CHAPTER VII 

 LIVING PARTS OF THE STEM; WORK OF THE STEM 



115. 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 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 XI). 

 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. But 

 in the trunk of a large tree 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, from which 

 the ordinary stoppers for bottles are made, sometimes 

 cling for years after they are dead and useless except as a 

 protection for the parts beneath against mechanical injuries 

 or against cold. But in many cases, as in the shell-bark hick- 

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

 in birches it finally peels off in bands around the stem. 



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