28 



SOIL CONDITIONS AND PLANT GROWTH 



supply increases. The relationship between food supply and water 

 requirements is very interesting but not easily explained. The amount 

 of soluble nutrient salts a plant takes up, and presumably also the con- 

 centration of the cell sap, increases with the concentration of these salts 

 in the surrounding medium. It might be supposed that, as the con- 

 centration of the cell sap increases, so its vapour tension decreases and 

 the amount of water lost by evaporation decreases also. 1 Drabble and 

 Drabble's experiments, however, are against the view that transpiration 

 is much influenced by the vapour pressure of the foliar cell sap [8 50]. 



Lawes at Rothamsted (163) found that about 250 units of water 

 were transpired for every unit weight of dry matter formed ; Hellriegel 

 at Dhame (128) obtained higher results, 300-350, Wollny at Munich 

 still higher, 600-700 (318), and Leather at Pusa (167) the highest 

 of all. The effect of variations in water and food supply was also 

 studied by Hellriegel, and more recently by von Seelhorst at Gottin- 

 gen (256-260), who more than any one else has worked at the various 

 water relationships of plants. His results with oats are given in 

 Table VI. : 



TABLE VI. EFFECT OF VARYING WATER SUPPLY 2 AND FOOD SUPPLY ON THE WATER 

 REQUIREMENTS OF OATS. VON SEELHORST (258). 



Similar results have been obtained by Wilfarth (Table VII.), (308), 

 with sugar beets grown in pots of soil containing known but varying 

 amounts of nitrate : 



1 Fitting (97) has shown that the osmotic pressure of the sap of desert plants is ex- 

 tremely high ; the vapour tension is therefore correspondingly low and the plant requires 

 remarkably little water. A different result, however, was obtained by Livingstone (178). 



z The variations in water supply are : 



where 100 = saturation of the soil, 



