o Epidermis (epiblema): The outermost layer. Many cells of epiblema protrude as unicellular root hairs.
o Cortex: It consists of several layers of thin-walled parenchyma cells with intercellular spaces.
o Endodermis: Innermost layer of the cortex. It comprises a single layer of barrel-shaped cells without intercellular spaces.
The tangential and radial walls of the endodermal cells have a deposition of suberin (water impermeable, waxy material) in the form of casparian strips.
o Stele: All tissues on the inner side of the endodermis together constitute stele. They include
§ Pericycle: A few layers of thick-walled parenchyomatous cells next to endodermis. Initiation of lateral roots and vascular cambium during the secondary growth takes place in these cells.
§ Pith: Innermost region of the stele. It is small or inconspicuous.
§ Conjunctive tissue: The parenchymatous cells which lie between the xylem and the phloem.
§ Vascular bundles: 2-4 xylem & phloem patches. Later, a cambium ring develops between the xylem & phloem.
- It has epidermis, cortex, endodermis, pericycle, vascular bundles and pith.- There are usually more than six (polyarch) xylem bundles.
- Pith is large and well developed.
- Monocot roots have no any secondary growth.
o Epidermis: Outermost protective layer. Covered with a thin layer of cuticle, it may bear trichomes & few stomata.
o Cortex: Multiple layers of the cells arranged in between epidermis and pericycle. It consists of 3 sub-zones:
§ Hypodermis: Outer zone. It consists of a few layers of collenchymatous cells just below the epidermis. It provides mechanical strength to the young stem.
§ Cortical layers: Below hypodermis. They consist of rounded thin walled parenchymatous cells with conspicuous intercellular spaces.
§ Endodermis: Innermost layer. Cells are rich in starch grains. So it is also called starch sheath.
o Stele: Consists of pericycle, vascular bundles, medullary rays & pith.
§ Pericycle: It is present on the inner side of the endodermis and above the phloem in the form of semi-lunar patches of sclerenchyma.
§ Medullary rays: These are few layers of radially placed parenchymatous cells in between vascular bundles.
§ Vascular bundles: Large in number. Their ring arrangement is a characteristic of dicot stem. Each vascular bundle is conjoint & open. Protoxylem is endarch.
§ Pith: Central portion of the stem. It has many rounded, parenchymatous cells with large intercellular spaces.
- It has a sclerenchymatous hypodermis, many scattered vascular bundles, each surrounded by a sclerenchymatous bundle sheath, and a large, conspicuous parenchymatous ground tissue.
- Vascular bundles are conjoint & closed. Peripheral vascular bundles are smaller than centrally located ones.
- The phloem parenchyma is absent, and water-containing cavities are present within the vascular bundles.
The vertical section of a dicot leaf through lamina shows 3 main parts: Epidermis, mesophyll & vascular system.
o Epidermis: It covers both the upper surface (adaxial epidermis) and lower surface (abaxial epidermis) of the leaf. It has a conspicuous cuticle.
Abaxial epidermis generally bears more stomata.
In adaxial epidermis, stomata are fewer or absent.
§ Palisade parenchyma: It is adaxially placed. Made up of elongated cells arranged vertically and parallel to each other.
§ Spongy parenchyma: The oval or round and loosely arranged. It is situated below the palisade cells and extends to the lower epidermis. There are numerous large spaces and air cavities between these cells.
o Vascular system: It includes vascular bundles. They can be seen in the veins and midrib.
Size of vascular bundles is dependent on the size of the veins. The veins vary in thickness in the reticulate venation of dicot leaves. Vascular bundles are surrounded by a layer of thick walled bundle sheath cells.
o Stomata are present on both surfaces of the epidermis.
o Mesophyll is not differentiated into palisade and spongy parenchyma.
o In grasses, certain adaxial epidermal cells along the veins modify into large, empty, colourless cells. These are called bulliform cells. When they have absorbed water and are turgid, the leaf surface is exposed. When they are flaccid due to water stress, they make the leaves curl inwards to minimise water loss.
o Parallel venation is reflected in the near similar sizes of vascular bundles (except in main veins).