I2O PRINCIPLES OF AGRICULTURAL CHEMISTRY 



ground, or if it is too large to be weighed, the evaporation of 

 water may be estimated by determining the quantity of 

 water in the vessel at the beginning and at the end of the experi- 

 ment (by analysis of the soil), and measuring the quantity of 

 water received by the vessel during the course of the experiment. 

 In the transpiration experiments of the Bureau of Soils, the 

 plants are grown in wire baskets covered with paraffin. Before 

 the measurements of transpiration are begun, the pot is sealed 

 with a sheet of paper coated with paraffin so as to exclude 

 evaporation as much as possible, a suitable hole being left for the 

 plants. The pots are weighed, and the loss of water is restored 

 daily. 



Factors of Transpiration. The amount of water transpired 

 by plants depends upon several factors : 



Humidity of Air. Transpiration decreases as the humidity 

 of the air increases, for evaporation into dry air is much more 

 rapid than into moist air. At the Nebraska Experiment Station, 1 

 plants were grown in an open greenhouse, in which the air was 

 dry, and in a closed greenhouse in which floors and benches were 

 kept wet, and water atomized into the air. The weights of water 

 transpired per gram of dry weight, were as follows : 



In dry greenhouse 340 



In moist greenhouse 191 



Available Water. Plants appear to transpire more as the 

 available water increases. The following figures show the 

 amounts of water transpired from the same soil containing differ- 

 ent quantities of water : 



Grams water used 



Per cent, saturation per grams dry 



weight produced 



IOO 290 



80 262 



60 239 



45 229 



35 252 



The plants grown with 35 per cent, saturation of the soil, did 

 not grow normally. 



1 Montgomery, Proc. Am. Soc. Agron., 1911, p. 276. 



