Epidermal Tissue System
This tissue system forms the outermost covering of the plant body. It is in the direct contact with the external environment. The epidermal tissue system comprises
- Epidermal cells,
- Epidermal structures, i.e., stomata
- Epidermal appendages the trichomes and hairs
Epidermis is the outermost layer of the primary plant body. It is usually single-layered, i.e., composed of single layer of epidermal cells. However, it may be multilayered, ie., composed of several layers of epidermal cells, e.g., Nerium, Ficus.
The epidermal cells are the living cells having cytoplasm and a nucleus. These are the elongated cells which are compactly arranged leaving less space between themselves. They form a continuous layer of epidermis which is interrupted by the epidermal structures, i.e., stomata in leaves and certain stems. The epidermal cells are the parenchymatous cells which possess a large central vacuole and peripheral cytoplasm lining the cell wall. The wall of these cells vary in structure. In a transversely cut stem or root, the outermost cell layer represents the epidermis.
The outside of the epidermis is often covered with a waxy thick layer called the cuticle which prevents the loss of water because of deposition of a waxy substance called cutin on the outer walls of cells. Cuticle is absent in roots and hydrophytes.
- Specialised upper epidermal cells called bulliform cells are present in certain monocct leaves which will be discussed later in the anatomy of monocot leaves.
- Root epidermis is termed as epiblema or piliferous layer or rhizodermis.
(b) Stomata (Singular Stoma)
Stomata are the minute structures present in the epidermis of leaves. They are absent in the epidermis of roots. Each stoma is composed of
- Two bean-shaped or kidney-shaped cells known as guard cells, and
- A tiny stomatal pore.
The pore surrounded by two guard cells together form a stoma.
Guard cells: Guard cells are called so because they ‘guard’ the opening and closing of the stomatal pore. They are the living cells. They possess chloroplasts and regulate the stomatal movement. These cells are generally bean-shaped or kidney-shaped but in grasses, the guard cells are dumb-bell shaped.
The walls of the guard cells towards the stomatal pore are called inner walls and away from the stomatal pore are called outer walls. The outer walls of the guard cells are thin while the inner walls are highly thickened. The thickening is limited to the middle part in the dumb-bell shaped guard cells.
Function of guard cells: Guard cells control the opening and closing of the stomatal pore. When they are turgid (swollen), the stomatal pore is open and when they are flaccid (shrunken), the stomatal pore is closed.
Subsidiary cells: These are the specialised epidermal cells. In some flowering plants, the epidermal cells surrounding the guard cells become specialised in their shape and size and are called the subsidiary cells. They are also called accessory cells. They provide support to the guard cells. They do not possess chloroplasts and do not carry out the photosynthesis.
Function of stomata: Stomata regulate the process of transpiration and gaseous exchange transpiration is the loss of water by plants through the process of evaporation. Gaseous exchange involves the exchange of oxygen and carbon dioxide between plant and the environment. The loss of water and entry and exit of gases occurs through these epidermal structures called stomata.
Usually, the lower surface of a dicotyledonous leaf has a greater number of stomata than the upper surface while in a monocotyledon leaf, they are about equal in number on both the surfaces.
Stomatal apparatus: The stomatal aperture (pore), guard cells and the surrounding subsidiary cells are together called stomatal apparatus.
(c) Epidermal appendages
An appendage can be defined as an outgrowth from the external surface of the body. Epidermal appendages are the outgrowths from the epidermis layer. These are commonly present on the roots and shoots.
(i) Root hairs: Root hairs are the epidermal appendages of roots. They arise from the cells of epidermis. These are the unicellular elongations of the epidermal cells or we can say that each root hair is a single cell having its cytoplasm and nucleus. The root hairs help in absorption of water and minerals from the soil. They increase the surface area for absorption in the roots. Root hairs are the thin-walled structures into which the water alongwith minerals enters by diffusion.
(ii) Trichomes: The epidermal hairs on the stem are called trichomes. They are also the epidermnal outgrowths like root hairs. They are usually multicellular. They may be unicellular also.
- They may be branched or unbranched.
- They may be soft or stiff.
- Trichomes mainly help in preventing the water loss due to transpiration. The trichomes cover the evaporating surfaces and prevent the evaporation by blocking the flow of air across the plant surface.
- They may even be secretory in some plants. The sticky secretions of trichomes help in trapping the insects for killing and obtaining the nutrition from them.
Functions of epidermal tissue system
All the functions of the above discussed structures are the functions of epidermal tissue system which can be summarised as
- Protection: It provides protection from external injuries, excessive evaporation, scorching solar radiations, etc. with the help of cuticle, trichomes, etc.
- Secretion: Secretion of many sticky substances is facilitated by trichomes.
- Gaseous exchange: By stomatal apparatus.
- Control of transpiration: By stomatal apparatus, trichomes and cuticle. Nb o ute
- Absorption of water and minerals: By root hairs.