90 TILLAGE— MOVEMENTS OF SOIL WATER [chap. 



The divergencies in these results are intelligible, if 

 we consider that the " transpiration " process by which 

 the water is lost, and the "assimilation" process by 

 which the plant gets heavier, have no necessary con- 

 nection, though both become active under the same 

 stimuli of light and warmth. Some leaves transpire 

 rapidly as a means of maintaining a low temperature 

 whilst absorbing large amounts of radiant energy from 

 the sun; other plants which have to resist drought 

 reduce the transpiration by a thickened cuticle, or by 

 a more concentrated cell sap. Dr H. Brown has shown 

 that of the radiant energy falling upon a sunflower leaf 

 on a bright August noonday, about 95 per cent, was 

 consumed in evaporating the transpiration water; of 

 the energy falling upon the same leaf in bright diffuse 

 daylight, only 28 per cent was used up in evaporation. 

 Comparing in these two cases the water transpired with 

 the carbohydrate produced (and this will be about -£$ of 

 the total dry matter) we find in the sunlight the ratio 

 was 347 to 1, in the diffuse daylight 234 to 1. Further 

 investigations are desirable ; but, taking the whole 

 group of observations, we shall be justified in assuming 

 that our ordinary field crops transpire about 300 lbs. 

 of water for each lb. of dry matter produced. It now 

 remains to translate this approximate figure into tons 

 of water per acre required to grow the ordinary crops. 

 The following table shows the weight at harvest of a 



