The Stem is composed of three tissue systems
1. The Epidermal System
2. The Ground Tissue system
3. The Vascular Tissue System
Epidermis is the outermost covering of the stem. It is represented by a single layer of compactly arranged, barrel-shaped parenchyma cells. Intercellular spaces are absent. The cells are slightly thick walled. Epidermis shows the presence of numerous multicellular projections called trichomes. Externally, a thin transparent waxy covering called cuticle, which prevents excessive evaporation of water, surrounds the epidermis. The epidermis also contains numerous minute opening called stomata, which are mainly involved in transpiration.
Cross-Section of a Dicot Stem
2. The Cortex
The Cortex can be divided into three regions
b. General Cortex
Hypodermis is a region lying immediately below the epidermis. It is represented by a few layers of collenchyma cells with angular thickenings. The cells are compactly arranged without any intercellular spaces. Hypodermis provides mechanical support and additional protection.
Cortex is the major part of the stem represented by several layers of loosely arranged parenchyma cells. Intercellular spaces are prominent. Cortex is the major storage organ in the stem.
Endodermis is the innermost layer of cortex represented by compactly arranged barrel shaped cells, without any intercellular spaces. The endodermis is wavy in appearance. The cells are richly deposited with starch grains and hence, endodermis is commonly described as starch sheath.
Stele is the central cylinder of the stem, consisting of pericycle, medullary rays, pith and vascular bundles.
Pericycle is the outermost covering of the stele, which lies immediately below the endodermis. It is represented by a few layers of compactly arranged sclerenchyma cells. Above each vascular bundle, the pericycle forms a distinct cap-like structure known as bundle cap.
found in between the vascular bundles. They are meant for the storage of food.
They are eight in number, arranged in form of a broken ring. The vascular bundles are conjoint, collateral and open. Xylem is on the inner surface and phloem on the outer surface. Xylem is described as endarch.
Pith is the innermost part of the stem formed by a group of loosely arranged parenchyma cells. Intercellular spaces are prominent. The pith is also meant for storage of food.
Following are some of the diagnostic features of a young dicot stem
* Presence of cuticle and trichomes.
* Presence of stomata.
* Presence of a hypodermis made up of collenchyma.
* Presence of a wavy endodermis containing numerous starch grains.
* Presence of a bundle cap above each vascular bundle, formed by sclerenchyma.
* Presence of eight vascular bundles, arranged in the form of a broken ring.
* Presence of conjoint, collateral and open vascular bundles with an endarch xylem.