152 Our Field and Forest Trees 



of white birch-bark as a decorative pattern of 

 bright brown streaks. 



Each lenticel is a lens-shaped opening in the 

 bark, filled in with cork cells. But instead of being 

 squared at the corners, and arranged in regular 

 rows, like the cells which make the trees' union 

 undergarment, these are 

 rounded, and look as if they 

 were tumbled together in a 

 loose mass. 



Between them are many 

 chinks and spaces, and through 

 these air gets into the wood, 

 while moisture and gases 

 breathed away by the tree find 

 their way to the outside air 

 (Fig. 38). 



Trom Farmer's Bulletin I\o. 17 3. 



Fig. 38. Cross section of wood 

 and bark of the western yellow pine, 

 showing two of the deep cracks^ in 

 tlie _ bark, at the bottom of wliich 

 lenticels are placed. 



As autumn draws 

 near, seals of cork 

 grow under these lit- 

 tle vents, so that the 

 tree's union undergarment is no longer porous. 

 These seals help to protect the wood from sudden 

 and bitter frost. 



But their chief purpose, it seems, is to check 

 the breathing away of the tree's moisture. The 

 cold season is vacation time for the rootlets that 

 were so busy all summer taking up water from the 

 ground. Water is not coming into the tree in 

 winter, so water must not go out of it. 



