ROOTS OF PLANTS 



45 



the structure of the root and the size of the plant that it 

 anchors, and, on the other hand, between the size of the root 

 and its food-accumulating, or its absorbing, activity (Fig. n). 



70. Tap-roots. In many plants the main root continues to 

 grow downward into the soil as long as the plant lives and 

 as long as the tip of the root remains uninjured. Such a main 

 descending root is called a tap-root. The fleshy roots that have 



FIG. ii. Forms of roots 



/,tap-root of dandelion ; 2, fibrous root of buttercup ; ^, bundle (or " fascicled ") root of 

 dahlia ; 4, fleshy root of beet 



been mentioned are all tap-roots ; and a number of trees, as 

 certain kinds of maples, also produce tap-roots. When a tap- 

 root is injured or cut off, some of the side roots turn and grow 

 downwards, although in a few cases the tip of the . tap-root, 

 when not too much injured, can regrow a new tip and continue 

 the main line of growth. 



71. Root pressure. We have found that when osmosis takes 

 place in a root there is likely to be an excess of movement 

 in one direction. We should expect more to come into the 



