CHAPTER VIII 

 LIVING PARTS OF THE STEM; WORK OF THE STEM 



85. Active portions of the stems of trees and shrubs. In 



annual plants generally, and in the very young shoots of shrubs 

 and trees, there are stomata (singular stoma, meaning mouth), 

 or breathing pores, which occur abundantly in the epidermis, 

 serving for the admission of air and the escape of moisture, 

 while the green layer of the bark answers the same purpose 

 that is served by the green pulp of the leaf (Chapter xn). For 

 years, too, the spongy lenticels, which succeed the stomata and 

 occur scattered over the external surface of the bark of trees 

 and shrubs, serve to admit air to the interior of the stem. The 

 lenticels at first appear as roundish spots, of very small size; but 

 as the twig or shoot on which they occur increases in diameter, 

 the lenticel becomes spread out at right angles to the length of 

 the stem, so that it sometimes becomes a longer transverse slit 

 or scar on the bark, as in the cherry and the birch and the 

 elder. But in the trunk of a large tree often no part of the 

 bark except the inner layer is alive. The older portions of 

 the bark, such as the highly developed cork of the cork oak, 

 sometimes cling for years after they are dead and useless ex- 

 cept as a protection for the parts beneath against mechanical 

 injuries or against cold. But in many cases, as in the shellbark 

 hickory and the grapevine, the old bark soon falls off in strips ; 

 or as in birches it finally peels off in bands around the stern. 



The cambium layer is very much alive, and so is the young 

 outer portion of the wood. Testing this sapwood, particularly 

 in winter, shows that it is rich in starch and proteids. 



The heartwood of a full-grown tree is hardly living, unless 

 the cells of the medullary rays retain their vitality, and so it is 



71 



