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 for 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. 
