Hydrotropic Responses of Roots in Soil

Seven thousand seven hundred sixty-three seedlings representing 29 genera and 14 families have been grown in a great number of experiments to determine the hydrotropic response of primary roots of seedlings growing on a steep soil moisture gradient. The steep gradient has been obtained by placing in contact a layer of soil with a moisture percentage below the hygroscopic coefficient and a layer with a moisture percentage slightly below the moisture equivalent or field coefficient. This gradient was intended to provide optimum conditions for a hydrotropic response, and is steeper than will ordinarily be encountered in the field where the wilting percentage normally represents the low moisture content of the areas into which roots might grow. Under these conditions the growth of seedlings in the moist soil has been normal and a sharp moist-dry line between the two soil layers has been maintained over a period of one to two weeks. In contrast to the normal growth of roots in the moist layer, growth has been completely checked in roots which have penetrated more than approximately 1 cm. into the dry soil. 2. The primary roots of many plant species have shown no tendency to bend when growing from the moist soil layer with available moisture into the dry soil with insufficient moisture to maintain growth. The typical response in this group of plants has been a growth along the lines determined by the position of the seed and the normal response to gravity, until the root has grown into the dry soil. Under these conditions branching was stimulated in the portions of the root remaining in the moist soil. These branches grew normally outward and down without showing hydrotropic response, and continued in the moist soil or passed from it into the dry soil, depending upon their initial direction of growth. Those which grew into the dry soil, with favorable food conditions, again formed branches which repeated the response of the earlier roots. 3. A second group of plants showed weak hydrotropic responses. A small percentage of the primary roots bent at an angle of 45⚬ from the vertical and followed a moist-dry soil line. Normally a somewhat larger percentage showed some bending toward the more moist soil but did not follow the line at an angle of 45⚬. 4. A third and smaller group of plants showed responses in soil comparable with those obtained in air with the Sachs experiment, with from one-half to three-fourths of the roots following the moist soil at an angle of 45⚬ and another fraction bending toward the moisture but not following the 45⚬ line. Secondary roots as a group showed no clear-cut hydrotropic response in any of these experiments. 5. The data indicate that hydrotropism in roots is by no means comparable in distribution and intensity of reaction with geotropism in roots and stems or with phototropism in stems. The results with several species of the Cucurbitaceae and Leguminosae suggest that hydrotropic responses may depend upon a genetic factor present in some plants and absent from closely related species or varieties. 6. Apparently these results demonstrate the possibility of a response in soil similar to that which has been obtained in the air. On the other hand, they seem to show equally clearly that hydrotropism is not a universal and probably under field conditions not a common plant response.