124 WILLIAM H. BROWN 



of the plants used, except those of Vicia faba, were grown in 

 the open until the beginning of the experiment. 



The plants . were grown either in ordinary five-inch flower 

 pots or in zinc cans. Either method is open to objections. The 

 flower pots, being porous allow water to evaporate from their 

 sides, so that the soil near the outside may become dryer than 

 that nearer the center. This objection was largely obviated 

 by using the plants while they were small, when the roots were 

 still chiefly in the interior of the soil mass; and by taking the 

 soil sample from top to bottom, 2 cm. from the center of the pot. 

 A cork-borer was used for this purpose, the upper 2.5 cm. of the 

 core being discarded. The use of the zinc cans had the disad- 

 vantage that the temperature of the soil varied with the intensity 

 of the sunlight, while the temperature in the pots, wherein 

 the soil was cooled by evaporation, was comparatively free from 

 this influence. For this reason the pots were used almost exclu- 

 sively, and were employed in all cases except where cans are 

 mentioned. 



Each soil sample, as soon as taken, was put into a weighed 

 bottle; weighed, dried for three days at 101° C, reweighed and 

 the non-available water content calculated on the basis of the 

 dry weight of the soil. 



The evaporation rate was taken every hour by means of a 

 porous cup atmometer, and the results were afterwards reduced 

 to the standard which has been employed b}^ Livingston. 5 The 

 evaporation rate given with each experiment is for the hour 

 immediately preceding wilting. 



The soil used for growing the plants was a mixture of equal 

 parts by dry volume, of clay-loam and sand. Both soil com- 

 ponents had been passed through a 3 mm. sieve. The water 

 holding capacity of the mixture was 31.4 per cent of the weight 



5 Livingston, B. E., The relation of desert plants to soil moisture and to evapo- 

 ration. Publication 50 of the Carnegie Institution of Washington. 1906. 



Idem, A simple atmometer. Science, N. S. 28: 319-20. 1908. 



Idem, A rain-correcting atmometer for ecological instrumentation. Plant 

 World 13: 79-82. 1910. 



Idem, Paper atmometers for studies in evaporation and plant transpiration. 

 Plant World 14: 281-9. 1911. 



