diffusion means to disperse.
imbibe/ adsorb water and hold it. This is called Diffusion can be defined as the movement of
‘hygroscopic water’. Roots cannot absorb it. ions/ atoms/ molecules of a substance from the
Water present in the form of hydrated oxides region of their higher concentration to the region
of silicon, aluminum, etc., is called ‘combined of their lower concentration. The movement
water’. It is also not available to plants for is due to the kinetic energy of the molecules.
absorption. Some amount of water is held in Diffusion continues till an equilibrium is
pores present between the neighbouring soil reached. Thus, water passes into the cell by
particles, due to capillarity. This is called diffusion through a freely permeable cell wall.
capillary water that is avilable for absorption. Water is now at the interface of cell wall and
plasma membrane.
6.4 Absorption of water by roots from soil:
Root hair absorbs water by employing three Diffusion results in the diffusion pressure
(D. P.) which is directly proportional to the
physical processes that occur sequentially- viz. number of diffusing particles. Diffusion
imbibition, diffusion and osmosis. pressure of pure solvent (pure water) is always

Activity :

Try this at your home.
A. Take 10 ml of pure water in a suitable glass vessel and put 2 - 3 raisins in it. Observe the

changes in raisins since the time you put them in water till they become fully swollen i.e.
turgid. Why did raisins become turgid?
B. Take 10 ml of pure water and add 5 gms of either sugar or salt to it. Let it dissolve and then
put the same turgid raisins in it and observe the changes in raisins. What changes did occur
in raisins and why? Discuss your observations with your teachers.

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more than the diffusion pressure of solvent c. Osmosis : It is a process by which water enters
in a solution. The difference in the diffusion into the cytoplasm of the root hair cell. Osmosis
pressures of pure solvent and the solvent in a is a special type of diffusion of solvent through
solution is called Diffusion Pressure Deficit a semipermeable membrane. The cytoplasm
(DPD) or Suction Pressure (SP). The term of root hair cell contains minerals, sugars,
was coined by B.S. Meyer (1938). Now a etc. In other words, solution inside the cell is
days, term water potential is used for DPD. In more concentrated (stronger) than outside the
colloquial language, the term DPD is actually cell (weaker). Therefore, solvent from weaker
the thirst of a cell with which it absorbs water solution enters into cytoplasm (i.e. to stronger
from the surroundings. Water arround cell solution) of cell through a semipermiable
wall has more diffusion pressure than cell plasma membrane. This migration of solvent is
sap. Due to this, water moves in the cell by called Osmosis.
diffusion. Diffusion is significant in plants in
the absorption of water, minerals, conduction Thus, water at the interface of cell wall and
of water against the gravity, exchange of gases plasma membrane, enters into the cytoplasm of
and transport and distribution of food. the root hair cell due to osmosis.

Outside of cell Water molecules Water-selective With respect to the concentration and
pore (aquaporin) osmotic migration, three types of solutions are
recognized viz,
Cytoplasm Membrane bilayer i. Hypotonic (weak solution or strong

Fig. 6.2 : Diffusion of water into plant cell solvent) having low osmotic concentration.
across the plasma membrane ii. Hypertonic (strong solution or weak

Use your brain power solvent) having high osmotic concentration.
iii. Isotonic having such a concentration of
1. When you burn an incense stick in one
corner of room, its fragrance spreads all solution where there is neither gain nor
over the room in a short time. How does loss of water in an osmotic system. In other
it happen? words, concentration outside and inside the
cell is same.
2. How does the water come out through Osmosis is of two types viz, Exo-osmosis
the surface of porous earthen pot? and Endo-osmosis.
Exo-osmosis : It is the migration of solvent
from the cell outside. It causes flaccidity of cell.

Endo-osmosis : It is the migration of the
solvent into the cell. It causes turgidity of
cell i.e. cytoplasm becomes turgid. Turgidity
increases the turgor pressure (T. P.) of the cell.
T. P. is the pressure exerted by turgid cell sap on
to the cell membrane and cell wall. In a fully
turgid cell, DPD is zero. Cell wall being thick
and rigid, exerts a counter pressure on the cell
sap. This is called Wall pressure (W. P.). In a
fully turgid cell, T. P. = W. P. but operating in
opposite direction.

Osmotic pressure (O. P.) : The pressure
exerted due to osmosis is osmotic pressure.

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Osmotic pressure is a pressure of the solution,∴ 6.5 Water Potential (ψ):
which is required in opposite direction, so as to According to the principle of
stop the entry of solvent molecules into the cell.
More simply, osmotic pressure of a solution thermodynamics, every component of a system
is equivalant to the pressure which must be is having a definite amount of free energy
exerted upon it to prevent flow of solvent across which is used to do work. Osmotic movement
a semipermeable membrane. of water is on the basis of free energy. Free
energy per molecule in a chemical system, is
Therefore, D. P. D. = O. P. - T. P. called its chemical potential.
( T. P. = W. P.)
Chemical potential of water is called water
∴ D. P. D. = O. P. - W. P. potential. It is represented by Greek letter psi
In a flaccid cell, T. P. is zero ∴ DPD = OP (ψ). Water potential of protoplasm is equal but
In a turgid cell, DPD is zero ∴ TP = OP opposite in sign to DPD It has negative value.
The unit of measurement is in bars/ pascals/
Do you know ? atmospheres.

Improtance of T. P. : It keeps cells and Water potential of pure water is always
organelles stretched; provides support to zero. Addition of any solute in it, decreases its
the non-woody tissues; essential for cell psi (ψ) value. Therefore, it has negative value.
enlargement during growth; maintains shape
of cell and facilitates opening and closing of D. P. D. is now termed as water potential.
stoma. O. P. is now termed as osmotic potential.
Improtance of Osmosis : It is responsible T. P. is now termed as pressure potential. It
for absorption of water into root; maintains has always positive value.
turgidity of cell; facilitates cell to cell
movement of water; offers resistance to Water always flows from less negative
drought, frost, etc; also helps in the drooping potential to more negative water potential
of leaflets and leaves in vicinity of “touch me (i.e. from high water potential area to low
not” plant. water potential area). Difference between
water potential of the adjacent cells decides
Facilitated diffusion : The passive absorption movement of water through plasmodesmata
of solutes when mediated by a carrier, is across the cells.
called Facilitated diffusion. Particles that
are lipid soluble can easily diffuse through Factors affecting water absorption:
lipoproteinous cell membrane. The diffusion i. Presence of capillary water is essential.
of hydrophilic solutes has to be facilitated ii. Rate of water absorption is maximum at
because their diffusion across the membrane
is difficult. Membrane proteins provide soil temperature between 200 to 300C.
such sites for facilitated diffusion. These iii. High concentration of solutes in soil water
proteins are aquaporins and ion- channels.
These proteins help move substances across reduced the rate of absorption of water.
membranes without the expenditure of energy. iv. Poorly aerated soil shows poor absorption
Concentration gradient must be present for the
molecules to be diffused through facilitated rate.
diffusion. v. Increased transpiration accelarates the rate

of absorption of water in the irrigated soil.

