HAP. iv.] WATER REQUIRED FOR GROWTH 



101 



The divergencies in these results are intelligible, if 

 reconsider that the "transpiration " process by which 

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

 rhich the plant gets heavier, have no necessary con- 

 ection, though both become active under the same 

 timuli of light and warmth. Some leaves transpire 

 ipidly as a means of maintaining a low temperature 

 hilst absorbing large amounts of radiant energy from 

 le sun ; other plants which have to resist drought 

 iduce the transpiration by a thickened cuticle, or by 

 more concentrated cell sap. Dr H. Brown has shown 

 lat of the radiant energy falling upon a sunflower leaf 

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

 Dnsumed in evaporating the transpiration water ; of 

 le energy falling upon the same leaf in bright diffuse 

 aylight, only 28 per cent, was used up in evaporation, 

 omparing in these two cases the water transpired with 

 le carbohydrate produced (and this will be about T %- of 

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

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

 .vestigations are desirable ; but, taking the whole 

 oup of observations, we shall be justified in assuming 

 tat our ordinary field crops transpire about 300 lbs. 

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

 mains to translate this approximate figure into tons 

 water per acre required to grow the ordinary crops, 

 he following table shows the weight at harvest of a 



