Amount of Water Used by Plants 43 



house and two others in the field, one pair on irrigated ground 

 and the other on ground not irrigated. 



The table below shows the variations in the rate of evapora- 

 tion observed in the three localities : 



Plant house Irrigated field Field not irrigated 



These rates of evaporation took place upon a surface of 27 

 square inches of wet filter paper. 



The relative humidity observations were as here given: 



Plant-house Irrigated field Field not irrigated 



PER CENT 



49 55 



57 62 



48.5 49 



51.5 55.3 



So far as these figures may be relied upon, it would appear 

 that the rate of evaporation in the plant-house may even have 

 exceeded that. in the field, and if this was true during the time the 

 dry matter of the plant -house experiments was being produced, 

 then the indications are still less marked pointing toward an 

 increase in the amount of water being required for a pound of 

 dry matter in a dry, rapidly changing atmosphere, than is 

 required under stiller and more humid conditions. 



It may be true that in the dry air a more rapid loss of mois- 

 ture from the plant does take place, and that this loss stimulates 

 a proportional increase of dry matter. This is merely a suppo- 

 sition, however, with no experimental evidence to bear it out, 

 but such a tendency would give relations approaching those 

 recorded above. So, too, if the rate of evaporation is automatic- 



