28 



SOIL CONDITIONS AND PLANT GROWTH 



sap increases so its vapour tension decreases and the amount of water 

 lost by evaporation decreases also.^ 



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

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

 atDhame (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^ 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 :— 



Table VII.- 



-Effect op Varying Food Supply on the Water Requirements of 

 Sugar Beet. Wilfarth. 



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 (177). 



" The variations in water supply are : — 



where 100 = saturation of the soil. 



