344 CHAPTER XXI 



The variation of transpiration ratio according to the crop 

 is shown by the following figures obtained at experimental 

 stations in the Western States : — 



Expressed otherwise, one ton of dry lucerne transpired 

 1,068 tons, or from 9 to 10 inches of water; one ton of rye 

 transpired about 7 inches, and so on, for the weight of one 

 acre-inch of water is roughly 110 tons. 



The transpiration ratios given above are not likely to be 

 the same in South Africa, because our evartoration is about 

 70 inches per annum, while at these stations in America it 

 was considerably less. The relative order, however, would 

 probably correspond. 



In practice, a study of the transpiration ratios of plants 

 is important w^hen, for example, we realise that a crop of 

 millet utilises about half the amount of water that is required 

 by a barley crop of equal weight ; also, that kaffir corn re- 

 quires considerably less water than maize, which, without 

 taking into consideration its other drought-resistant qualities, 

 such as dormancy, shows why sorghums can be grown under 

 drier conditions than maize. 



The more humid the climate the smaller is the water 

 requirement of plants generally. Thus, in North Dakota, 

 with an annual evaporation of 30 inches, the transpiration 

 ratio of lucerne w^as found to be 518, while in Texas, with an 

 evaporation of 54 inches, the transpiration ratio was 1,006. 

 This means that tw^ice the amount of lucerne could be grown 

 with the same amount of water in North Dakota as in Texas. 



The fertility of the soil has a direct influence on the 

 amount of water lost by transpiration. Hellriegel obtained 

 the following results in pot-cultures : — 



Units of Ca (NOJ, Dry Transpiration 



applied. matter. ratio. 



1,111 724 



4 8,479 399 



8 13,936 347 



12 18,288 345 



16 23,026 302 



20 25.504 292 



