Anatomy of Dicotyledonous Stem:
In young Dicotyledonous stems there are three distinct regions- the epidermis, the cortex and the stele.
Epidermis:
The epidermis consist of single layer of cells and is the outermost layer of the stem. It contains stomata and produces various types of trichomes. The outer cell walls are greatly thickened and heavily cutinized. The cells are compactly arranged and do not possess intercellular spaces. In trasverse section the cells appear almost rectangular. It serves mainly for recruiting the rate of transpiration and for protecting the underlying tissues from mechanical injury and form disease producing organisms.
Cortex:
The region that lies next to the epidermis is the cortex. The innermost layer of the cortex is the endodermis, known also as the starch sheath. It consist of a single layer of cells which surrounds the stele and contains numerous starch grains. Frequently it is most easily distinguishable from the surrounding tissue by the presence of these starch grains. The part of the cortex situated between the epidermis and the endodermis is generally divided into two regions, an outer zone of collenchyma cells and an inner zone of parenchyma cells.
Collenchyma:
On the inside of the epidermis there is usually a band of collenchyma. The cells of the collenchyma are modified parenchyma cells with cellulose walls thickened at the angles where three or more cells are in contact.
Parenchyma:
The parenchyma cells are generally regular in shape, have comparatively thin walls, and are not greatly elongated in any direction. They are living cells and contain a moderate amount of protoplasm.
Sclerenchyma:
The sclerenchyma cells are found in the cortex of some stems, there are two varieties of these sclerenchyma cells short Or irregularly shaped cells, known as a stone cells and sclerenchyma fibres. Sclerenchyma fibres are long, thick walled dead cells and serve as strengthening material. Stone cells give stiffness to the cortex.
Endodermis:
The innermost layer of the cortex is the endodermis consisting of barrel shaped, elongated, compact cells, having no intercellular spaces among them. Usually the cells contain starch grains and thus the endodermis may be termed as starch sheath.
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| T. S of Dicotyledonous stem |
Stele:
The part of the stem inside of cortex is known as the stele. The stele consist of three general regions 1) Pericycle 2) The Vascular Bundle region and 3) The Pith
Pericycle:
The region between the vascular bundles and the cortex is known as the pericycle. It is generally composed of parenchyma and sclerenchyma cells, but the sclerenchyma cells may be absent.
Vascular bundles:
The vascular bundles as seen in cross section, are arrenged in the general form of a broken ring. Each vascular bundle consists of three parts. That nearest the centre of the stem, contain thick walled cells and is known as xylem. The peripheral portion of the bundle is composed of thin walled cells called phloem. The xylem and phloem are separated by a cambium layer, which is composed of meristematic cells. By division the cambium layer increases the size of vascular bundles by forming xylem cells on the innerside and phloem cells on the outer side. In some stems the bundles are separate and run the length of the internode. In others they are more or less united and form a hollow cylinder in which the medullary rays occur as radiating plates with slight vertical extension.
Xylem:
The xylem which is formed before the activity of the cambium has begun to produce xylem and phloem cells is called primary xylem. It is composed of two parts. The xylem formed first is nearest to the centre of the stem and is called protoxylem. The more peripheral part of the primary xylem is known as metaxylem.
Phloem:
The primary phloem of the Dicotyledonous stems consists of three types of cells sieve tubes, companion cells and phloem parenchyma.
Cambium:
There lies a layer of the meristematic cells between the xylem and the phloem which is known as the cambium. The cambium consists of a single layer of cells which, by division gives rise to xylem cells towards the centre of the stem and phloem cells towards the periphery. At first the cambium is confined to the bundles, but later the parenchyma cells of the pith rays which lie between the edges of the cambium is confined to the bundles, but later the parenchyma cells of the pith rays which lie between the edges of the cambium in the bundles divide and form a layer of cambium which reaches across the pith rays and connects that in the bundles, so that the cambium becomes a continuous cylinder.
Pith rays:
The vascular bundles are separated from each other by radial rows of parenchyma cells known as pith rays. The pith ray cells are usually elongated in a radial direction. They serve primarily for the conduction of food and water radially in the stem and for the storage of food.
Pith:
In a Dicotyledonous plant the centre of the stem is composed of thin walled parenchyma cells and is known as the pith. The cells have distinct intercellular spaces.