6.6 Plasmolysis:
Exo-osmosis in a living cell when placed

in hypertonic solution, is called plasmolysis.
During plasmolysis, protoplast of cell shrinks
and recedes from cell wall. Thus, cell becomes

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flaccid. In a plasmolysed cell, a gap is Absorption of water being a continous
developed between cell wall and the protoplast. process, a sort of hydrostatic pressure is
This gap is filled up by outer solution. developed in living cells of root. This is called
root pressure. It is due to root pressure, water
In a plasmolysed cell, T. P. is always zero. from pericycle is not only forced into the xylem,
When such cell is placed in hypotonic solution, but also conducted upwards against the gravity.
endo-osmosis occur, making cell turgid. This
is called deplasmolysis. In a fully turgid cell Pathway of water across the root essentially
T. P. = O. P. hence, DPD is always zero. occurs in two ways viz, apoplast and symplast.

6.7 Path of water across the root (i.e. from Apoplast Pathways
epiblema upto xylem in the stelar region) : Symplast

Water is absorped by root hair cell Vacuolar Transmembrane
through imbibition diffusion osmosis,
sequentially. Consequently the cell becomes When some amount of water passes
turgid. Its turgar pressure increases, but its DPD across the root through the cell wall and the
value decreases. However, the immidiately intercellular spaces of cortical cells of root, it
adjacent cortical cell inner to it, has more DPD is then called apoplast pathway. This pathway
value, because its O. P. is more. Therefore, occurs up to endodermis.
cortical cell will suck water from the turgid root
hair cell. It then becomes turgid. The flaccid The apoplastic (non-living) pathway
root hair cell now absorbs water from soil. provides a route toward the vascular stele
through free spaces and cell walls of the
Water from the turgid cortical cell is sucked epidermis and cortex. An additional apoplastic
by inner cortical cell and the process goes on. route that allows direct access to the
Thus, a gradient of suction pressure (DPD) is xylem and phloem is along the margins of
devloped from cells of epiblema to the cortex secondary roots. Secondary roots develop
of the root. Consequently water moves rapidly from the pericycle, a cell layer just inside the
across the root through loosely arranged endodermis. The endodermis is characterized
living cells of cortex, followed by passage by the Casparian strip, a suberized layer that
cells of endodermis and finally into the cell of forces all to move in the symplast in order to
pericycle. Protoxylem is in the close proximity enter the vascular system. Since secondary
with pericycle. roots grow through the endodermis, a direct
pathway to the xylem and phloem is available
Endodermis Pericycle Phloem that bypasses the Casparian strip and allows
to enter the vascular system without moving
Symplastic into the symplast (living tissue).
path

Cortex Casparian Xylem When water passes across from one living
strip cell to other living cell through plasmodesmata,
Apoplastic then it is called symplast pathway. It is also
path called transmembrane pathway.

Fig. 6.3 : Pathways for water uptake 6.8 Mechanism of absorption of water :
by the root Mainly, there are two ways/ modes of

absorption of water viz, passive absorption and
active absorption.

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a. Passive absorption : a hydrostatic pressure, called root pressure,
It is the main way of absorbing water is developed in root cells. This root pressure
forces water from pericycle to xylem and then
through the roots and not by the roots from soil upwards to the stem.
into the plant. The driving force is transpiration 2. Non-osmotic absorption : Kramer and
pull and it thus proceeds through DPD gradient. Thimann (1959) proposed this theory.
There is no expenditure of energy (ATP) as Sometimes, water is absorbed from soil against
water moves in accordance to the concentration the concentration gradiant. Such absorption
gradient. Hence, it is passive absorption. About requires an expenditure of energy released
98% of the total water absorbed in plants, occur during respiration, directly. Poor supply of
passively. Passive absorption occurs during oxygen retards water absorption. Moreover
day time when transpiration is in progress. It low temperature retards water uptake because
stops at night when transpiration stops. of decrease in the rate of respiration. Use of
metabolic inhibitors also retards the rate of
Rapid transpiration creates a tension in the respiration and thus the water uptake.
xylem vessel due to negative water potential.
This tension is transmitted to xylem in the 6.9 Translocation of water:
roots. Consequently water is pulled upwards The transport of water with dissolved
passively.
minerals from root to other aerial parts like
During passive absorption, no ATP is stem and leaves, against the gravity, is called
utilized. Obviously, the rate of respiration is not translocation or ascent of sap.
affected. In plants, water is mainly absorbed
passively. Translocation of water occurs through
the lumen of conducting elements of xylem-
b. Active absorption : tracheids and vessels, in all vascular plants.
Here, water is absorbed due to activity Ringing experiment has proved that xylem is
the path of ascent of sap.
of roots. Root cells play active role in the
absorption of water. The driving force is the root Several mechanisms/ theories have
pressure developed, in the living cells of root. been put forth to explain the mechanism of
Active absorption occurs usually at night when translocation of water. The theories include-
transpiration stops due to closure of stomata. As vital force theory, relay pump theory, physical
water absorption is against the DPD gradient, force theory, root pressure theory, etc. We shall
there is expenditure of ATP (energy) generated consider following three theories :
through the respiratory activity of cells.
a. Root Pressure Theory (Vital Theory) :
Active absorption may be of two kinds According to this theory, the activity
viz, osmotic and non-osmotic :
of living cells of root is responsible for
1. Osmotic absorption : Atkins and Priestly translocation of water. J. Pristley proposed this
(1922) proposed that water is absorbed from soil theory. When a stem of potted plant is cut few
into xylem of the root according to the osmotic inches above the soil by a sharp knife, xylem
gradient. To create osmotic conditions, there is sap is seen flowing out/ oozing out through the
an expenditure of energy. But such absorption cut end. This exudation at the cut end of stem is
does not directly require an expenditure of a good proof for the existence of root pressure.
energy. As water absorption by roots is constant and
continous process, a hydrostatic pressure is
A gradient of DPD develops from cell of developed in the living cells of cortex of root.
epiblema to pericycle due to activity of living
cells of root. As the process is continuous,

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This is termed as root pressure by S. Hales. It is b. Capillarity theory (physical force theory):
due to root pressure water along with dissolved
minerals is not only forced into xylem but it is According to this theory, physical forces
also conducted upwards against the gravity. and dead cells are responsible for ascent of sap.

