BUDS AND BRANCHES 103 



93. Lenticels. On the general surface of the bark of many 

 kinds of twigs and young branches for example, of birch, 

 cherry, elder, and sumach there are found many dots, or 

 markings with a rough surface, known as lenticels. These are 

 nearly circular on the younger twigs, but on branches of 

 moderate size they become lengthened at right angles to the 

 length of the branch. In many kinds of birch and most 



FIG. 89. Lenticels, wild black cherry 

 From a tree fifteen or twenty years old. One and one-half times natural size 



cherries the lenticels finally become narrowly oblong or lens- 

 shaped (Fig. 89). This is due to the fact that as the branch 

 increases in diameter the lenticel is drawn out by the trans- 

 verse expansion of the bark. 



Lenticels originate as stomata (Sect. 14) in the epidermis 

 of the young shoot. On growing older the interior of the 

 lenticel becomes filled with a spongy mass of thin-walled cells. 

 Air is admitted into the interior of the stem and gases can 

 pass out through the lenticels far more freely than through 

 other parts of the bark. 



