Epidermal tissue system : It is the outermost protective tissue of the plant organs. It is derived from protoderm and consists of epidermis and epidermal outgrowths.
(i) Epidermis : (a) Epidermis is usually single layered, but in the leaves of some tropical plants and the velamen of the roots of epiphytic plants, it is more than one layer in thickness.
(b) There is a layer of cutin, on the outer side of the epidermis of aerial parts of the plants called cuticle.
(c) Epidermis along with cuticle protect the inner cells against loss of water, mechanical injury, attack of pathogens and leaching effect of rain.
(d) The epidermis of grasses and cereals has a deposition of silica.
(ii) Epidermal outgrowths : (a) The epidermis of most plants often bear outgrowths called epidermal outgrowths.
(b) They are of two types-Trichomes or hairs emergences or prickles.
(c) The epidermal hairs of aerial parts may be unicellular, branched or unbranched. They enclose stationary air and protect the plant organ against water loss and sudden fluctuation in atmospheric temperature. In some plants, the hairs are glandular. They secrete essential oil and provide characteristic odour to the plants e.g., Citrus, Ocimum etc.
(d) The root epidermis bears unicellular outgrowths called root hairs. It is therefore, also called piliferous layer or epiblema. It is mainly concerned with the absorption of water and minerals from the soil.
(e) The emergences or prickles are multicellular, sharp and stiff epidermal outgrowths, which contain some inner tissues. They protect the plant against grazing and excessive loss of water