Root pressure seems to be largely an This theory was put forth by Bohem in
osmotic phenomenon and its development is an (1863). Wick dipped in an oil lamp, shows
active process. The value of root pressure is +1 capillarity due to which oil is raised upwards.
to +2 bars which is enough to pump water to The conduction of water in a straw dipped in
a height of 10 to 20 meters. The factors like water, is raised to a certain height because of
oxygen, moisture, temperature of soil, salt capillarity. The height to which water is raised
contents, etc. influence the root pressure. depends on the diameter of the straw.

Manometer Manometer to measure Capilarity is because of surface tension,
exudation pressure from and forces of cohesion (attraction between like
molecules) and adhesion (attraction between
cut stump unlike molecules). Xylem vessel/ tracheid with
its lumen is comparable with straw. Water
Final level column exist because of combined cohesive
Initial level and adhesive forces of water and xylem wall,
due to capillarity. It is because of capillarity
Stump water is raised or conducted upwards against
the gravity, to few centimeters only.

Objections/ Limitations of capillarity theory:

Fig. 6.4 : Experiment to demonstrate root Few important objections are :
pressure
i. Capillary tube (xylem) must be continously
Objections/ limitations of root pressure and completely hollow from one end to the
theory: other end but tracheids in the xylem show
closed end-walls.
Although, ascent of sap takes place due
to root pressure, there are certain objections ii. The lower end of capillary tube i.e. xylem
raised, such as - must be in direct contact with soil water.
However, there exists a barrier of root
i. It is not applicable to plants taller than 20 cortex between xylem and soil water.
meters.
iii. Narrower the capillary tube, greater is the
ii. Ascent of sap can also occur even in the height to which water column is raised.
absence of root system. Thus, taller trees should show xylem
vessels with very narrow bore (diameter).
iii. Root pressure value is almost nearly zero in However, in nature the tall trees show
taller gymnosperm trees. xylem vessels having wider bore.

iv. In actively transpiring plants, no root Hence, to sum up capillarity can not be the
pressure is developed. sole mechanism to explain ascent of sap in all
the plants of varying heights.
v. Xylem sap under normal condition is under
tension i.e. it shows negative hydrostatic c. Cohesion- tension theory (Transpiration
pressure or high osmotic pressure. pull theory) :

To sum up therefore, root pressure is not This is presently widely accepted theory
the sole mechanism explaining the ascent of explaining ascent of sap in plants. It was put
sap in all plants of varying heights..

125

forth by Dixon and Joly (1894). This theory is iii. If plant is smeared with vaseline in order to
based on two principles. stop transpiration, even then ascent of sap
occurs.
Cohesion and adhesion, and transpiration
pull : iv. Ascent of sap also occurs in deciduous
plants that have shed all of their leaves.
A strong force of attraction between water
molecules, is called cohesive force. While These observations point to the fact that
a strong force of attraction between water besides physical forces, activity of living cells
molecules and lignified wall of lumen of xylem seems to be necessary for lifting the water
vessel, is called adhesive force. column up.

Due to combined cohesive and adhesive 6.10 Transport of mineral ions :
forces a continous water column is developed Soil serves as main source for minerals.
(formed) in the xylem right from root upto the
tip of the topmost leaf in the plant. Minerals constitute most commonly occuring
solid, crystalline inorganic materials obtained
Transpiration pull : The transpiration pull from earth’s crust. Minerals play an important
developed in the leaf vessel is transmitted down role in the day to day life of plant. Minerals are
to root and thus accounts for the ascent of sap. absorbed by plants in the ionic (disolved) form,
mainly through roots and then transported.
Excess water is lost in the form of vapour,
mainly through the stomata found on leaf. Do you know ?
This water loss increases D. P. D. of mesophyll
cells. These cells withdraw water ultimately • Minerals that play important role in
from xylem in the leaf. In otherwords, due to the day to day life, are called essential
continous transpiration, a gradient of suction elements. About 36 to 40 elements are
pressure (i.e. D. P. D.) is developed right from incorporated in the plant’s life.
guard cells up to the xylem in the leaf. This
will create a tension (called negative pull or • Some minerals like C, H, O, P, N, S, Mg
transpiration pull) in the xylem. Consequently, required in large quantity, are called
water column is pulled out of xylem. Thus, macro elements. While minerals like Cu,
water is pulled upwards passively against the Co, Mn, B, Zn required in small quantity,
gravity leading to the ascent of sap. are called micro elements.

Objections/ Limitations of transpiration pull The analysis of plant ash demonstrates
theory: that minerals are absorbed by plants from soil
and surroundings. Absorption of minerals is
i. For transpiration pull to operate, water independent of that of water.
column should be unbroken and continous.
However, due to temperature fluctuations Absorbed mineral ions are pulled in
during day and night, gas bubbles may enter upward direction along with xylem sap because
in water column breaking the continuity. of transpiration pull. This could be understood
when the ascending sap is analysed. Mineral
ii. This mechanism assumes that tracheids are ions are needed in the areas of the plant viz.
more efficient than the vessels, as their end apical, lateral, young leaves, developing
walls support water column. flowers, fruits, seeds and storage organs.
Hence, from the source (root), these are pulled
However, vessels are more evolved and transported ascendingly through the sap
than tracheids and are more efficient. and gets unloaded by fine veins through the

126

process of diffusion in the vicinity of cells. food from one part to the other part, is called

Cells uptake them actively. translocation of food.

Soil would not be the only source for mineral Path of translocation : Food is to be
uptake. Mineral ions can be remobilized within translocated to longer distances in higher
the parts of the plant. Older parts (like leaves in plants. Hence plants must have adequate
deciduous plants) export their ions to younger channels for the transport of food. Sieve tubes
leaves before the fall. Most readily mobilized and vessels are structurally ideally suited
ions are like phosphorus, sulphur, nitrogen for longitudinal (vertical) translocation. The
and potassium but the ions from structural ringing experiment, structure and distribution
components like calcium is not remobilized. of phloem, chemical analysis of phloem sap
and use of isotope 14C, clearly point out that the
Internet my friend phloem tissue is primarily responsible for flow
of food in longitudinal downward direction.
1. The general roles of minerals in the life The horizontal (lateral) translocation occurs
of plants. from phloem to pith or phloem to cortex via
medullary rays in the stem.
2. The role of essential minerals in the day
to day life of plants. Food is always translocated in the form of
sucrose (soluble form) and always along the
Analysis of xylem exudate also shows concentration gradient from source to sink. The
that some nitrogen travels as inorganic ions transport of food occurs in vertical and lateral
whereas much of it is carried in the organic direction.
form like amino acids and related compounds.
Small amount of inorganic molecules of Vertical translocation : In vertical
phosphorus and sulphur are also carried. It was (longitudinal) transport, food is translocated
a belief earlier that xylem transports inorganic in downward direction from leaves (source) to
and phloem transports organic molecules. stem and root (sink). It also occurs in upward
However, it is not correct because some direction during germination of seed, bulbils,
exchange of materials also occurs between corm, etc. Upward translocation also occurs
xylem and phloem. from leaves to growing point of stem, to
developing flowers and fruits situated near the
Do you know ? ends of the branches of stem.

