SPECIALISED CELLS

NOTES

LEARNING OUTCOMES

a) outline the structures, functions and
distributions of unspecialised cells found
in plants (meristematic cells);

(b) describe the structures, functions and
distributions of specialised plant cells found
in epidermal, ground and vascular tissue;

Plant Tissues

Meristems

A plant meristem is a group of cells which retain
the ability to divide by mitosis.
There are three types of plant meristems:

Apical meristems
Lateral meristems
Intercalary meristems

Meristems

Apical meristems
Apical meristems are located at the growing shoot and
root apex and are responsible for primary growth.
Lateral meristems (cambium)
Lateral meristems occur as cylinders in the older parts
of the plant and are responsible for the secondary
growth of dicotyledons plants.
Intercalary meristems
These meristems occur at the nodes of plant e.g. the
nodes of many monocotyledons.

Meristems

Structure of meristematic cells

The cells are small,
thin-walled
have a central large nucleus and dense cytoplasm
with small vacuoles.
The cells are rectangular
closely packed with no intercellular air spaces.

Meristem function

Meristematic cells retain the ability to divide by
mitosis to produce new cells.
The cells elongate and differentiate to form
specialised cells to carry out specific functions.
Some examples are growth, reproduction and
replacement of old and damaged cells.
Apical shoot and root meristems produce new cells
for growth of shoot and root.

Meristem function

Vascular cambium produces new cells to increase the
diameter of stems and roots during secondary
growth.
Cork cambium (phellogen) produces the outer cork
layer (phellem) which consists of suberised cells.
The cork layer reduces evaporation of water from the
plant and protects against entry of pathogens.
The intercalary meristems allow growth and increase
in length in regions other than the tip.

Permanent tissue

Parenchyma

Parenchyma tissue consists of living cells.
They are usually isodiametric or elongated cells.
However their shape may be distorted by pressure
from adjacent cells.
Parenchyma cells have thin cell walls containing
cellulose, hemicellulose and pectin.
There is no secondary walls.
The walls are permeable to water and permit the
passage of solutes.

Parenchyma

have a large, central vacuole with nucleus and a
thin layer of cytoplasm pushed to the periphery
loosely packed together with many large
intercellular air spaces to allow exchange of gases.
Some parenchyma cells contain chloroplasts,
chromoplasts and leucoplasts. Cells containing
chloroplasts are also called chlorenchyma,



Functions of parenchyma:

They are unspecialised tissue which form the major
component of ground tissue of stems and roots
especially in herbaceous plants.
When the cells are turgid and tightly packed they
provide support for herbaceous plants.
Some parenchyma cells like mesophyll cells contain
chloroplasts and help carry out photosynthesis.

Functions of parenchyma

They store food substances such as starch,
proteins and lipids. For example, potato tuber cells
have many amyloplasts which contain starch and
malic acid that are stored temporarily in the
vacuoles of CAM parenchyma cells.

They are potentially meristematic and can form
secondary meristematic tissue such as vascular
cambium and cork cambium (phellogen).

Functions of parenchyma:

The parenchyma cells in flowers and pericarp contain
chromoplasts to attract pollinating agents and help in
the dispersal of fruits and seeds.
The parenchyma tissues can be modified or
differentiated to form specialised cells to carry out
specific functions.
These cells, for example, are epidermis, mesophyll,
endodermis, pericycle, aerenchyma and secretory
cells









Collenchyma

Collenchyma tissue consists of living cells.
Cells are polygonal-shaped and elongated.
The cells are closely packed together with very small or
no intercellular air spaces.
The cell walls are unevenly thickened with deposits of
cellulose, pectin and hemicellulose.
The thickenings usually occur at the corners of the cell
walls (angular collenchyma).
Pits are present in the walls
Collenchyma tissues are usually found in herbaceous
plants below the epidermis, midrib of the leaves and
leaf petioles.





Functions of collenchyma:

It acts as a supporting tissue to provide the herbaceous
plant with mechanical strength and flexibility.
It allows the cells to expand and be stretched as the
young stem grows.
Some of the collenchyma cells contain chloroplasts
which can carry out photosynthesis.

Sclerenchyma

There are two types of sclerenchyma:
Sclerenchyma fibres
Sclereids (stone cells)

Sclerenchyma fibres

Elongated, polygonal-shaped with tapering ends
Average length is about 1-3 mm.
In certain plants such as hemp and jutet the length is
about 20-550 mm.
Mature sclerenchyma tissue consists of dead cells
tightly packed together with no intracellular air spaces.

Sclerenchyma fibres

(Acidified phloroglucinol can be used for staining
lignin. The lignified tissue is stained red)
Sclerenchyma cells have thick, lignified secondary cell
walls impermeable to water, solutes and gases.
The cells have no protoplasm with narrow empty
lumen. Pits are present in the cell walls.
Sclerenchyma fibres are found below the epidermis of
stems or roots, in or around the vascular bundles and
in the midribs of leaves.

