92 BRIGGS: THE LIVING PLANT AS A PHYSICAL SYSTEM 



The lowest water requirement so far observed in the case of a 

 field crop was obtained at Arlington, Virginia, in 1915, where 

 corn and sorghum gave values of 151 ± 2 and 150 ± 2 respec- 

 tively. If we substitute these values in equation (5) we find 

 that the efficiency of the plants with the lowest water require- 

 ment so far measured is only about 5 per cent, while in the case 

 of plants with a water requirement of 1000, the efficiency is less 

 than 1 per cent. In other words, the fuel value of the plant, 

 i.e., the potential energy stored in the plant substance, repre- 

 sents only from 1 to 5 per cent of the energy dissipated during 

 the growth of the plant. 



Since the efficiency is approximately inversely proportional 

 to the water requirement it is of interest to consider briefly how 

 the latter may be reduced. Two procedures are open, namely, 

 plant selection and reduction in the evaporation rate. Dif- 

 ferent species of plants under the same environment show wide 

 differences in water requirement. For example, Shantz and the 

 writer have found the water requirement of alfalfa to be ap- 

 proximately three times that of millet, when the two species are 

 grown in large pots side by side. It is thus evident that in a 

 region where the rainfall is the limiting factor in plant production, 

 the fuel value of the plant material produced will vary widely 

 according to the water requirement of the plant. It would be 

 of much interest in this connection to know something of the 

 efficiency of different forest trees, but no water requirement 

 measurements of such plants are yet available. 



The second means of reducing the water requirement consists 

 in the selection of habitats where the evaporation-rate is low. 

 Here again little is known quantitatively as to what extent the 

 efficiency may be increased in this way, and the subject affords a 

 wide field for exploration. It is evident that if the absorption 

 of water from the soil is so far reduced by increasing the hu- 

 midity that the plant receives an inadequate supply of nutri- 

 ents from the soil, growth would be retarded from this cause. 

 Again, if we reduce the energy dissipated in evaporation by 

 shading the plants, a limit will be set by the minimum quantity 

 of radiant energy necessary for the photosynthetic processes. 