• Different modes of passive absorption and Sieve plate Sieve
active absorption of minerals in plants. Companion cell tube
element
• Carrier concept of active absorption. Sieve tube

6.11 Transport of food : Cytoplasm
All the plant parts require continous supply
Fig. 6.5 : L. S. of sieve tube
of food for nutrition and developement. In
higher plants, there is a great differentiation and
division of labour. Chloroplasts are confined to
green cells of leaves where food is synthesized.
The non-green parts like root and stem must
received food from leaves. The part where food
is synthesized is called source and while part
where it is utilized, is called sink. Food has to
travel from source to sink. This movement of

127

Lateral translocation : It occurs in the root and 6.12 Transpiration :
stem. When food is translocated from phloem Plants absorb water constantly and
to pith, it is called radial translocation and
from phloem to cortex, it is called tangential continously. Hardly 5% of the total water
translocation. absorbed by roots that is utilised for cell
expansion and plant growth. Remaining 95%
The transport of food through phloem is water becomes surplus which is then lost into
bidirectional. Phloem sap contains mainly the atmosphere, through its aerial parts. Hardly
water and food in the form of sucrose. But 1% of surplus water is lost in the form of liquid
sugars, amino acids and hormones are also and 99% of surplus water, is lost in the form of
transported through phloem. vapour. The loss of water in the form of liquid
is called guttation. It occurs through special
Mechanism of sugar transport through structures called water stomata or hydathodes.
phloem : The loss of water in the form of vapour is called
transpiration that occurs through leaves, stem,
Several mechanisms/ theories like flowers and fruits. Most of the transpiration
diffusion, activated diffusion, protoplasmic occurs through the leaves (called foliar
streaming, electro-osmosis, pressure-flow, etc. transpiration). The actual water loss during
are put forth. The most convincing theory is transpiration occurs through three main sites
Munch’s pressure flow theory or mass flow - cuticle, stomata and lenticels. Accordingly,
hypothesis. three types of transpiration are recognized viz.
cuticular, stomatal and lenticular.
Ernst Munch proposed that photosynthetic
cell synthesizes glucose. Hence, its osmotic Water evaporates
concentration increases. Due to endo-osmosis from the leaves
water from surrounding cells and xylem,
is absorbed. The cell becomes turgid. Due Veins carry water
to increase in turgor pressure, sugar from into the leaves
photosynthetic cell is forced ultimately into the
sieve tube of the vein. This is called loading of Water is drawn up
Vein. the stem to the leaves

At the sink end, root cell utilizes sugar Roots take up water
and also polymerizes excess sugar into the from the soil
starch. Its osmotic concentration is lowered.
Exo-osmosis occurs. Water in the root cell is Fig. 6.6 : Transpiration
lost to surrounding cells, thereby decreasing i. Cuticular transpiration:
the turgidity of cell. Turgor pressure is lowered.
Hence, a turgor pressure gradient is developed Cuticle is a layer of waxy substance- cutin,
from sieve tube in the leaf to the root cell. present on outer surface of epidermal cells of
Consequently, food is translocated along the leaves and stem. Cuticular transpiration occurs
concentration gradient, passively. This is Vein by simple diffusion and contributes 8-10% of
unloading. At the sink end sugar is used and the total transpiration. Cuticular transpiration
excess water exudes into the xylem.

Main objection to this theory is that this
mechanism does not explain bidirectional
transport of food. More over, according to
Munch, pressure flow is purely a physical
process.

128

occurs throughout the day and its rate is Do you know ?
inversely proportional to thickness of cuticle.
• The number of stomata per unit area of
ii. Lenticular transpiration: leaf, is called stomatal frequency.
Lenticels are small raised structures
• The correlation between the number of
composed of loosely arranged complementary stomata and number of epidermal cells
cells. Each lenticel is a porous tissue consisting per unit area, is called stomatal index (I).
of cells with large intercellular spaces in
the periderm of the secondarily thickened 6.13 Structure of stomatal apparatus :
organs and the bark of woody stems and roots Typical stomatal apparatus consists of two
of dicotyledonous flowering plants. Lenticels
are present in bark of old stem and pericarp guard cells, stoma and accessory cells.
of woody fruits but are absent in leaves.
Lenticular transpiration contributes only Epidermal cells Subsidiary cell
about 0.1-1.0% of total transpiration. Rate of
Inner thick
Lenticular pore wall

Loose complementary Nucleus
cells Stoma pore

Guard cell

Fig. 6.8 (a) : Structure of guard cell

Guard cells

Fig. 6.7 : Structure of lenticel Stomatal
pore
lenticular transpiration is very slow. It also
occurs throughout the day. Chloroplast

iii. Stomatal transpiration: Fig. 6.8 (b) : Open and closed stoma
Stomata are minute apertures formed of
Stomata are minute, elliptical pores
two guard cells and accessory cells. They are bounded by two kidney/ dumbbell shaped
located in the epidermis of young stem and guard cells. Guard cell is a type of epidermal
leaves. Leaves generally show more number tissue which may be called as modified,
of stomata on the lower surface. Depending epidermal parenchyma cell. They are kidney-
upon distribution of stomata on leaves, leaves shaped in dicotyledons and dumbbell-shaped
are categorized into three types namely in grasses.
epistomatic- on upper epidermis (Hydrophytes-
e.g. Lotus), hypostomatic- on lower epidermis Epidermal
(Xerophytes- e.g. Nerium) and amphistomatic- cells
on both surfaces (Mesophytes- e.g. Grass).
Stomatal transpiration occurs only during Subsidiary
daytime. (Exception: Desert plants). cells

90 to 93% of total transpiration occurs Chloroplast
through stomata and that too during day time
only. Guard cells

Stomatal
aperture

A. Kidney shaped B. Dumbbell shaped

Fig. 6.9 : Types of guard cells

129

In Cyperus, both kidney- and dumbbell- According to theory of proton transport
shaped guard cells are present. (Levitt-1974), stomatal movement occurs due
to transport of protons H+ and K+ ions. During
Guard cells are living, nucleated cells with daytime, starch is converted into malic acid.
unevenly thick walls. Inner wall (wall facing Malic acid dissociates to form Malate and
stoma) of guard cells is thick and inelastic, and protons. Protons are transported to subsidiary
its lateral wall is thin and elastic. Guard cells cells and K+ ions are imported from them.
contain few chloroplasts which are capable of Potassium malate is formed that increases
poor photosynthesis. Guard cells have ability to osmolarity and causes endosmosis. Uptake of
change their size and form due to which stoma K+ ions is always accompanied with Cl¯ ions.
opens (widens) or closes (narrows).
At night, uptake of K+and Cl- ions is
Stoma is an elliptical pore formed due prevented by abscissic acid, changing the
to specific arrangement of guard cells. It is permeability of guard cells. Due to this guard
through the stoma, excess water is lost in the cells become hypotonic and thereby become
form of vapour. flaccid.
Accessory cells : These are specialized
epidermal cells surrounding the guard cells. Significance of Transpiration :
Their number is variable and are the reservoirs
of K+ ions. These are also called subsidiary Advantages:
cells.
Opening and Closing of Stoma : i. It removes excess of water.

