1910] DACHNOWSKI—ARID HABITATS 337 - 
and that, hence, from the point of view of the economic importance 
of these results, it may be found that a certain species, that is, that one 
crop, is better adapted than another to withstand the effects of this 
type of soil. Certainly an opportunity exists through such studies for 
an increase in the utility of these soils, and for a better understanding 
as to the character and the nature of functional responses of plants 
adapted to them. 
Leaving out of consideration for the present studies of plants 
which deal with functional variability and with selection, we may 
speak with much more confidence of the fact which environmental 
factors exert upon diversity of function. This effect can be much 
more readily studied and shows itself much more conspicuously. In 
fact, a knowledge of drought resistance in plants within a physiologi- 
cally arid habitat will be gained mainly by securing data as to the rela- 
tion of plants to environmental factors. It is quite generally agreed 
that transpiration is one of the most important criteria in physiological 
ecology. Transpiration data are more conveniently obtained, if not 
more serviceable in this respect, than data on respiration differences 
or on green and dry weight of plants. Aside from the physiological 
responses of roots and leaves, the most significant physical conditions 
which more than others directly affect transpiration in plants, are 
the supply of available water to the roots, and the evaporation power 
of the air. On account of this distinctive feature, the problem separates 
itself into two phases, each of which must be considered by itself. 
The one relates to the influence of the varying edaphic conditions, _ 
the other to the effects of climate. The two are quite different, 
and each is of relatively great importance. Taken by themselves 
neither should be unduly drawn upon in the interpretation of drought 
or other resistance. The futility of the point of view which disregards 
all but one or two influences is seen clearly enough in the laboratory. 
But the inadequacy of such proposals is still more apparent when we 
attempt field work. 
A consideration of the physiological causes underlying the con- 
ditions stated has led to the conclusion that for the local study exten- 
sive investigations in the heat requirements of bog plants and the 
location of their minimum temperature below which growth ceases 
(Merriam, also SwINGLE) do not have the importance which must 
