Stems and Their External Features 115 



not only by the bud scars, but also by whorls of branches. 

 Differences in the color of the bark and in its texture will also 

 help to distinguish successive annual stem segments in most 

 trees. 



Lenticels. All living cells require energy. This is largely 

 obtained from respiration. Therefore, in addition to a con- 

 stant food supply, the cells of the stem must have access to 

 oxygen. As in the leaves the oxygen is supplied through the 

 intercellular spaces, so in stems there must be sufficient in- 

 tercellular spaces to permit oxygen to diffuse inward and 

 carbon dioxid to diffuse outward. There must also be open- 

 ings through the epidermis or bark to connect these inter- 

 cellular spaces with the outside atmosphere. 



The young green stems of all plants have stomata. Peren- 

 nial stems, however, soon develop a corky layer beneath the 

 epidermis, which cuts the cells in the interior of the stem off 

 from the stomata. While this layer is developing, masses of 

 round, loose cells form beneath some of the stomata, pushing 

 out and tearing the epidermis above them. These open places 

 are the lenticels. They permit gas exchanges, and in older 

 stems take the place of the stomata. The lenticels of most 

 trees and shrubs are closed in the late autumn by the growth 

 of a thin layer of cork beneath them. The following spring 

 loose cells are again formed at the same point, the cork is 

 burst open, and the lenticels again permit gas exchanges. 



In the cherry and birch the lenticels persist for many years 

 and become elongated transversely, forming granular rings 

 part way around the' stem. In the trunks of thick-barked 

 trees, the lenticels occur in the furrows of the bark. 