Opening and closing of stoma is controlled ii. It helps in the passive absorption of water
by turgor of guard cells. During day time, and minerals from soil.
guard cells become turgid due to endosmosis.
Thus turgor pressure is exerted on the thin iii. It helps in the ascent of sap.
walls of guard cells. Being elastic and thin,
lateral walls are stretched out. Due to kidney iv. As stomata are open, gaseous exchange
or dumb-bell like shape, inner thick walls are required for photosynthesis and respiration,
pulled apart to open (widen) the stoma. During is facilitated.
night time, guard cells become flaccid due to
exosmosis. Flaccidity closes the stoma almost v. It maintains turgor of the cells.
completely. Endosmosis and exosmosis occur
due to diurnal changes in osmotic potential of vi. Transpiration helps in reducing the
guard cells. Different theories are proposed to temperature of leaf and in imparting
explain diurnal changes in osmotic potential. cooling effect.

According to starch-sugar inter- Disadvantages:
conversion theory (Steward 1964), during day
time, enzyme phosphorylase converts startch Excessive transpiration leads to wilting
to sugar, thus increasing osmotic potential of and injury in the plant. It may also lead to the
guard cells cosing entry of water there by gaurd death of the plant.
cells are stretched and stoma widens. The
reverse reaction occures at night brining about Transpiration is ‘A necessary evil’ :
the closure of stoma.
For stomatal transpiration to occur, stoma
Phosphorylase (Day) must remain open, during day time. When
stomata are open then only the gaseous exhange
Starch Sugar needed for respiration and photosynthesis, will
take place. If stomatal transpiration stops, it will
Night (Stoma closes) directly affect productivity of plant through the
(Stoma opens) loss of photosynthetic and respiratory activity.
Hence for productivity, stomata must remain
open. Consequently transpiration can not be
avoided. Hence, Curtis (1926) regarded
transpiration as ‘a necessary evil’.

130

Activity :

Prepare stomatal frequency chart for any six angiospermic plants in your area.

Plant Name Details

131

Exercise

Q. 1 Multiple Choice Questions 8. Water absorption takes place through
1. In soil, water available for absorption
by root is ................... .............
a. gravitaional water
b. capillary water a. lateral roots b. root cap
c. hygroscopic water
d. combined water c. root hair d. primary root

9. Due to low atmospheric pressure the

rate of transpiration will .............

2. The most widely accepted theory for a. increase
ascent of sap is ..............
a. capillarity theory b. decrease rapidly
b. root pressure theory
c. diffusion c. decrease slowly
d. transpiration pull theory
d. remain unaffected

10. Osmosis is a property of ..................

a. solute b. solvent

3. Water movement between the cells is c. solution d. membrane

due to ............. Q. 2 Very short answer questions.

a. T. P. b. W. P. 1. What is osmotic pressure?
2. Name the condition in which protoplast
c. DPD d. incipient plasmolysis
of the plant cell shrinks.
4. In guard cells, when sugar is converted 3. What happens when a pressure greater
into starch, the stomatal pore .............
a. closes almost completely than the atmospheric pressure is applied
b. opens partially to pure water or a solution?
c. opens fully 4. Which type of solution will bring about
d. remains unchanged deplasmolysis?
5. Which type of plants have negative root
5. Surface tension is due to .............. pressure?
6. In which conditions transpiration pull
a. diffusion b. osmosis will be affected?
7. Mention the shape of guard cells in
c. gravitational force d. cohesion Cyperus.
8. Why do diurnal changes occur in
6. Which of the following type of solution osmotic potential of guard cells?
9. What is symplast pathway?
has lower level of solutes than the

solution?

a. Isotonic b. Hypotonic

c. Hypertonic d. Anisotonic

7. During rainy season wooden doors warp Q. 3 Answer the following questions.
1. Describe mechanism for absorption of
and become difficult to open or to close water.
2. Discuss theories of water translocation.
because of ............... 3. What is transpiration? Describe
mechanism of opening and closing of
a. plasmolysis b. imbibition stomata.

c. osmosis d. diffusion

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4. What is transpiration? Explain role of Q. 4 Long answer questions.
transpiration. 1. Describe structure of root hair.
2. Write on journey of water from soil to
5. What is significance of transpiration? xylem in roots.
Explain root pressure theory and its 3. Explain cohesion theory for translocation
limitations. of water.
4. Write on the mechanism of opening and
6. Explain capillarity theory of water closing of stoma.
translocation. 5. What is hydroponics? How is it useful in
identifying the role of nutrients?
7. Why is transpiration called ‘a necessary 6. Explain the active absorption of
evil’? minerals.
7. Write on macro- and micro nutrients
8. Explain movement of water in the root. required for plant growth.

9. Define and or explain the terms: Project :
Osmosis, diffusion, plasmolysis,
imbibition, guttation, transpiration, 1. Prepare powerpoint presentation for
ascent of sap, active absorption, DPD, different types of transpiration.
turgor pressure, water potential, wall
pressure, root pressure.

10. Distinguish between a) Osmotic pressure
and turgor pressure b) Diffusion and
osmosis.

11. Enlist macronutrients and micronutrients
required for plant growth.

12. How are the minerals absorbed by the
plants?

133

7 Plant Growth and Mineral Nutrition

Can you recall? Do you know ?

