ABSORPTION OF WATER 143 



urated with water, solution C will draw water through it, quite 

 independently of the magnitude of the osmotic pressure in 

 cell A. 



Priestley's scheme naturally requires that solution C come in 

 contact with vessel B through cell A. Hence, there can be no dif- 

 fusion of the solution through the cell walls. But the walls of cells 

 are commonly made up of cellulose which is readily permeable to 

 diffusing substances. Priestley, therefore, supposes that the role of 

 an impermeable septum is played by the endoderm 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 seemed to be a puzzle. It 

 also allows the measurement of the suction power of roots and the 

 determination of 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, already referred to, have shown that 

 the suction tension of the roots of our common crop plants approxi- 

 mates 1.5 to 2 atmospheres; at any rate, it cannot be much above 

 this value. 



45. Absorption of Water from the Soil. The Water-retaining 

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

 absorption of water from the soil presents a much more compli- 

 cated and difficult problem than the simple imbibition of water by 

 roots immersed in a glass container. A number of forces are 

 found in the soil which counteract the suction 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, it shows suction tension. If 

 for convenience Priestley's supposed mechanism is adopted, then it 

 can be readily seen that with an equal concentration of the solution 

 in the soil and in the vessels of the plant, all suction 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 

 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 



