80 



THE WATER-SUPPLYING POWER OF THE SOIL 



Finally, to obtain a preliminary idea of the moisture-supplying 

 power of soils when they fail to support turgidity in plants rooted 

 therein, osmometer C was operated against a soil in which a Phaseolus 

 vulgaris plant had just attained the condition of permanent wilting.^ 

 The plant, well grown and apparently healthy, had developed from 

 seed in a sheet-metal cylinder. It had been sealed and allowed to 

 wilt thoroughly in the glass box, with high humidity, employed by 

 Shive and Livingston. The plant was lifted, allowing the roots to 

 bring away as much soil as they would. The bottom of the opening 

 thus formed in the soil was smoothed by removing loose soil and the 



Table 13. — Absorption rates of osmometer C, operating against soil in which Phaseolus plant 

 had wilted with very low evaporation intensity. 



No. of 



hour 



period. 



Hourly 

 rates, for 

 IS-inin. 

 periods. 



c.c. 



0.06 



0.15 



0.06 



0.03 



0.03 

 0.06 

 0.03 

 0.03 



0.03 

 0.00 

 0.00 

 0.03 



0.06 

 0.03 

 0.03 

 0.06 



Average 

 houriy 

 rates. 



0.08 



0.04 



0.02 



0.05 



No. of 



hour 



period. 



Hourly 

 rates, for 

 15-imn. 

 periods. 



c.c. 



0.03 



0.03 



0.03 



0.00 



0.03 

 0.03 

 0.06 

 0.03 



0.03 

 0.03 

 0.03 

 0.03 



0.03 

 0.03 

 0.03 

 0.03 



Average main- 

 tained rate*. . 



Average 

 hourly 

 rates. 



0.02 



0.04 



0.03 



0.03 



0.03 



♦After hour 1. 



osmometer was appressed against it. Cotton was packed about the 

 instrument in the usual manner and the whole cylinder was then 

 sealed with plastUine. The results of this test are given in table 13. 

 The temperature range during the test was from 27.8° to 31.0°. 



At the end of this experiment the soil directly beneath the osmometer 

 was found to have a moisture content of 4.2 per cent, on the basis of 

 dry weight. It is probably safe to suppose that this soil had as low a 

 water-supplying power (0.03 c.c. per hour through 10 sq. cm. of cross- 

 sectional area) as any in which ordinary plants can usually maintain 

 fohar turgidity. This single test is sufficient here to show the manner 

 in which such osmometers as were here employed may be used for 

 physiological inquiry. 



'Shive and Livingston, 1914. This soil was from one of the wilting experiments of these authors. 



