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 
oragainst 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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