HYDROTROPISM 663 



port. This movement usually occurs within a few minutes, and in the 

 tendrils of some species may take place in less than a minute. The speed 

 of the reaction is such as to suggest turgor changes in the cells rather than 

 growth. However, since the resulting changes in cell size are irreversible 

 the movement is properly classed as a tropism. Once the tendril becomes at- 

 tached to some object further growth in length ceases. As a result of in- 

 equalities in growth in the basal region the tendril becomes spirally coiled 

 so that it resembles a coil spring. Secondary wall formation then follows, 

 transforming the delicate thin-walled tendril into a firm supporting organ. 



A young tendril reacts readily to contacts with very light solids provided 

 the solid surface is not perfectly smooth and that contact with the tendril 

 is made at more than one place. No growth movements occur in tendrils 

 as a result of contacts with liquids or perfectly smooth solids. If a very 

 light thread weighing a small fraction of a milligram be moved along the 

 surface of the tendril curvatures will result, but drops of mercury many 

 thousand times heavier, or rain drops bring about no reaction. The mechanism 

 of thigmotropic movements cannot be satisfactorily explained at the present 

 time. 



Hydrotropism. — The roots of plants do not grow into soils in which 

 the water content is at or below the wilting percentage but usually do grow 

 into soils of higher water content. When the soil in small pots containing 

 growing plants is watered by means of porous clay irrigators the roots of plants 

 are often matted heavily around the surface of the irrigators and are more 

 sparsely distributed in the surrounding soil (Hendrickson and Veihmeyer, 

 1931). Such observations have often been cited as examples of positive 

 hydrotropism in roots. 



The work of Loomis and Ewan (1936) suggests, however, that curvatures 

 of growing root tips toward regions of higher water content are not as com- 

 mon as was once supposed. These investigators filled shallow boxes half full 

 of moist soil, placed the seeds to be observed upon the surface of the moist 

 soil, and then filled the box with dry soil. The moist soil was near its 

 field capacity and the dry soil had a moisture content well below the wilting 

 percentage (Chap. XVI). The soil was held firmly in position with paraffined 

 paper and the box placed in a moist chamber in such a way that the boundary 

 between the moist and dry soil made an angle of 45° to the vertical and so 

 that the dry soil was on the lower side of this boundary. These conditions 

 made it possible to determine the relative influence of soil moisture and 

 gravity upon the direction of root growth. A positive hydrotropic reaction 

 would result in a bending of the primary roots toward the moist soil. Ex- 

 amination of the root growth of thousands of seedlings representing 26 species 



