NATURE OF PLANTS 



83 



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 some plants the air 

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

 sufficient to bring about an adequate interchange of gases but in 

 the majority of plants special devices are required to bring the 

 living cells of the stem into more direct communication with the 



Fig. 51. 

 Later development of a lenticel: 



Fig. 52. 

 c, cork cambium; i, intercel- 



FiG. 51 

 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. 



air. This work is effectively accomplished in herbaceous stems 

 by the stomata, but in stems characterized by the formation 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 

 localized in a layer of cells situated on the inside of the lenticel 

 (Fig. 51). This layer of cells is a part of the cork cambium, 

 but strangely enough only loose cells are added to the lenticel 

 during each year's growth and consequently a passage way is 

 kept open to the living cells within the stem. The lenticels 



