PERMANENT WILTING IN PLANTS 



113 



resenting the five main averages as does that of figure 1. The 

 present generahzed curve was plotted to pass through the point 

 representing average (1), using the constant ratio 2, as hereto- 

 fore, and the constant difference 0.55, with the logarithmic equa- 

 tion already stated. 



It is obviously not far from the truth to assert that in this 

 series each increase of about 0.55 in the magnitude of the per- 

 centage of soil moisture residue is accompanied by a doubling 

 of the evaporation intensity as here measured. 



Fig. 4 



Series V. In this series plants of Zea were employed, three 

 plants in a culture, eight weeks old at the beginning of the work. 

 The soil used was the natural clay loam above referred to, with- 

 out admixture of sand. The water holding power of this soil was 

 found to be 57.32 per cent., from which the calculated soil moisture 

 residue at permanent wilting (from the formula of Briggs and 

 Shantz) becomes 12.52 per cent, of the dry weight of the soil. The 

 experiment was begun on August 21, 8 a.m. The first permanent 

 wilting occurred in the open, in the chamber in the open and in 

 the shelter, on the first day. On August 22, permanent wilting 



THE PLANT WORLD, VOL. 17, NO. 4, 19U 



