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 
Fic. 134. — The radicle or primary root of the Sunflower growing downward 
in response to gravity. After Osterhout. 
horizontally and, when strictly horizontal, are neither positively 
nor negatively geotropic. What is shown in Figure 134? 
Roots, especially primary roots, are sensitive to moisture and 
grow towards it when more moisture is needed. The tropism 
induced by water is called hydrotroptsm. Most roots are to a 
greater or less extent positively hydrotropic. Notice what is 
shown in Figure 185. In response to the water influence, the 
roots of most cultivated: plants grow deeper in the soil during 
Fie. 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. 
