i8o 



STEMS 



terested in the use of it in making canoes, a use which 

 shows very well the impermeability of bark to water. 

 Birch trees also show very well those structures of the 

 bark called lenticels. In birch bark the lenticels form con- 

 spicuous markings which run crosswise of the trunks. 

 (See Figure 62.) On other trees than birch, especially 



upon the younger 



bark, lenticels are 

 frequent, but they 

 are not so conspicu- 

 ous as upon the 

 birch. Lenticels, 

 though quite differ- 

 ent in structure, are 

 similar in function 

 to the stomates of 

 the leaves ; they per- 

 mit the passage of 

 gases. In them the 

 cork cells, such as 

 are present in the 

 rest of the outer 

 bark, are replaced by 

 loose cells through 

 whose intercellular spaces it is possible for gases to pass. 

 You will recall that elsewhere in this section it was men- 

 tioned that the cortex, as the stem grows older, usually is 

 compelled to give up its earlier work of photosynthesis. If 

 it were not for the lenticels, the cortex would have to give 

 up photosynthesis much more quickly than it does, for if 

 the covering of cork were continuous, the exchange of gases 

 which is necessary to photosynthesis could not occur. In 



Fig. 62. — Trunks of birch trees showing many 

 transverse lenticels. 



