52 THE WATER-SUPPLYING POWT^R OF THE SOIL 



It now appears that water passes from roots to foliage because there 

 is a progressive tendency towards desiccation in the latter and because 

 tension in a liquid is (like pressure) transmitted equally in all directions. 

 Thus, transpiration or growth appears to produce an increased satu- 

 ration deficit ("Sattigungsdefizit," Renner, 1911) or incipient drying 

 (Livingston and Brown^) in the exposed cell walls of the leaves, followed 

 by a corresponding increase in this deficit in all cell walls abutting 

 upon gas. It was Dixon who first pointed out that this, and this alone, 

 is a sufficient explanation for the entrance of water through the root 

 periphery, and his ideas in this connection have been supported and 

 augmented by the ingeniously derived experimental evidence set forth 

 by Renner,^ wherein he shows that the water-absorptive power of roots 

 is directly related to the degree of saturation deficit obtaining in the 

 leaves above. As Renner has mentioned, this absorptive power seems, 

 indeed, to be a direct function of the tendency of the exposed cell walls 

 of roots to become less thoroughly saturated or imbibed, through 

 migration of water from these organs to higher parts of the plant. 



What may be the importance of vacuolar osmotic pressure in root 

 cells, as far as water absorption is concerned, remains to be determined, 

 but it may be supposed at present that the influence of this pressure is 

 only an indirect one, perhaps being mainly effective in maintaining the 

 form of the roots and their contact with the water films of the sur- 

 rounding soil. It is already clear that roots do not take up water at 

 rates proportional to their vacuolar osmotic pressures and that the 

 absorbing sj^stem may operate equally well with high or low turgidity 

 of roots, so long as these organs are not deformed, as by plasmolysis.^ 



The rate at which water enters a plant is obviously a function of the 

 absorbing power of the roots and of the supjilying power of the soil. 

 If the water-supplying power of the soil be low and the absorbing power 

 of the roots be high, the subterranean environment as a whole must 

 act to retard water entrance. Statically, such a resistance may be 

 measured in terms of pressure, and its value might be equated to 



^Livingston, B. E., and W. H. Brown, Relation of the daily march of transpiration to variations in 



the water content of foliage leaves. Bot. Gaz. 53: 309-30. 1912. 

 ^Renner, O., Ver.suche zur Mechanik der Wasserversorgung. 1. Der Druck in den Leitungsbahnen 

 von Freilandpflanzcn (Vorliiufige Mitteilung). Ber. deutsch. Bot. Ges. 30: 57(3-641. 

 1912. 



, Versuche zur Mechanik der Wasserversorgung. 2. Uebcr Wurzeltatigkeit. Ber. deutsch. 



Bot. Ges. 30: 642-8. 1912. 

 'The relation of internal osmotic pressure to various forms of external resistance to water 

 absorption has been dealt with in the following paper: Livingston, B. E., Osmotic pressure and 

 related forces as environmental factors. Plant World 16 : 165-86. 1913. 



On some aspects of the relation Ijctween the osmotic pressure of roots and water absorption, 

 see: Bovie, W. T., Effects of adding salts to the soil on the amount of nonavailable water. Bull. 

 Torr. Bot. Club. 37: 273-92. 1910. Livingston, B. E., The relation of the osmotic pressure of 

 the cell sap in i)lants to arid habitats. Plant World 14: 153-64. 1911. -Vlso see: Renner, O., Ver- 

 suche zur Mechanik der Wasserversorgung, 2. 1912. 



On the relation of root plasmolysis to absorption, see: Caldwell, J. S., The relation of environ- 

 mental conditions to the phenomenon of permanent wilting in plants. Physiol. Res. 1 : 1-56. 

 1913. 



