WATER RELATIONS OF PLANTS 287 



Priestley's scheme naturally requires that solution C come in 

 contact with vessel B only through cell A. There should be no 

 diffusion of the solution through the cell wall. But the walls 

 of cells are commonly made up of cellulose, which is readily 

 permeable to diffusing substances. Priestley, therefore, sup- 

 poses that the role of an impermeable septum is played by the 

 endoderm, the Casparian strip, whose corky radial walls seem 

 to be formed for this function. 



This scheme explains quite cleverly the mechanism of root 

 pressure, as well as the role of the endoderm in certain parts 

 of the root, the function of which before this was a puzzle. It 

 also allows the measurement of the suction tension of roots by 

 means of determining the concentration of the solution exuded 

 from the cut stump of bleeding plants. According to Priestley's 

 idea, the concentration of this solution controls suction tension. 

 The determinations by Sabinin have shown that the suction 

 tension of the roots of our common crop plants approximates 

 1.5 to 2 atmospheres. 



66. Absorption of Water from the Soil. Water -holding Forces 

 of the Soil. Available and Unavailable Water. — The absorption 

 of water from the soil presents a much more complicated and 

 difficult problem than the simple imbibition of water by roots 

 immersed in a glass container. A number of forces exist in the 

 soil that counteract the suction tension of the roots. The sum 

 total of all the forces that retain water in the soil may be called 

 the '^ water-holding capacity" of the soil. 



It is not pure water but a solution of a certain concentration 

 that is found in the soil and, therefore, of itself it exhibits a force 

 that tends to hold water. If for convenience Priestley's mecha- 

 nism is adopted, then it can be seen readily that with an equal 

 concentration of the solution in the soil and in the vessels of the 

 plant, all absorption must cease. But here, likewise, a special 

 regulating mechanism exists in the plant, which indicates that 

 the higher the concentration of the soil solution, the greater also 

 is the concentration of the solution in the vessels of the plant. 

 This mechanism, however, works only within certain limits. The 

 majority of plants are unable to grow in strongly saline soils, 

 where the concentration of the soil solution is very high. In 

 such places a special flora develops, the so-called ''halophytes." 

 These plants have the faculty of accumulating an enormous 