1. Do you think that the growth is property of Apical meristem : In vascular plants, growth
living beings only? is restricted to the apices of root and shoot. It
is responsible for growth in length/ height and
2. Is there any difference between plant the differentiation or cell types. It contributes
growth and animal growth? to the primary growth.
Intercalary meristem : It is located at the
7.1 Plant growth : node or at the base of internode of stem. It
Growth is one of the characteristic features is primarily responsible for increasing length
of internodes and also for formation of leaf
of living organisms. Growth as a phenomenon primordia and lateral buds.
has two aspects viz. quantitative and Lateral meristem : It is located laterally along
qualitative. Quantitative aspect speaks for an the axis of dicotyledons and gymnosperms. It
increase in the length, breadth, size, volume, is located as strip in the vascular bundles of
body mass or dry weight and number of cells. stem of dicots. It is called vascular cambium.
Growth as a quantitative change is a final end It is responsible for increase in the girth of the
product of successive metabolism. stem due to addition of secondary vascular
tissues.
Qualitative aspect talks about the change
in the nature of growth where development 7.2 Phases of growth :
is an ordered change or progress while The cells in the meristem divide, enlarge
differentiation leads to higher and more
complex state. Growth thus can be defined as and get differentiated. Corresponding to these
permanent, irreversible increase in the bulk of three stages, there are three phases of growth :
an organism, accompanied by the change of A. Phase of cell division/ formation : Cells
form. of meristem are thin walled, non-vacuolated
having prominent nucleus and granular
In multicellular (vascular) plants, growth cytoplasm. Meristematic cell undergoes
is indeterminate and occurs throughout mitotic division to form two new cells. One
the life indefinitely. It is restricted to some cell remains meristematic and the other cell
specific region called meristems which are undergoes enlargement and differentiation. In
the regions where new cells are constantly and this phase, rate of growth occurs at a slower
continuously produced. Meristems are of three pace (Lag phase).
types based on location viz. Apical, Intercalary
and Lateral. B. Phase of cell enlargement/ elongation :
The newly formed cell becomes
Do you know ?
vacuolated, osmotically active and turgid due
Root and stem show indefinite or to absorption of water. The turgidity results in
indeterminate growth. However, in organs the enlargement of cell - both lengthwise and
like leaves, flowers and fruits, growth is breadthwise. In this phase new wall materials
determinate. i.e. they grow up to certain and other materials are synthesized to cope
genetically destined size. In unicellular up with the enlargement. The growth rate
plants, growth is uniform and determinate. in this phase occurs at an accelerated pace

(exponential or Log phase).

134

C. Phase of Cell maturation/ differentiation : temperature, oxygen, Carbon/ Nitrogen ratio,
The enlarged cell now becomes specialized gravitational force, light and growth hormones.
The chief conditions are explained below :
to perform specific fuction and attains maturity
- both morphological and physiological. In this Water is the essential component of
phase, rate of growth slows down and comes to protoplasm and maintains turgidity of the cell.
a steady state (stationary phase). It acts as aqueous medium for biochemical
reactions. Microelements and Macroelements
Plumule are nutrients required for the proper growth
of the plant. Optimum Temperature ranges
Zone of mature between 25-350C. Oxygen is essential for
cells respiration and the release of energy. Light is
very much essential for germination of seed and
Zone of cell photosynthesis. Gravitational force decides
differentiation the direction of growth of the shoot and root.

A Zone of cell 7.4 Growth Rate and types of growth :
elongation Growth rate :

Zone of cell It is the increased growth per unit time. It
B formation is also called efficiency index. Rate of growth
can be measured by an increase in the size and
Fig. 7.1 Phases of growth in root - area of different plant organs like leaf, flower
Position of radicle at the begining (A) and at and fruits.

the end (B) The ratio of change in the cell number (dn)
7.3 Conditions for Growth : over the time interval (dt) is called Absolute
growth rate (AGR). Alternatively, it is the
The different environmental and measurement and comparison of total growth
physiological conditions necessary for the per unit time.
growth include - Water, supply of nutrients,

Growth in plants can be measured in terms of....
1. Increase in the number of cells produced - e.g. single maize root apical mesistem can give rise

to more than 17,500 new cells/Hour.
2. Increase in surface area of the leaf - e.g. growth of dorsiventral leaf.
3. Increase in length - e.g. growth of pollen tube.
4. Increase in volume of a fruit - e.g. In watermelon flower, ovary after fertilization increases in

its size/ volume by upto 3,50,000 times.
5. Increase in girth of shoot.
6. Increase in dry weight of organ.

Various methods for the measurement of linear growth of stem and radicle are as follows:
1. Direct method : It can easily be measured with the help of ordinary measuring scale. It is a

simple method.
2. Horizontal microscope : It is used to measure growth in fields.
3. Auxanometer : This equipment is used for precise measurement of linear growth of shoot.

There are two types of an auxanometers viz, a) Arc auxanometer b) Pfeffer’s auxanometer
4. Crescograph : It records primary growth very accurately. It magnifies growth upto 10,000

times giving information of growth per second. It is developed by sir J. C. Bose.

135

AGR = dn initially but later on there is a rapid growth
dt at exponential rate. Geometric growth can be
expressed mathematically by an equation.
The AGR, when divided by total number of
a
cells present in the medium, gives Relative b

growth ratio (RGR). Alternatively, RGR refers Height of the plant (L)

to the growth of a particular system per unit

time, expressed on a common basis or it is the

ratio of growth in the given time/ initial growth.

RGR = AGR
n
AGR and RGR are useful in describing the

dynamics of cell growth in culture.

Types of growth : Time (t)
There are two types of growth viz,
Graph 7.2 : Constant linear growth
arithmetic growth and geometric growth.
W1 = Wo ert
a. Arithmetic growth : Here, rate of the W1= Final size , Wo = initial size
r = growth rate, t = time of growth
growth is constant and an increase in the e = base of natural logarithm

growth occurs in arithmetic progression. i.e. 2,

4, 6, 8 cms etc.In this type of growth, the rate of

growth is constant.

After mitosis one of the daughter cell a. Arithmetic b. Geometric

continues to divide and the other cell takes

part in the differentiation and maturation.

e.g. elongation of root at a constant rate, best

explains arithmetic growth. Linear curve is

obtained when growth rate is plotted against

the time. Arithmetic growth is expressed c. Zygote divided

mathematically by an equation, Geometric phase:
all cells divide
It is expressed as,

Lt = Lo + rt

Where Lt = Length at time ‘t’

Lo = Length at time ‘Zero’

r = Growth rate

t = Time of growth

When graph of length (L) is plotted against Arithmetic phase : = Cells capable of
the time (t), a linear curve is obtained as These cells divide division
indicated in the diagram.
This cells do not = Cells that have
b. Geometric growth : divide lost capacity to
Cell divides mitotically into two. Here,
divide
both the daughter cells continue to divide and
redivide repeatedly. Such growth is called Digrammatic representation of : a. Arithmetic, b.
geometric growth. Here, growth rate is slow Geometric grwoth and c. Stages during embryo
development showing geometric and arithmetic phases

Fig. 7.3 : Growth rate

136

We can also observe quantitative Population size J-shaped curve
comparison between the growth of living
system in two ways. Year

Measurement and comparisons of total Graph 7.5 : Exponential Growth curve
growth per unit time is called the Absolute
growth rate(AGR) whereas the growth of Stationary phase
the given system per unit time expressed on
a common basis per unit initial parameter is
called the Relative growth rate(RGR).

