WHERE FOOD COMES FROM 



115 



uninjured. Such a main descend- 

 ing root is called a taproot. The 

 fleshy roots that have been men- 

 tioned are all taproots ; and a 

 number of trees, as certain kinds 

 of maples, also produce taproots. 

 When a taproot is injured or cut 

 off, some of the side roots turn 

 and grow downward. In a few 

 cases the tip of the taproot, 

 when not too much injured, can 

 grow a new tip and continue the 

 main line of growth. 



The first function of a root 

 may be said to be the absorp- 

 tion of water and of dissolved 

 substances. This work is car- 

 ried on by the "root hairs" 

 (see Fig. 66). 



94. Structure of roots. To 

 understand the structure of 

 roots, use a carrot or parsnip 

 root that has been standing 

 for twenty-four hours or 

 longer with its tip in water 

 containing red ink. Cut slices 

 both crosswise and length- 

 wise, and use a magnifying 

 glass (see Fig. 67). 



The fiber and vessel cells 

 can grow, but they cannot 

 divide. In the young root 

 there appear layers of cells 

 which separate the water- 

 carrying bundles from the food-carrying bundles. These cam- 

 bium cells are capable of producing new fiber and duct cells of 

 the two kinds— the water-carrying, or wood, and the food- 



Fig. 66. The tip of a young root 



The root hair is a single cell formed by the 

 outward prolongation of one of the skin 

 cells. The root hairs are the actual ab- 

 sorbing organs. Each root hair lives but a 

 short time and then shrivels up. As the tip 

 of the root grows on, new root hairs are 

 formed. The older skin cells of the root 

 die and their contents dry out. Together 

 with the shriveled root hairs these skin 

 cells form a protective covering through 

 which water does not pass very readily. 

 As the plant becomes older and uses up 

 more water, the absorbing area of the root 

 is increased by the formation of many side 

 roots and by the branching of the roots. 

 But it is always in the region near the 

 growing tip of the main root and of the 

 many branch rootlets that absorption takes 

 place. The rootcap re protects the grow- 

 ing point 