Sclerenchyma fibres

Sclerenchyma Functions:

It acts as supporting tissue. Collectively, the tightly
packed sclerenchyma fibres with thick lignified walls
provide the plant with mechanical strength and
rigidity.
The tapered ends of the sclerenchyma fibres overlap
and interlock with one another, further increasing
their combined strength.

Sclereids (stone cells)

Different shapes (Figure 2.28) but are usually shorter
than sclerenchyma fibres.
Consist of dead cells with thicker lignified walls.
Simple branching pits are present in the walls
Sclereids are found singly or in groups in stems, leaves,
fruits e.g. pears, guava, in the hard endocarp of
coconuts and seeds e.g. testa of bean seeds.
Functions:
Protective tissue which gives strength and support to
the plant structures or organs.

Sclereids (stone cells)

Sclereids (stone cells)

Xylem

Xylem tissue consists of four types of cells:
Vessels
Tracheids
Fibres
Xylem parenchyma

Xylem vessels

These are only found in the xylem tissue of flowering
plants.
Most water travels in the xylem vessels.
Xylem vessels are formed from a column of tubular cells.
The end cross walls break down so that the cells (xylem
elements) combine to form a continuous long tube.
The walls of these tubes become strengthened by the
deposition of lignin,
The lignified walls are impermeable to water, solutes and
gases.
Xylem vessels are composed of dead cells.
There is no protoplasm in the xylem vessels and hence,
more water can flow through the hollow continuous tube
with less friction.

Primary xylem

The first xylem tissue formed in growing plants.
There are two type of vessels in the primary xylem,
protoxylem and metaxylem
(a) Protoxylem is the first xylem vessel to develop,
just behind the apical meristem in the shoot and root.
There is incomplete lignification in the walls
Lignin is deposited in rings to form annular vessels
or in spirals to form spiral vessels.
These annular and spiral vessels can be stretched to
provide support for the vessels during elongation and
growth of the young stems and roots.

Metaxylem & secondary xylem

(b) As growth proceeds, new vessels are formed with more
extensive lignification. These are called metaxylem.
(c) Metaxylem vessels cannot be stretched. The lignified
walls have high tensile strength and prevent the vessels
from collapsing when water flows through it under tension.
Metaxylem have bigger lumen and are able to transport
more water and mineral ions to the older plant.
The presence of pits in metaxylem vessels allow lateral
movement of water and mineral ions to the surrounding
living cells.
Secondary xylem is formed from the activity of the
vascular cambium of dicotyledonous plants during
secondary thickening

















Tracheids

generally longer but more narrow than vessel elements.

elongated spindle-shaped single cells with tapering end
walls.

The tapered ends of the tracheids overlap and interlock
with one another further increasing their combined
strength.

The cell walls are lignified. When mature, tracheids are
dead cells with empty lumens.

Pits are present in the walls to allow water to move from
one tracheid to another and also to the surrounding living
cells

Tracheids transport water and mineral ions but are less
efficient than vessels. They do not have large perforated or
open ends between the cells and water has to move
through smaller pits.

Xylem fibres & xylem parenchyma

Xylem fibres
These are similar in structure to sclerenchyma fibres.
They are dead cells with thickened lignified walls.
They help to support the plant body and are not
involved in water transport.

Xylem parenchyma
The xylem parenchyma are living cells.
They act as packaging tissue in the primary xylem.
In secondary xylem, the parenchyma cells form radial
medullary rays which function in radial transport of
food and water, gases and food storage.

Functions of xylem

To transport water and mineral salts from the roots
to the aerial part of the plant.
To give mechanical support to the plants. The
lignified cell wall enable xylem vessels and tracheids to
resist compression and tension.

Phloem

Phloem is a component of the vascular tissue.
In angiosperms, phloem consist of sieve tubes,
companion cells, phloem parenchyma, fibres and
sclereids.





A sieve tube

consists of sieve elements (sieve cells) joined together to form a
long tube.
end walls are perforated forming sieve plates with sieve pores.
This allows cytoplasmic connections between the sieve elements
and the flow of liquid from one sieve element to the next.
The cells are alive, with thin cellulose cell walls and protoplasm.
As the cells mature some of the organelles for example,
The nucleus, ribosomes and the Golgi apparatus degenerate.
This leaves a narrow layer of cytoplasm containing few small
mitochondria and endoplasmic reticulum at the periphery to
facilitate smooth flow of soluble food.
In some plants the sieve tube elements also contain fibrous
phloem proteins

Companion cells

Found only in the angiosperms.
Adjacent and closely associated with the sieve tube
elements.
have a large nucleus, dense cytoplasm with small
vacuoles.
metabolically active
have numerous mitochondria and ribosomes.
linked by numerous plasmodesmata to sieve elements.
move sugars such as sucrose and amino acids into the
sieve elements