ELONGATION OF TREE TRUNKS, ETC. 113 



in addition, all the growing force is concentrated 

 here to drive the point of the root through the soil. 

 Besides, if elongation took place for any consider- 

 able distance back from the point, the root hairs, 

 which are the chief agents in its absorption of food, 

 would be torn off by the pushing of the root through 

 soil; for these reasons the growing part of roots 

 is necessarily more restricted than that of stems, 

 which have in the air nothing to prevent their ex- 

 pansion at any point. In the leaves and stems of 

 grasses there are various ways by which the grow- 

 ing part is protected. In the leaves of June- grass, 

 for example, there is a narrow space near the base 

 of the leaf where growth takes place, so that the 

 leaf continues to elongate, even if eaten off above. 

 In all grasses the main growth of the stem itself 

 takes place just above each joint, where it is pro- 

 tected by the sheath of the leaf. 



