SOIL MOISTURE AND WILTING 131 



The data given in table 8 show that in these experiments 

 Vicia faba, like the other three species, wilted with a greater 

 percentage of water in the soil when the rate of evaporation was 

 high than when it was lower. 



The results of the above experiments, with Martynia, Physalis, 

 Tropaeolum and Vicia, clearly show that determinations of the 

 percentage of soil moisture at the time of wilting, — at least for 

 these plants and for the soil here used, — are of no value unless 

 the evaporating power of the air, at least for the period just 

 preceding the time of wilting, is taken into account, and it is 

 quite probable that this is also true for all ordinary plants and 

 soils. 



Another interesting and apparently fundamentally important 

 point is brought out by a comparison of the data of tables 2, 

 4, 6, and 8. The moisture content of the soil at the time of 

 wilting is seen to vary among the different plants within but 

 comparatively narrow limits for any given evaporating power 

 of the air during the ho.ur preceding wilting. Thus, with an 

 evaporation rate from the standard atmometer of from 7.2 to 

 7.5 cc. per hour, the percentage of residual water content of the 

 soil here used was 11.9 to 17.5 for Martynia, 12.7 to 19.9 for 

 Physalis, and 17.5 to 17.8 for Tropaeolum. With the evapor- 

 ation rate between 0.9 and 1.6 cc. per hour these limits are 5.2 

 to 6.6 for Martynia, 5.7 to 7.8 for Physalis, 5.3 to 7.4 for Tro- 

 paeolum and 7.5 to 8.3 for Vicia. The most marked exception 

 to this generalization is shown by Vicia. (table 8, experiments 

 1 to 10) with an evaporation rate of 4.2 cc, in which case wilt- 

 ing occurred with a soil water content of from 17.8 to 36.4 per 

 cent. Under high evaporation rates Vicia faba is able to main- 

 tain its form for but a short time, as has already been noted, 

 and we may suppose that these extraordinarily high residual 

 moisture contents are due to the fact that the limits for the 

 general life activities of this plant have been surpassed and that 

 the physical system of its body is somehow in process of break- 

 ing down. Some of the results obtained Avith Tropaeolum sug- 

 gest a similar interpretation. But it appears to be fairly clear 

 from the general nature of the data here given that, within a 



