Aspects of Ecology. 
241 
series occur in bright dry habitats and in shady damp habitats 
respectively. In spite of much comparison of xerophyte sun leaves 
with hydrophyte shade leaves, it is not yet certain which structural 
features are a correlation with water-factors and which with light- 
factors. 
Dr. Clements regards transpiration as a functional response to 
water-stimulus and variation in functional leaf-area as the chief 
structural adaptation to the same stimulus. The processes of 
adjustment and adaptation which are to be regarded as two aspects 
of the same response he styles “hydroharmose.” Dr. Clements 
soundly recommends the study of transpiration in the actual habitat 
and by the best method, that of weighing plants potted in the natural 
soil, or in prepared soil with controlled amounts of water. When 
he comes to correlate transpiration with structure he would like to 
be able to measure the surface of all the mesophyll cells that abut 
on the air-spaces in the leaf as the real source of the vaporisation. 
Failing the possibility of this, he resigns himself to correlating the 
function to the area of the leaves, multiplied by a factor for the 
number of stomata. He hopes by introducing further factors for 
light, heat and humidity to get an exact mathematical expression 
for the amount of transpiration for any plant, and so make it 
possible “to compare species of different habitats on an exact 
basis.” 
In spite of the desire for great exactitude it seems to be 
overlooked that it is not purely the number of stomata which affects 
transpiration, but also the diameter of their pores, which is ignored. 
According to the measurements of Weiss there is a general 
tendency for crowded stomata to have small pores, and sparse ones 
to have large pores, and thus variations in number tend to be 
neutralised out. The closing of stomata in the drier grades of air, 
which has been already referred to, would appear in itself to be 
enough to throw out any calculation of transpiration based on 
external factors. The temperature of the transpiring leaf is the 
fundamental factor, since it provides the energy for the evaporation, 
and this also in ordinary field-work remains quite indeterminable, 
being, in direct insolation, much above the temperature of an 
adjacent thermometer. 
As regards Adaptation to water stimulus, Dr. Clements admits 
that it is not yet possible “ to connect each adaptation quantita¬ 
tively with the corresponding adjustment,” and so only sketches the 
well known varied ways of reducing transpiration. 
