SOIL MOISTURE AND TO EVAPORATION. 19 



Another experiment, showing similar results, was performed with 

 the same soil after the author's return to Chicago. This extended over 

 a period of only ten days. The results are given at B, figure 2 (p. 15) . 

 It is seen to be the same form of curve as the previous one. 



RESISTANCE OFFERED BY THE SOIL TO WATER ABSORPTION BY ROOTS. 



There are in general three conditions under which plant roots fail 

 to absorb water from the soil. First, the soil may not contain an 

 adequate supply; second, the supply may be adequate but the solutes 

 dissolved in the water may not permeate the protoplasm of the root 

 hairs and may be of so great a concentration that plasmolysis occurs; 

 and, third, the soil may contain poisonous substances which injure the 

 roots and make absorption impossible, even though the physical concen- 

 tration of the soil solutions may not be great. Although the soils of 

 Tumamoc Hill contain a rather high percentage of soluble salts, it 

 is not probable that the second of the conditions just mentioned is ever 

 effective here to prevent water absorption. As the soil dries out, how- 

 ever, plants finally wither from lack of moisture, and this may be due 

 to either or both of the other two conditions. 



The first condition, lack of adequate water supply, may be effective 

 to check absorption in two ways. First, the actual water content of the 

 soil may be too low, and, second, there may be sufficient water in the 

 soil to supply the plants in question for many days, and yet the plants 

 may suffer because the rate of movement of this water may not be 

 sufficiently high to supply the soil layers immediately surrounding the 

 roots as fast as these layers are exhausted by absorption. These two 

 conditions are closely related and difficult to separate. Also, as water is 

 removed from the soil, the concentration of the soil solution may 

 increase, so that it is somewhat difficult to distinguish, as the critical 

 point is approached, between actual paucity of water and the effects 

 of high osmotic pressure. 



No attempt was made to analyze these factors by experiment, but 

 some interesting data were obtained in regard to the tenacity with 

 which this soil holds water against the osmotic pressure of a sugar 

 solution. Whether or not absorption in roots is primarily a phenomenon 

 of osmosis, we may be sure that the osmotic condition of the root hairs 

 is of fundamental importance in the process. If the root hairs are 

 plasmolyzed absorption can not proceed normally. Therefore it is of 

 the utmost importance to study the relations existing between an osmotic 

 cell and soils which contain various amounts of water, and it was along 

 this line of inquiry that experiments were instituted. 



