88 NATURE OF LENTICELS 



some trees the cork cells break off about as fast as they are formed, 

 so that a comparatively thin layer of cork cells remains attached 

 to the trunk. Such trees have a smooth bark, as in the beech. 

 In other cases the cork cells are formed in great abundance, 

 and owing to the adhesion of the cells, thick layers of cork are 

 formed, as in the oaks, and giant trees of the Pacific. 



It is evident that this mantle of impervious cork cells would 

 tend to prevent the access of the atmosphere to the stem. We 

 have seen that all living cells respire. In many instances the air 

 spaces extending from the leaves to all regions of the stem are 



Fig. 51. Fig. 52. 



Fig. 51. Later development of a lenticel: d, cork cambium; i, intercel- 

 lular space. 



Fig. 52. Surface view of lenticels: A, lenticels on branch of horse-chestnut 

 appearing as minute brownish swellings. B, old lenticels on white .birch 

 appearing as dark lens-shaped streaks. 



sufHcient to bring about an adequate interchange of gases but in 

 young stems where growth is vigorous and where therefore 

 respiration is considerable, special devices are required to bring 

 the living cells of the stem into more direct communication with 

 the air. This work is effectively accomplished in herbaceous 

 stems by the stomata, but in stems characterized by the forma- 

 tion of cork it is noticed that the cells just below the stomata 

 begin to divide and form a rather loose mass of cells that lift up 

 and finally rupture the tissues about the stomata, thus forming a 

 small, lens-shaped outgrowth on the surface of the young stem, 

 called a lenticel (Figs. 50; 52, A). Soon this growth becomes 