A1 B1 Size/ weight of the organ
10 cm2 55 cm2 Exponential phase

5 cm2 50 cm2
AB

Fig. 7.4 : Growth rate comparison Lag phase

In the above example, two leaves ‘A’ Time
and ‘B’ are of different sizes but show same
absolute increase in area in a given time. Both Graph 7.6 : Sigmoid Growth curve
leaves grow and increase their area by 5cm2
to produce ‘A’ and ‘B’ leaves. ‘A’ leaf of 5cm2 The total time (period) required for all
in size grows 5cm2/ day then its RGR would phases to occur, is called Grand Period of
be 100%. If the leaf is 50cm2 in size and the Growth.
growth rate/day is 5cm2 then its RGR would
become 10%. 7.6 Differentiation, De-Differentiation, Re-
Differentiation :
7.5 Growth curve : a. Differentiation :
It is a graphic representation of the total
It is maturation of cells derived from apical
growth against time. There are three types of meristem of root and shoot. Permanent change
curves viz, Linear, Exponential and Sigmoid. in structure and function of cells leading to
Arithmetic growth curve is linear while maturation, is called differentiation. During
Geometric growth curve is exponential. cell differentiation, cell undergoes few to
major anatomical and physiological changes
Corresponding to three distinct phases e.g. Parenchyma in hydrophytes develops
of growth, growth rates differ. In Lag phase, large schizogenous interspaces for mechanical
growth rate is slow. In Exponential (Log) support, buoyancy and aeration. The maturation
phase, growth rate is faster and reaches its is at the cost of capacity to divide and redivide.
maximum. In Stationary phase, growth rate
gradually slows down. When a graph of rate of b. Dedifferentiation :
growth against time is plotted for three phases The living differentiated cell which has
of growth, a sigmoid curve is obtained.
lost the capacity to divide, may regain the same
as per the need and divide. Thus, permanent
(mature) cell undergoes dedifferentiation and
becomes meristematic e.g. interfascicular

137

cambium and cork cambium are formed from stage show different forms of leaves in the same
parenchyma cells between vascular bundles plant e.g. heterophylly in cotton, coriander,
and inner most layer of cortex, respectively. larkspur (Delphinium). The environmental
heterophylly is shown by Ranunculus flabellasis
c. Redifferentiation : (butter cup). The intrinsic plasticity is found in
The cells produced by dedifferentiation coriander and cotton. Heterophylly is exhibited
in the same plant in different growth phases or
once again lose the capacity to divide and under different environmental conditions.
mature to perform specific function. This
is called redifferentiation e.g. secondary 1 2 3
xylem and secondary phloem are formed
from dedifferentiated cambium present in the Juvenile Adult Terrestrial habitat
vascular bundle.

7.7 Development :
It refers to the ordered or progressive

changes in shape, form and degree of
complexity. It includes all the changes occurring
in sequence from the germination of seed upto
the senescence or death during life cycle of
plants. Thus development includes growth,
morphogenesis, maturation and senescence.

Seed germination 4
Meristem
Water habitat
Cell division
Fig. 7.8 Heterophylly

Plasmatic growth Can you tell?

Cell elongation 1. Is there any relation between phases of
growth and regions of growth curve?
Cell maturation
2. Which plant organ does show both
Mature cell arithmetic and geometric growth?

Senescence 3. Differences between arithmetic and
geometric growth.
Death
7.9 Growth Hormones :
Fig. 7.7 Flow chart of development The term ‘hormone’ was coined first by
7.8 Plasticity :
Starling (1906) in animal physiology. The
It is the capacity of being moulded, formed internal factors that influence growth are called
or modeled. It is the ability of plants to form growth hormones or growth regulators as they
different kinds of structures (i.e. to change) in inhibit, promote or modify the growth. Growth
response to different environmental (external) promoters are auxins, gibberellins (GA) and
or internal stimuli, in various phases of life. cytokinins (CK). Growth inhibitors in plants
are ethylene and abscissic acid (ABA). All
In many plants, juvenile stage and mature phytohormones are growth regulators.

138

Know the Scientist : According to Thimann and Pincus (1948)
“Plant hormones are organic substances
The auxin is the first hormone to be produced naturally in higher plants affecting
discovered in plants. Discovery of auxins growth or other physiological functions at a site
dates back to 19th century when Charles remote from its place of production and active
Darwin (1886) was studying tropism in in very minute (optimum) amount”. Hormones
plants. He exposed canary grass coleoptile to are transported through phloem parenchyma
unilateral light. He concluded that a growth (Phillips 1971).
stimulus is developed in the coleoptile tip
and transmitted downwards to the growth a. Auxins (Auxien = to grow):
zone. This has caused bending of the tip F. W. Went in 1931, used this term first.
towards light.
Auxin was isolated from urine of a person
The Danish plant physiologist Boysen suffering from Pellagra (Kogl and H. Smit
- Jensen (1910) cut off the colepotile and 1931). In plants, it is synthesized in growing
inserted thin plate of gelatin beween the tips or meristematic regions of plants from
tip and the cut stump. He observed that where it is transported to other plant parts. The
coleoptiles tip still bends towards unilateral most common and important natural auxin is
light. Indole-3-acetic acid (IAA). Tryptophan is the
primary precursor of IAA in plants. It is the
Paal (1919) cut off the tip of colepotile first hormone to be discovered in plants and
and replaced it asymmetrically on the is primarily responsible for cell elongation. It
cut coleoptile stump. He observed that shows polar transport - Basipetal transport in
the colepotile tip bent away from the side stem. Now synthetic auxins like IBA (Indole
bearing tip even in dark. butric acid), NAA (Naphthalene acetic acid),
2, 4-D pichloro (Phenoxy acetic acid), etc. are
F.W. Went (1928) successfully isolated used.
natural auxin from Avena coleoptile tips.
He cut off the tip and placed them on small Physiological effects and applications of
agar blocks. Then after certain period of auxin:
time placed the agar blocks asymmetrically
on cut coleoptile stump that caused bending. The primary effect is cell enlargment. In
He demonstrated the presence of substance most of the higher plants, growing apical bud
which could diffuse into agar blocks. Went inhibits the growth of lateral buds. This is called
named this substance as auxin. as apical dominance. Auxin stimulates growth
of stem and root. Auxin induces multiplication
coleoptile bends tip removed tip removed coleoptile tips of cells, hence used in tissue culture experiments
on agar to produce callus. It stimulates formation
of lateral and adventitious roots. These are
top of coleoptile gelatine tip placed on tips removed, marketed as synthyetic herbicides. e.g. 2, 4-D
removed (no inserted one side agar cut into (2,4 dichlorophenoxy acetic acid). It kills dicot
bending) blocks weeds without affecting monocot crop plants.

