H7 
The Co-Efficient of Humidity. 
values m t and m 2 mark the limits of the association and are 
probably determined by competition with other associations whose 
M values are different. 
(3.) There will also he a supra-maximal (wj) and a sub-minimal 
(w 2 ') value of in for any association or its component members, 
determined not by competition, but by the physiological relations of 
the root-systems with the soil-water. When these values are 
reached and maintained the association can no longer continue to 
exist. 
Opportunities for determining these supra-maximal and sub- 
minimal values may not be frequent, and usually they may not be 
distinguishable from the maximal and minimal values. But there 
are occasions when they cannot be ignored. The case already 
quoted where Mercurialis was wilting in September, 1911 is one in 
point, where the sub-minimum had been reached, for though the 
plants recovered it was only because the existing state was not 
maintained, and clearly there was no competitive factor. Again 
the gradual accumulation of peat may of itself raise the coefficient 
to such a supra-maximal value that the existing association can no 
longer maintain its position, just as artificial drainage may lead to 
the extinction of the association by reducing the coefficient to its 
sub-minimal value. 
(4) . The sub-minimal value of the coefficient for any association 
will represent, and he the appropriate expression of the echard. 
(5) . The soil moisture index of an association will be fully 
determined by finding the values of m lt m 2 , m' lt ui 2 and M. 
These considerations only hold good at present in cases such 
as have been considered, where the humus practically holds all the 
soil water, i.e., where there is no “free” water in the substratum, 
nor sufficient colloid clay to affect the balance. 
It only remains for me to express my thanks to Mr. A. G. 
Tansley for helpful criticism of the paper, for calling my attention 
to the work of Briggs and McLane and for the suggestions utilized 
in the closing paragraphs. 
