. Botany for agricultural students . Botany. 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 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,
. Botany for agricultural students . Botany. 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 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 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 Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Martin, John N. (John Nathan), b. 1875. New York : John Wiley & Sons, Inc.
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