light-proof bends bending in agar block placed The seedless fruits like orange, lemon,
cap on tip (no towards absence of on one side and grapes, banana etc. are produced by application
of auxin (i.e. induced parthenocarpy). Auxins
bending) light light bending occurs promote cell division in cambium and also cause
early differentiation of xylem and phloem. It
Darwin Boysen-Jensen Paal Went

139

promotes root elongation in low concentration by synthesizing hydrolysing enzyme amylase
and shooting at higher concentration. It to produce sugar. The most striking effect of
also hastens early rooting in propagation by it, is the elongation of stem where internodes
‘cutting’. increase in length. It also promotes bolting i.e.
elongation of internodes just prior to flowering
Foliar spray of NAA and 2,4-D induces in plants those with rosette habit e.g. beet,
flowering in litchi and pineapple. Likewise, it cabbage. It causes parthenocarpy in tomato,
prevents premature fruit drop in apples, pear apple and pear, and flowering in long day
and oranges, and also prevents formation plants. It is used to increase the fruit size and
of abscission layer. Auxins play a role in bunch length of grapes. When gibberellins
elongation of cell. It is known to increase rate are applied on genetically dwarf plants
of respiration. Auxins break dormancy in seed like maize, the stem rapidly elongates and
and promote quick germination. acquires the height of normal tall varities of
maize. Application of gibberellins overcomes
Do you know ? the requirement of vernalization. Usually, it
inhibits growth of root, delays senescence and
Agent orange : Mixture of two phenoxy prevents abscission. It also breaks dormancy of
herbicides in easter form. 2, 4-D and 2,4, 5-T seed and hastens germination. Application of
(dioxin) is known as agent orange used in gibberellin causes production of male flowers
Vietnam war for defoliation of forests. on female plant.

b. Gibberellins : c. Cytokinin :

It is another growth promoting hormone It is another growth hormone that promotes
and is abundant in root tip and developing cell division. Letham coined the term cytokinin.
seeds. It shows non-polar transport through The first cytokinin was discovered by Skoog
vascular tissue. and Miller (1954) during investigation of
nutritional requirements of callus tissue
Gibberellins were first isolated from the culture of Nicotiana tabacum (Tobacco).They
fungus Gibberella fujikuroi by Japanese scientist observed that the callus proliferated when
Kurosawa (1926). He observed that when the nutrient medium was supplemented with
rice plant was infected by fungus Gibberella coconut milk and degraded sample of DNA
fujikuroi, it shows extensive stem elongation (obtained from herring sperm). They named
called‘bakane disease’. The crystalline form it as kinetin. Chemically kinins are 6-furfuryl
of Gibberellins were isolated by Yabuta and amino purine. First natural cytokinin was
Sumiki (1938) from the fungus culture. They obtained from unripe maize grains by Letham
named it as gibberellin. It is synthesized in et al. It is known as Zeatin. 6-benzyl adenine is
young leaves, seeds, roots and stem tips. These a synthetic cytokinin hormone. Seven different
are synthesized from mevalonic acid. More types of cytokinins are recorded from plants.
than 150 chemical types are known so far. Natural cytokinins are also reported from
GA3 is most common and biologically active plants like Banana flowers, apple and tomato
form. Chemically it contains a gibbeane ring - fruits, coconut milk, etc.
a cyclic diterpene with four isoprene units.
Physiological effects and applications of
Physiological effects and application of cytokinin :
Gibberellins :
Besides cell division, it also promotes cell
Dormancy of bud can be broken by enlargment. High cytokinin promotes shooting.
gibberellin treatment. It can promote seed
germination in cereals like barley and wheat

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A low ratio of cytokinin to auxin induces root the enhancement of process of senescence of
development but a high ratio causes buds and plants organs. It inhibits flowering in most of
shoot to develop. Cytokinin and auxin ratio the plants except pineapple. It causes epinasty
and their interactions controls morphogenic (drooping) of leaves and flowers. It increases
differentiation. It promotes the growth of activity of chlorophyllase enzyme causing
lateral buds and controls apical dominance by degreening effect in banana and Citrus fruits.
cell division. It delays the senescence or ageing
and abscission processes in plant organs. This Internet my friend
was reported by Richmond and Lang (1957).
Formation of interfasicular cambium and Degreening: It is the process of decomposition
expansion of cells are other functions. It also of green pigment in fruits usually by applying
breaks dormancy and promotes the germination ethylene. This method is called trickle
of seeds. Cytokinin reverses apical dominance degreening. Collect more information about
effect. It induces RNA synthesis and formation degreening.
of interfascicular cambium.
e. Abscissic Acid :
d. Ethylene : It is a natural growth inhibiting hormone.
It is the only gaseous growth regulator.
Carns and Addicott (1961-65) observed that the
Denny (1924) reported ethylene is effective shedding of cotton balls was due to a chemical
in fruit ripening. Gane (1934) established that substance abscisin I and II. Wareing (1963)
plants naturally synthesize ethylene. Crocker isolated a substance from buds of Acer that can
(1930) proposed that ethylene is the plant induce bud dormancy and named it dormin.
hormone responsible for fruit ripening. It is a These two identical chemical substances were
simple gaseous hydrocarbon with essential given the common name abscissic acid. It is
role in the fruit ripening. The most widely used synthesized in leaves, fruits, roots, seeds etc.
compound as a source of ethylene is ethephon. Chemically, it is a 15-carbon sesquiterpenoid
It is synthesized in roots, shoot apical meristem, and is synthsized from mevalonic acid.
ripening fruits etc.
Physiological effects and application of ABA:
Do you know ? It promotes abscission of leaves and

Ethret / Ethephon is a 2-chloroethyl induces dormancy in many plants. It controls
Phosphoric acid, which releases ethylene the dormancy in buds and seeds by inhibiting
after dissolving in water. growth processes. It accelerates the senescence
of leaves, flowers and fruits. It inhibits and
Physiological effects and application of delays cell division and cell elongation and
ethylene : suppresses cambium activity by inhibiting
mitosis in vascular cambium. ABA could cause
It promotes ripening of fruits like bananas, efflux of k+ ions from the guard cells and
apples and mangoes. It stimulates initiation of result in closure of stomata. So, it is known as
lateral roots in plants and breaks the dormancy an antitranspirant. It acts as a stress hormone
of bud and seed. by inducing the plant to bear the adverse
environmental conditions. It inhibits flowering
It accelerates the abscission activity in long day plants but stimulates flowering in
in leaves, flowers and fruits by forming of short day plants.
abscission layer. Ethylene inhibits the growth
of lateral buds and causes apical dominance
and retards flowering. It is associated with

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