180 DRY-FARMING 



Plant-fnod and transpiration 



It has been observed repeatedl)' by students of 

 transpiration that the amount of water which actuaUy 

 evaporates from the leaves is varied materially by 

 the substances held in solution by the soil-water. 

 That is, trans]3iration depends upon the nature and 

 concentration of soil solution. This fact, though not 

 commonly applied even at the j^resent time, has 

 really been known for a very long time. Woodward, 

 in 1699, observed that the amount of water tran- 

 spired by a plant growing in rain water was 192.3 

 grams; in s])ring water, 16.'!. G grams, and in water 

 from the River Thames, 159.5 grams; that is, the 

 amount (jf water transpired by the plant in the com- 

 jjaratively pure rain water was nearly 20 per cent 

 higher than that used l)y the ])lant gnjwing in the 

 notoriously impure water of the River Thames. 

 Sachs, in 1859, carried (.)n an elaborate series of ex- 

 periments on transjjiration in which he showed that 

 the addition of p(.itassium nitrate, ammonium sul- 

 phate or common salt to the solution in which plants 

 grew reduced the transjjiration; in fact, the reduc- 

 tion was large, varying from 10 to 75 per cent. This 

 was confirmed by a number of later workers, among 

 them, f(jr instance, Buergerstein, who, in 1875, 

 showed that whenever acids were added to a scjil or to 

 water in which plants are gi'owing, the transpiration 



