222 LIGHT, VEGETATION AND CHLOROPHYLL 



these conditions, even if nothing is added, the increase of 

 growth and yield will be proportional to the quantity of water 

 supplied. The circumstances are such that the yield obtained 

 from water alone is maximum. 



When a good field of maize is in full growth, it evaporates 

 in a day 24 tons of water per acre. If the soil can provide it 

 with only 16, the growth will be proportionately limited and 

 instead of 198 lb. of dry matter, only 132 lb. will be elaborated; 

 the deficit of 8 tons of water will be shown by a deficit of 

 66 lb. of dry matter, which represents in chemical energy 

 120,000,000 calories. 



Suppose that the dam is situated in an excellent position 

 for the generation of electricity, with a possible fall of 

 100 metres. Each litre of water represents a potential of 

 100 kilogrammetres, i.e., 234 calories; the 8 tons in question 

 therefore represent 1,872,000 calories. The transformation 

 into electricity will not be made without losses, but never- 

 theless it is reasonable to expect in kilowatt-hours the 

 equivalent of 1,200,000 calories. 



It is obvious that in the service of chlorophyll water can 

 give a return in energy a hundred times greater than that of 

 turbines, only ten times if the possible fall is 1,000 metres, 

 but such falls will never be very numerous. On this basis, 

 irrigation is therefore fifty to one hundred times more 

 economic than the generation of electricity. 



The results of irrigation are obviously more uncertain 

 than those of hydro-electric generation, but they become less 

 so when irrigation is not indiscriminate but is directed only to 

 the points which lack water. All crops are not as productive 

 as maize, but the coefficient of transpiration never falls below 

 a quarter of the value for maize and an adequate and 

 indisputable margin still remains in favour of irrigation, even 

 if the possible fall were 1,000 metres. 



Again, the water used by the plant is immediately returned 

 to the air in the form of water vapour. This water vapour 

 transforms the climate of the region; the atmosphere is more 

 humid and more quickly saturated by passing clouds which 



