The Co-Efficient of Humidity. 131 
tuents may be neglected. Leaving on one side the possibility that 
in other types of soil colloid clay may have a similar function, it 
remains to see how far the formula the water-content is a function of 
the humus-content provides in the coefficient of humidity a satisfactory 
instrument for gauging the soil moisture of a plant habitat. 
Turning to the examples given above it is seen that in Nos. 34 
and 66 the values of the coefficient in the two layers are practically 
identical. The natural interpretation of this is (1) that the humus 
is the sole cause for the difference in water-content; (2) that both 
layers are equally humid, in the same sense that the atmosphere 
may be equally humid on two days when the actual “water-content” 
is widely different. Its absolute magnitude is determined in the one 
case by the temperature of the air, in the other by the humus of the 
soil. So the coefficient measures the relative humidity of the soil, 
whilst the water-content measures its absolute humidity. 
In No. 13 the coefficient values are not quite so consistent, but 
the case is partly chosen for that reason. The uppermost layer, 
above the roots, in spite of its high water-content has the lowest 
coefficient, which thereby reveals the superficial drying of the peat 
during the summer—a fact that could not be demonstrated in any 
other way. It is also noteworthy that the two habitats represented 
by Nos. 13 and 34, occupied by the same plants, exhibit very similar 
edaphic conditions. 
Before turning to other examples in confirmation of these con¬ 
clusions it may be as well to look at another possible explanation of 
the varying water-content in these soils. It may be argued that we 
have here merely a variable holard, or total water-content, of no 
physiological importance and that in spite of this variation the 
chresard or available water-content may be the same throughout, 
viz., 20% or thereabouts. This implies a high echard (non-available 
water-content) in the case of peat, which in itself is not unreasonable, 
as there are other grounds for considering that bog plants (bog 
xerophytes) have difficulty in taking up water from wet peat. But 
this view appears untenable. There is no evidence that as a peat 
dries out until the wilting point is reached there is any change in 
the equilibrium between it and the sub-peat as regards the humidity. 
The evidence all goes to show that drying proceeds pari passu in 
peat and sub-peat. When each has lost all its physiological water, 
the echard, it is true is considerably higher in the peat than in the 
sub-peat, but the water is still distributed between the two according 
to the humus-content of each. The coefficient of humidity is lower 
