Ch. V, 7] SELF-ADJUSTMENTS OP ROOTS 247 



7. The Self-adjustments of Roots to Prevailing 

 Conditions 



Roots possess in remarkable degree that property of in- 

 dividual adjustment to the peculiarities of their immediate 

 surroundings, such as was earlier described in the photo- 

 tropism of leaves and the geotropism of stems. 



Geotropism, indeed, is no less characteristic of roots than 

 of stems. The first root which issues from the germinating 

 seed always grows over to point directly downward, no 

 matter in what position the seed happens to lie (Fig. 119). 

 It is described as positively geotropic, or progeotropic, 

 the main stem being negatively geotropic, or apogeotropic. 

 The secondary or side roots possess transverse geotropism, 

 growing out horizontally, or nearly so, and are described as 

 diageotropic. The tertiary roots, however, those which 

 grow from the side roots, are hardly geotropic at all, and 

 therefore respond more freely to the other influences next to 

 be mentioned. The adaptive explanation of such geotropic 

 growth is obvious, for thus the main root is brought in the 

 quickest way to the water supply, essential to the further 

 growth of the young plant ; the side roots are spread at angles 

 which take them into the widest area of soil, while giving 

 them angles advantageous to their anchorage function; 

 and the tertiary roots are left free to wander wheresoever 

 the materials needed by the plant are most abundant. 



Especially characteristic of roots is their hydrotropism, 

 or sensitive adjustment to moisture in the soil. Roots 

 not only grow towards soil moisture, but branch and grow 

 more profusely in moist than in dry places. A practical 

 exemplification thereof is found in the filling of drain pipes 

 by tree roots (Fig. 174). The adaptive explanation of 

 hydrotropism is sufficiently clear; since the primary func- 

 tion of roots is the absorption of water, they need to find the 

 most abundant supply. The actual operation seems to 

 be this, — the tip of the root is sensitive to differences in 



