ROOTS AND RHIZOIDS 



499 



falls " a common sight. The effectiveness of toots 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 of roots to grow downward (i.e. to ex- 

 hibit progeotropism) 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 

 through an arc of 180, and grow directly down 

 into the soil. But while the tap root usually grows 

 straight down in this fashion, the numerous side 

 roots branch out in almost all directions. The 

 advantage of such a habit is clear enough, but the 

 cause is obscure. If the growing part of a tap 

 root is removed, some of the stronger side roots 

 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. ?io). In nature similar 



FIG. 710. A seed- 

 ling of maize (Zea 

 Mays) that has been 

 grown at the edge of 

 a funnel whose sur- 

 face has been kept 

 moist ; note that the 

 root is prohydrotropic, 

 following the moist- 

 ened surface, instead 

 of growing vertically 

 downward. 



