Roots. 



43 



33. Clinging Roots. — The roots growing on the stems 

 of the poison ivy and trumpet creeper (Fig. 17), for in- 

 stance, do not serve an absorbing function, but are merely 

 employed in holding the slender stems upright against a 

 support. If these plants are growing near a tree or wall 

 they find themselves shaded on one side, and their roots 

 grow away from the side of greater 

 illumination and toward the object 

 which is shading them ; in this way 

 they are quite certain to find a suit- 

 able support. 



34. The Various Directive Forces. — 

 Thus we see that roots have quite 

 diverse functions to perform and that 

 they show a marvelous capacity for 

 employing various forces to direct 

 them in their growth. If they are 

 to grow into the soil, gravity is chosen 

 as a guide ; if into the body of some 

 host, as in the case of dodder, the 

 stimulus of contact is selected ; if 

 toward some object of support, either 

 light or gravity is chosen, as is most practicable. Even to 

 any given force the various parts of the root system may 

 react differently ; the main roots growing in the soil are 

 directed more or less downward ; the lateral roots spring- 

 ing from these make greater or less angles with the line 

 of gravity ; while the ultimate branches may grow in any 

 direction, apparently without respect to gravity. In this 

 way all parts of the soil within the range of the roots are 

 fully occupied, which would not be the case if the entire 

 root system were impelled by gravity in one direction. The 

 water in the soil, also, has a directive influence on the 



Fig. 17. 



Portion of a stem of the 

 Trumpet Creeper, show- 

 ing its clinging roots. 



