96 



BOTANY: PRINCIPLES AND PROBLEMS 



Protective Layers. — The epidermal cells resemble those of 

 the leaf epidermis and require no special comment. In stems 

 which are growing in thickness, however, the epidermis is soon 

 sloughed off and its protective function is assumed by a layer 

 of corky cells formed directly under it and constantly renewed. 

 In these cells the protoplasm soon disappears and the normal 



Fig. 48. — Tranfe\ei-5e section of a thicc-\ c ii-oUl twig of the tiilip-tree {Lirio- 

 dendron). This is a t>pical wood> twig The hbro-vascular cylinder consists 

 of a solid ring of wood within and ba^t without, surrounding a central pith. 



cellulose wall becomes corky or suherized and is thus rendered 

 almost impermeable to air or water. The lenticels, which we 

 have already mentioned, are spots in this corky layer where the 

 cells are somewhat loose and spongy and thus allow the passage 

 of gases. 



Cortex and Pith. — The cortex and the pith are very similar 

 in constitution. Their cells usually remain alive, are roughly 

 spherical in shape, retain their cellulose walls and function 

 chiefly in the storage of food. To such undifferentiated tissues 

 the term parenchyma is often applied. In older woody stems the 

 pith often dries up and collapses; and the cortex, crushed by the 

 expansion of the wood underneath it, is finally sloughed off. 



