structure of stem
Aug 09, 2022, 16:45 IST
About structure of stem
The part of the axis of the plant which is usually ascending and aerial and also bears leaves and reproductive structures is called stem. The stem has conspicuous nodes and internodes and differs fundamentally from the root in its vascular structure. Vascular bundles are conjoint, collateral or bicollateral or even concentric, open or closed.
A transverse section of young stem shows the following structures:
It is represented by the outermost layer of the stem which is protective in function. It is made up of a single layer of closely packed rectangular cells with relatively thin primary walls. The epidermis is covered on the outer surface by a layer of cuticle. The continuity of epidermis is broken by the presence of stomata. Multicellular hairs (trichomes) are frequently found on the epidermis.
Fundamental (ground) tissue system
In dicotyledonous stems the ground tissue is clearly differentiated into cortex and pith. But in monocotyledonous stems there is no such distinction in the ground tissue. In dicotyledonous stems the cortex is usually differentiated into three distinct regions, an outer collenchymatous hypodermis, a middle parenchymatous general cortex and an inner endodermis.
Hypodermis: It consists of a few layers of collenchymatous cells immediately below the epidermis. It usually forms a continuous ring in cylindrical stems, but in angular stems, it occurs in patches (e.g., Cucurbita).
General Cortex: It is parenchymatous with polygonal, round or oval cells having intercellular spaces. Chloroplasts may be present in the outer layers of the cortex. Latex tubes, resin ducts, crystals of calcium oxalate and reservoirs of waste products are frequently present in the cortex.
Endodermis: It is the innermost limiting layer of the cortex which separates vascular tissue from the cortex. It is made up of vertically elongated cells which appear barrel shaped in cross section. These cells, however, do not develop casparian thickening like those of the true endodermis. The cells of this layer contain abundant starch and hence it is also called starch sheath.
Pericycle: The layer of cells which occur inner to the endodermis and outside the vascular tissue form pericycle.It is usually made up of closely fitting fibres, which occur in isolated patches or form a complete cylinder. It is multilayered and may be parenchymatous (e.g., most of the dicots) or sclerenchymatous (e.g., cucurbits) or both (e.g., Asteraceae)
Pith: It is the ground tissue present in the centre of the stem. It is made up of large parenchymatous cells with distinct intercellular spaces. Laticifers, secretory cells, sclereids, idioblasts, crystals of various kinds and other ergastic substances are present in the pith. The outer part of the pith is sometimes distinct in having smaller cells with thicker walls. Such a morphologically distinct outer pith is called perimedullary zone or medullary sheath.
Medullary rays: These are narrow, vertically elongated bands of parenchymatous cells which extends transversely from the pith towards the periphery. The first rays, formed as a result of the growth of the apical meristem, are known as primary medullary rays.
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