FACTORS INFLUENCING THE DIRECTION OF GROWTH 151 



response to the influence of gravity. Primary roots are positively 

 geotropic (growing toward the earth's center), while most stems 

 are negatively geotropic (growing directly away from the earth). 

 Lateral roots, as well as most branches of stems, grow more or less 



FIG. 134.; The radicle of Corn growing downward in response to gravity. 



After Hayden. 



horizontally and, when strictly horizontal, are neither positively 

 nor negatively geotropic. What is shown in Figure 134f 



Roots, especially primary roots, are sensitive to moisture and 

 grow towards it when more moisture is needed. The tropism 

 induced by water is called hydrotropism. Most roots are to a 

 greater or less extent positively hydrotropic. Notice what is 

 shown in Figure 135. In response to the water influence, the 

 roots of most cultivated plants grow deeper in the soil during 



FIG. 135. An experiment to show the effect of moisture upon the direc- 

 tion of the growth of roots. The box containing moist sawdust in which the 

 Corn is planted has a bottom of wire netting. After the roots grew through 

 the meshes, thus coming in contact with dry air, they changed their direction 

 and grew along the bottom of the box, thus keeping in contact with moisture. 

 Adapted from Osterhout. 



a dry season than during a wet season. When there is abun- 

 dance of moisture in the soil, Corn roots may grow within 2 

 inches or less of the surface, but are 3 inches or more under the 

 surface when there is a lack of moisture, and usually penetrate 



