CORK AND LENTICELS 339 



become corky (a complex substance, suberin, of which 

 fatty acids are constituents, being deposited in them), 

 and a cellulose layer is laid down next the cell cavity 

 (Fig. 56, d). The cork cell then dies, for the corky cell 

 wall is waterproof and prevents the diffusion of water 

 and solutes into and out of the cell necessary to the 

 continued life of the protoplasm. Brown colouring 

 substances are usually formed both in the walls and in 

 the cavities of the cork cells. The thick layer of cork 

 cells with waterproof walls forms a very effective barrier 

 between the living tissues of the plant and the outer 

 world. It is effective not only in checking evaporation 

 and gaseous interchange in general, but also in stopping 

 the attacks of minute insects and the germ tubes of 

 fungal spores. Like the epidermis, however, which it 

 supersedes, it is interrupted at certain spots by openings, 

 and these are called lenticels. 



Lenticels are formed in the following way : Cork 

 formation typically begins below the stomata at a 

 somewhat deeper level of the cortex than over the rest 

 of the surface of the stem, and thence it spreads to the 

 outermost cortical layer (or to the epidermis itself) over 

 the general surface. The vigorous cell division below 

 the stomata raises the epidermis at these points and 

 eventually bursts it. The cork cells formed below the 

 stoma are loose, with abundant intercellular spaces 

 between them, and this loose brown tissue is pushed out 

 through the opening made by the bursting of the stoma, 

 and forms a purulent projection on the surface. The 

 whole structure is called a lenticel, and through the 

 air spaces between the cells of the lenticellar cork, 

 which communicate with the intercellular spaces of the 

 cortex below, diffusion of gases takes place between the 

 living cells of the stem and the outside air. 



