106 INTRODUCTION TO BOTANY 



In tree-like monocotyledons the leaves are not shed as they 

 are by the common trees of temperate climates. Frequently 

 the leaves persist for years but finally decay and leave the 

 trunk of the tree covered with a rough coating formed by 

 the persistent bases of the leafstalks, as is the case with the 

 date palm and many other kinds of palms. 



99. Lenticels. Along the twigs and younger branches of 

 most trees and shrubs there are found many dots or larger 

 areas of rough, spongy bark. These are called lenticels. They 

 are especially distinct on the bark of most birches and cherry 

 trees, and in these finally reach a rather large size. Each len- 

 ticel covers the position originally occupied by a stoma in the 

 epidermis of the very young bark. As the stoma grew older 

 its characteristic cells disappeared and were replaced by a 

 spongy mass of thin-walled cells. The lenticels serve for the 

 entrance of gases into the stem and for their passage out of it, 

 and the respiration of the stem is considerably aided by the 

 readiness with which an exchange of gases goes 011 through 

 these porous spots. 



