ROOTS AND THEIR USES 



175 



best for the work of absorption, is also as a rule best for the 

 support of an upright stem. So the need of support is an 

 additional reason for pushing the roots of a tree deeply and 

 widely into the soil. The roots of many plants, like the brace 

 roots of the Indian corn, shorten after they have grown to 

 some length apparently through a change in shape, of 

 some of their cells and pull the plant closer to, or more 

 deeply into, the soil, thus attach- 

 ing it more firmly. 



Not a few trees support them- 

 selves by roots of special sorts. 

 One form of supporting roots is 

 seen in the "screw pines" (Fig. 

 109), which are really not pines 

 at all, but are monocotyledons. 

 Some of them become good-sized 

 trees. Their trunks send out ad- 

 ventive roots at various heights 

 above the ground, which grow 

 downward on all sides into the 

 soil and become woody and firm. 

 The base of the stem and the 

 primary root often die, in which 

 case the body of the tree is sup- porting roots 

 ported entirely on the stilt-like 



adventive roots. Mangroves (Fig. no) are trees that grow 

 on low, swampy coasts, where the soil is moist and soft ; often 

 they live within the strip that is covered during part of each 

 day by the tide. They support themselves by roots that grow 

 downward from all parts of the trunk and branches. The 

 famous banyan trees of India support their lower horizontal 

 branches by similar prop roots that push into the soil and 

 grow to be thick and strong. These prop roots produce 

 adventive branches and look like so many additional trunks. 

 By means of these new branches and of the new prop roots 



FIG. 109. A screw pine 

 (Pandanus), showing the sup- 

 After Kerner. 



