ROOTS AND RHIZOIDS 



499 



falls " a common sight. The effectiveness of roots as anchorage organs 

 is well displayed along streams and shores, where erosive forces are 

 active. In such situations the earth often is held firmly in place by 

 matted roots and it is only by undermining the grasses and trees, which 

 may even overhang in such places, that the erosive forces are able 

 finally to dislodge the plants and to encroach farther upon the land. Even 

 more striking is the behavior of roots in regions of shifting sands along 

 seacoasts, where many grasses and other plants are able not only to 

 maintain themselves, but even to check the prog- 

 ress of the sand (fig. 707). Such plants are 

 known as sand binders, and are of great impor- 

 tance in preventing the encroachment of sand upon 

 villages and farms, even being planted for that 

 purpose in many places. 



Gravity and the direction of root growth. The 

 tendency cf roots to grow downward (i.e. to ex- 

 hibit progeotropisni) makes possible a favorable 

 relation to absorption and anchorage (figs. 691- 

 693). The geotropic reactions of roots are well 

 shown when seeds germinate on the soil surface; 

 if the root issues from the upper side, it may curve lin s of maize ( Zea 



, , - n ,. . j Mays) that has been 



through an arc of 180, and grow directly down grown at the edge of 

 into the soil. But while the tap root usually grows a funnel whose sur- 

 straight down in this fashion, the numerous side face has been ke P l 



, moist ; note that the 



roots branch out in almost all directions. 1 he root is pro hydrotropic, 

 advantage of such a habit is clear enough, but the following the moist- 

 cause is obscure. If the growing part of a tap e " ed surface > in f tead 



of growing vertically 



root is removed, some of the stronger side roots downward. 



commonly begin to grow straight down (figs. 708, 



709). Apparently there is something in the tap root which inhibits 



the expression of progeotropism by the side roots. The removal of the 



tap root removes this inhibition and the side roots change their growth 



direction. 



Water and the direction of root growth. Roots usually grow toward 

 moisture; that is, they are prohydrotropic. Water and gravity often com- 

 bine to cause downward growth in roots, but commonly the water in- 

 fluence is the stronger of the two. When seeds are planted in such a 

 way that the source of water is at one side rather than from beneath, 

 the roots grow laterally, not vertically (fig. 7 10 )- In nature similar 



FIG. 7 ro. A seed- 



