PERMANENT WILTING IN PLANTS 109 



The graph of figure 3 brings out this and other points, this graph 

 being plotted similarly to those for series I and II. 



Figure 3 shows a generalized curve, of logarithmic form, pass- 

 ing through the points representing averages (1) and (4); it is 

 seen that this curve falls but a little below the point representing 

 'average (5). Averages (2) and (3) lie farther from and below 

 the generalized curve. In plotting the latter, the constant geomet- 

 rical ratio was again taken as 2 and the value of the constant 

 difference as 0.5 per cent. 



Series III agrees, therefore, with series I, in furnishing main 

 averages that may be satisfactorily considered (within the hmits 

 of probable experimental error) as forming a logarithmetic curve 

 for which each doubling of the evaporation intensity is concom- 

 itant with an addition of 0.5 to the value of the percentage rep- 

 resenting the residual soil moisture at permanent wilting. Al- 

 though the observed residues approach the calculated value 

 (from the Briggs and Shantz formula) for low evaporation 

 intensities, they are all, as in both the preceding series, much 

 greater than the percentage demanded by the calculation. 



Series IV. Phaseolus plants were used in this series, seven 

 weeks old at the beginning of the experiment. Each culture 

 comprised three plants, the containers being of the same size as 

 those employed in series I and II. The soil mixture here used was 

 different from that of the series heretofore considered ; it consisted 

 of a mixture of equal volumes of sand and clay loam. Its water 

 holding power was found to be 38.16 per cent, of its dry weight, 

 thus giving a calculated moisture residue (according to the formula 

 of Briggs and Shantz) of 5.92 per cent, of the dry weight. The 

 series was started September 11, 8 a.m., and the first permanent 

 wilting was observed, in the chamber in the open and also in 

 the open, on the first day. Permanent wilting occurred in the 

 chamber in the shelter on September 12, it was attained in the 

 glass box on September 17, and in the shelter on September 18. 

 The numerical results of the series are presented in table V, in 

 in which the arrangement and notations are uniforni with those 

 of the preceding three tables. 



