CHAPTER VII. 



Living Parts of the Stem ; Work of the Stem. 



90. In annual plants generally and in the very young 

 shoots of shrubs and trees there are 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, which will be explained in Chapter XII. For 

 a good many years, too, the spongy lenticels, which 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 long 

 transverse slit or scar on the bark, as is readily seen in the 

 cherry and the birch. But in the trunk of a large tree no 

 part of the bark except the inner layers is alive. The older 

 portions of the bark sometimes cling for years after they are 

 dead and useless, except as a protection for the parts beneath 

 against mechanical injuries or against cold. A familiar 

 example of highly developed cork is in the bark of the cork 

 oak, from which the ordinary stoppers fpr bottles are made. 

 Trees which have been bruised or peeled so as to expose the 

 wood require a coat of paint or coal-tar on the injured parts 

 to keep out water and prevent decay. But in many cases, as 

 in the shellbark hickory and the grapevine, the old bark 

 soon falls off in strips ; in birches it finally peels off in bands 

 around the stem. 



