STEMS 



177 



he has been unlucky enough to split the cylinder of bark; 

 if he looks at the inside face of it, he will see other moist 

 particles. These are the outside cambium layer. 



Bordering each layer of cambium, many of the cells 

 are arranged in long 

 strings, or pipes, much 

 as our blood vessels are. 

 Through these pipes the 

 sap of the plant circu- 

 lates, rising from the 

 roots to the branches 

 through the outer wood, 

 and returning to the 

 roots in the inner bark. 1 



(3) The wood of the 

 stem supplies strength 

 and firmness to the tree, 

 as bones do to animal 

 bodies. In an old tree 

 the heart or center wood 

 is merely a mass of dead 

 cells, and serves no use 

 except to give strength. 

 But the sap wood, or 

 that nearest the cam- 

 bium, in addition to 

 affording stiffness to the 

 tree and to providing 

 pipes for the rise of sap, 

 serves as a storehouse where the sap may store surplus 

 food for the next season's growth. Each year a layer of 



BUDS IN THE AXILS OF LEAVES. 

 Branch of a cherry tree in July. 



1 A stem of any rapidly growing plant placed in red ink will show in a most 

 striking manner in what part of the stem the sap rises, and the rate at which 

 it moves. 



