52 RELATIONS OF SOIL TO WATER 



needful from the soil partly because a living plant contains 

 so large a proportion of water seldom less than three- 

 quarters of its weight ; but still more because the passage 

 of water through the plant is one of the most important 

 means of plant nutrition. Evaporation from the surface of 

 a living plant, chiefly from the leaves, takes place energetic- 

 ally whenever the amount of heat received from the sun, or 

 the state of dryness of the atmosphere, admits of water being 

 converted into vapour. The evaporation of water from the 

 leaf surface produces an upward stream of water from the 

 roots, by means of which the solution of plant food held 

 by the soil is carried to the leaves, and the substances which 

 it contains made use of for the production of organic matter. 

 Thus the greater the evaporation, the greater is the trans- 

 ference of plant food from the soil to the plant. 



So plain is the connexion between plant evaporation and 

 plant nutrition, that it has been thought probable that a 

 definite relation exists between the quantity of water evapo- 

 rated and the quantity of organic matter produced. The 

 relation in question appears to be fairly definite so long as 

 certain conditions remain constant, but to vary considerably 

 under a wider range of circumstances. The following table 

 gives the results arrived at by various investigators em- 

 ploying different methods of experiment (Lawes and Gilbert, 

 Jour. Hort. Soc. 1 850 ; Hellriegel, Exp. Station Record, iv. 

 532 ; King, Rep. Wisconsin Exp. Station, 1 894, 248 ; Wollny, 

 Exp. Station Record, iv. 532). 



That the conditions of the experiment have had a con- 

 siderable influence on the result is evident from a glance 

 at these figures. Barley, for example, has been experimented 

 with by each investigator, and has under various conditions 



