LIVING PARTS OF THE STEM TT 
strips. The cambium layer is very much alive, and so 
is the young outer portion of the wood. Testing this 
sapwood, particularly in winter, when it serves for food 
storage, 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 wood of this kind is useful to the tree mainly by 
giving stiffness to the trunk and larger branches, thus 
preventing them from being easily broken by storms. 
It is, therefore, possible for a tree to flourish, sometimes 
for centuries, after the heartwood has much of it rotted 
away and left the interior of the trunk hollow. This is 
well shown in the trunk of one of the old olive trees of 
Plate III. In the Sequoias, or big trees of California, 
there are sometimes cavities large enough to allow a two- 
horse covered wagon to drive inside; and the “ chestnut 
of a hundred horses” on Mt. Etna gets its name from the 
fact that the interior cavity would easily hold that number 
of horsemen. In this case, however, there is some doubt 
whether the whole was originally a single trunk. 
92. Uses of the Components of the Stem. — There is a 
marked division of labor among the various groups of cells 
that make up the stem of ordinary dicotyledons, partic- 
ularly in the stems of trees, and it will be best to explain 
the uses of the kinds of cells as found in trees rather than 
in herbaceous plants. <A few of the ascertained uses of 
the various tissues are these: 
The pith forms a large portion of the bulk of very 
young shoots, since it is a part of the tissue of compara- 
tively simple structure amid which the fibro-vascular bun- 
dles arise. In mature stems it becomes unimportant, though 
it often long continues to act as a storehouse of food. 
