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 

 hi 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 nbro-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. 



