upper epidermis palisade mesophyll bundle sheath vascular bundle lower epidermis Spongy mesophyll epidermis Xylem Xylem Phloem epidermis โครงสร้างพืชใบเลี้ยงคู่ pith cell Phloem vascular cambium vascular bundle Cortex Stele vascular bundle cortex pericycle endodermis