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 on through 
these porous spots. 
