LIVING PARTS OF THE STEM 77 



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 many old elms, sycamores, and 

 other trees. 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. 



