Protective Tissue in Plants

Protective tissues are generally present in the outermost layer of the plant body. These tissues are primarily protective in function. They are of two types:

(i) Epidermis

(ii) Cork

1.0Epidermis

It is the outermost layer of all organs of plant body which is formed from parenchymal cells. In epidermal cell outer walls are thicker than inner wall. It is mostly single layer but in desert plant it is multilayered for protection against water loss.

It protects the internal tissue from mechanical injuries and entry of germs.

Cuticle

The outer wall of epidermis of aerial parts of plant secretes and deposits a waxy substance, called cut in which form a water proof layer called cuticle.

It checks the loss of water by transpiration, mechanical injuries and invasion by parasitic fungi.

Stomata

Epidermis of leaves has large number of microscopic aperture called stomata. Each stomata is an elliptical aperture bounded by two kidney shaped guard cells which regulate opening and closing of stomata.

Guard cells contains chloroplast unlike other epidermal cells hence they can perform photosynthesis.

Transpiration (loss of water in the form of water vapours) takes place through stomata. Stomata helps in exchange of gases.

Root hairs: Epidermis of roots (epiblema) have root hairs which greatly increase their surface area for absorption of water and minerals.

Function of Epidermis 

(i) It gives mechanical support and strength to plants parts.

(ii) It protects the internal tissues from injuries and invasion by pathogens.

(i) It reduces the rate of transpiration by developing cuticle.

(iv) Exchange of gases and transpiration take place through stomata.

(v) It bears unicellular or multicellular appendages in the form of root hairs, stem hairs, glands etc.

2.0Cork or Phellem 

Cork is the peripheral tissue of old stems and roots of woody trees and is formed due to activity of cork cambium or phellogen (secondary lateral meristem). Cork cambium (phellogen) produces new cells on its both sides, thus, forming cork (phellem) on the outer side and the secondary cortex or phelloderm on the inner side.

It is made up of dead cells with thick wall but no intercellular spaces. 

Dedifferentiation takes place only in permanent cells. In this process the specialized cells regain the division of power and become meristematic e.g. cork cambium.

Commercial Importance of Cork

Commercial cork is obtained from the stem surface of cork oak (Quercus suber). 

Cork is light and highly compressible which does not catch fire easily.    

Cork is used in the making of a variety of sport goods such as cricket balls, table tennis, shuttlecocks, wooden paddles etc.

Functions of Cork

(i) Cork checks the entry of harmful microorganisms into plant parts.

(ii) Multilayered, impervious cork prevent loss of water by evaporation.

(iii) Cork provides protection against mechanical injury, extreme temperature and fire.

The walls of cork cells are heavily thickened by the deposition of an organic substance (a fatty substance), called suberin. Suberin makes these cells impermeable to water and gases and it also helps in conservation of water in the trees.

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