Tuesday, November 27, 2012

Internal Structure of Dicot Stem

Internal Structure of Dicot Stem

The ts of dicot stem shows following internal structure:

Epidermis
it is the outermost layer of stem.it consist of singlw layer of thinwalled ,compactly arranged parenchymatous cells.they may contain chloroplast.they are externally covered by cuticle.they possess multicellular hair.epidermis help in photosynthesis,protection and transportation.

Hypodermis 
it lie below the epidermis.hypodermis are collenchymatous.each cell contain pectin or lignin deposit in their cell wall.they are 2-4 layered.hypodermis help in mechanical sterngth ans store food.

Cortex
cortex lie below hypodermis.they are made up of parenchymatous cell.they poaasess inter cellular space and are richly oval in shape.they may contain chloroplast.they help in storage of food,conduction,photosynthesis and secretion.

Endodermis: This is innermost layer of the cortex consisting of barrel shaped cells and surrounding the stele. endodermis is parenchymatous.Endodermis is conspicuous outside the hard bast but often loses its identity on either side. It contains many starch grains and is also known as starch sheath.cell wall contain deposition of suberin and lignin.

Pericycle
pericycle are made up of sclerenchymatous cells.parenchymatous tissue also alters with sclerenchymatous tissue.they are 2-4 layered.they are also called bundle cap or bast cap as thet lie above vascular bundle.

Vascular bundles
vascular bundle of dicot stem are conjoint,collateral and open type.sometime bicollateral in cucurbita stem.vascular bundle are arranged in ring.vascular bundle consist of :

(a) Phloem: It is external and has (i) sieve tubes which are large elements (ii) companion cells which are smaller cells associated with sieve tubes and (iii) Phloem parenchyma which is remaining mass of small cells. They are all thin and living. phloem fibre is absent. phloem help in transportation of food material.
(b) Cambium: It is band of thin walled tissue lying between phloem and xylem. Its cells are in radial rows and are rectangular in shape, very small in size and very thin walled. Cambium is responsible for secondary growth,

(c) Xylem :

xylem is in endarch condition.xylem contains xylem parenchyma,xylem fibre,trachieds and xylem vessels.xylem is diffrentiated into proto and meta xylem.xylem help in transportation of water and minerals.

Medullary rays
radially elongated,thin walled,parenchymatous cells present between vascular bundle are called medullary ray.they help in radial translocation of food and water.

Pith
it is the central part of stem.it consist of round,oval,thin walled parenchymatous cells with intercellular space.they store food material.




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