72 WORK OF THE STEM 



probable that wood of this kind is chiefly useful to the tree 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 nourish, sometimes for 

 centuries, after the heartwood has much of it rotted away and 

 left the interior of the trunk hollow, as shown in Fig. 72. 



86. 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, particularly 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 structure 

 amid which the hbro-vascular bundles arise. In mature stems 

 it becomes rather unimportant, though it often continues for a 

 long time to act as a storehouse of food. 



The medullary rays in the young shoot serve as a channel 

 for the transference of water and plant food in a liquid form 

 across the stem, and they often contain much stored food. 



The vessels carry water upward through the stem in certain 

 plants. 



The wood cells of the heartwood are useful only to give stiff- 

 ness to the stem. Those of the sapwood, in addition to this 

 work, have to carry most of the water from the roots to the 

 leaves and other distant portions of the plant. 



The cambium layer is the region in which the annual growth 

 of the tree takes place. 



Sieve tubes form the most important portion of the inner bark, 

 carrying elaborated plant food from the leaves toward the roots. 



The green layer of the lark in young shoots does much toward 

 collecting nutrient substances, or raw materials, and preparing 

 the food of the plant from air and water, but this work may 

 be best explained in connection with the study of the leaf 

 (Chapter xn). 



