Epidermis:

The outer most layer of cells is called epidermis. The epidermis is usually made of a single layer of cells. The epidermis may thicker in plants of dry habitats.

Since, it has a protective role to play, cells of epidermal tissue form a continuous layer without intercellular spaces. Most epidermal cells are relatively flat. Often their outer and side walls are thicker than the inner wall.

Features:

(i) Epidermis may be thicker since protection against water loss is critical.

(ii) The entire surface of a plant has this outer covering of epidermis.

(iii) It protects all the parts of the plant.

(iv) Epidermal cells on the aerial parts of the plant often secrete a waxy, water-resistant layer on their outer surface.

(v) This aids in protection against loss of water, mechanical injury and invasion by parasitic fungi.

Functions Of Epidermis in plants:

(i) This aids in protection against loss of water,

(ii) Mechanical injury and invasion by parasitic fungi.

(iii) Its jelly like substances  forms a water resistant layer. 

Functions of Epidermis in roots:

Epidermal cells of the roots, whose function is water absorption, commonly bear
long hair-like parts that greatly increase the total absorptive surface area.

Role of Epidermis in Desert Plants:

Epidermis has a thick waxy coating of cutin (chemical substance with waterproof quality) on its outer surface.

Cutin: This is a chemical substance with waterproof quality, which is mainly found in epidermis of desert plants. 

Suberin: There is a chemical called suberin in the walls of outer protective
tissue (thick cork) or the bark of the tree that makes them impervious (harmless) to gases and water.

Stomata: There are many small pores in the epidermis (surface) of the leaf. These small pores are called stomata.

Protective Layer: As plants grow older, the outer protective tissue undergoes certain changes. A strip of secondary meristem replaces the epidermis of the stem. Cells on the outside are cut off from this layer. This forms the several-layer thick cork or the bark of the tree. Cells of cork are dead and compactly arranged without intercellular spaces

Functions of stomata:

(i) The process of transpiration takes place through stomata. 

(ii) The gaseous exchanges also take place through stomata.

Transpiration: This is a process of loss of water in the form of water vapour.

• Stomata are enclosed by two kidney-shaped cells called guard cells.

Function of guard Cells:

They are necessary for exchanging gases with the atmosphere.