126 
W. B. Crump . 
he has indicated a promising field for research in Ecology. But 
very little progress has been made in opening it out and this, 
apparently for two reasons. The direct determination of the 
available water-content in the field is impracticable under ordinary 
conditions, and when attempts have been made to determine the 
total water-content—the actual amount present as distinct from the 
maximum when the soil is saturated—the results have been 
discordant or inconclusive. So it comes about that whilst much 
attention has been given to soil physics and the relation between 
soil structure and water capacity, the particular information desired 
by the ecologist—the actual amount of water within the zone of 
root activities—is still non-existent. But the need of it is urgent. 
Even when definite soil types have been distinguished according to 
their chemical nature and physical structure and when the natural 
vegetation has been correlated with these types, the investigation 
is at least incomplete. To leave the water-content out of consi¬ 
deration is to omit the factor which links the two, the soil and the 
plant, together. Granted that the soil structure controls the water- 
content, still the latter may prove a much better index of the plant 
environment, either because it can be determined more simply, or 
expressed with greater precision, or because its relation to the plant 
is more direct and intimate. There may be cases where the nature 
of the soil is the controlling factor, but so long as the water-content 
is unknown its influence can neither be predicated nor denied. 
But it is necessary to consider at the outset whether it is 
possible to obtain a value for the water-content that will be in any 
way expressive of the habitat. Can it be regarded as a constant 
whose value for any given association may be determined by a 
series of analyses? How are the irregular fluctuations and the 
seasonal variation in the quantity of water present in the soil to be 
eliminated ? Is it the mean value, or the maximum in the wet 
season, or the minimum in the dry season, that is the more 
significant ? Is the range within such bounds as to afford a 
mean value that is in any way a criterion of the habitat ? Experi¬ 
mental results afford an answer to such questions and soon show 
that the investigation of a plant-habitat does not admit of the 
rigorous measurement applicable in physics and chemistry. If 
the water-content of the Calluna habitat, for example, is a 
constant of nature it differs radically from an atomic weight and 
can never be stated with the same exactitude, for it is of necessity 
subject to considerable variation. 
