LIVING PARTS OF THE STEM 105 
The cambium layer is very much alive, and so is the 
young outer portion of the wood. Testing this “sap- 
wood,” particularly in winter, 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 it may be that 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, as shown 
in Fig. 75. | 
116. 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, particu- 
larly 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 part of the bulk of very young 
shoots, since it is a part of the tissue of comparatively 
simple structures amid which the fibro-vascular bundles 
arise. In mature stems it becomes rather unimportant, 
though it often continues for a long time to act as a store- 
house of food. 
The medullary rays in the young shoot serve as a chan- 
nel for the transference of water and plant-food in a liquid 
form across the stem, and they often contain much stored 
food. 
