INTERNAL STRUCTURE OF YOUNG STEM

Internal structure of young stem 

Internal stucture of dicot stem :

Internal structure of a typical dicot (dicotyledonous) stem shows following features :-

1. Epidermis : Epidermis is the outermost protective layer of the stem It is made up of elongated compactly arranged cells. It is single layered and lack of chloroplasts.

Stomata, multicellular hairs (trichomes) are present on epidermis. Thin cuticle is present on epidermis

Epidermis plays a significant role in protection

Trichomes help in preventing water loss due to transpiration

2. Cortex : The cells arranged in multiple layers helmen epidemic, and periack constitute the corte in dicotyledon stem cortex is divided into three parts.or sub zunes.

(a) Hypodermis

(b) Endodermis

(c) General cortex

 (a) Hypodermis :It is pattent lust below the epidermis. İs multilayered ihick . It is composed of collenchyma and cole contain chloroplasts.

(b) General Cortex. : This part is composed.of rounded the walled cells.of parenchyma conspicuous intercellular spaces Storage of food is the main function of the cortex.Resin canals/ mucilage casals are present in it. These are schlagenous in origin. The innermost layer.of the.cotex in.called endodermis.

(c) Endodermis : It is single layered The cells of endodermis ae rich in starch grains, thus lus also known as "starch sheath".

3. Pericycle Pericycle is situated below the endodermis. The pericycle of stem is multilayered.

In sunflower stem, pericycle is made of alternate bands of parenchymatous and sclerenchymatous cells. The part of pericycle which is present infront of the vascular bundle is made up of sclerenchyma and remaining part is composed of parenchyma. Part of pericycle which is situated in front of vascular bundle is known as Bundle cap. In sunflower stem, pericycle is heterogenous in nature Sclerenchymatous part of period is also known as Hard bast.

Note - Pericycle is present above the phloem in the form of semilunar patches of sclerenchyma.

4. Vascular Bundles : The wedge shaped vascular bundles are arranged in a ring. The ring arrangement of vascular hurdles is a characteristic of dicot stem. Each vascular bundle is conjoint, collateral and open and xylem is endarch.

5. Pith: This is well developed region, present in the centre. The cells of this region are made up of parenchyma.

6. Medullary rays : Radially arranged parenchymatous in between vascular bundles called pith rays or medullary rays. The main function of pith rays is radial conduction of food and water.

INTERNAL STRUCTURE OF CUCURBITA STEM:

It contains five ridges and five furrows. The vascular bundles are arranged in two rings. Each ring has five vascular bundles. In this way the total 10 vascular bundles are present

The vascular bundles of outer ring are small in size and situated below the ridges while the vascular bundles of inner ring are large in size and located below the furrows

Vascular bundles are conjoint, bicollateral and open and xylem is endarch. Outer cambium is functional.

Hypodermis is mainly present in ridge region.

General cortex is Sclerenchymatous.

Pericycle is sclerenchymatous.

INTERNAL STRUCTURE OF MONOCOTYLEDONOUS STEM

Epidermis: Epidermis is the outer most single celled thick laver. It is cOVered with thick cuticle Multicellular hairs are absent.

Hypodermis: Hypodermis of monocotyledon stem is made up of sclerenchyma. It is 2-3 layered thick, In monocot stem rigidity is more in hypodermis whereas in dicot stem elasticity is more. It provides mechanical support to the plant.

Ground tissue: It is large conspicuous parenchymatous. There is no differentiation of ground tissue in monocotyledon stem. It means ground tissue is not differentiated into general cortex, endodermis, pericycle, pith & medullary rays.

Note : Sometimes in some grasses, wheat etc. the central portion of ground tissue becomes hollow and is called pith cavity (pith cavity is found in stems of Cucurbita. Ricinus amongst dicots).

Vascular Bundle: Mary vascular bundles are found scattered in the ground tissue and V.B. are generally oval (egg shaped). Peripheral vascular bundles are generally smaller then the centrally located ones. Vascular bundles which are situated towards the centre are large in size and less in number. Vascular bundles which are situated towards the periphery are small in size but more in number. Each vascular bundle is conjoint, collateral and closed and xylem is endarch. Each vascular bundle is smanded by sclerenchymatous bundle sheath. So vascular bundles are called fibro vascular bundles. Water containing cavities are present within the vascular bundles.

(a)Xylem - In xylem number of vessels is less. In metaxylem there occurs two large vessels while in protoxylem there occurs one or two small vessels. Vessels are arranged in the form of V or Y. Just beneath protoxylem vessels, there occurs a water cavity which is schizolysigenous in origin. In which major part of water cavity is lysigenous in origin (formed due to lysis of protoxylem elements and few part of water cavity is schizogenous (formed by separation of cells).

(b)Phloem - It consists of sieve tube elements and companion cells. Phloem parenchyma is absent.

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