239' 
Aspects of Ecology. 
of view the moving instrument is therefore more accurate, but it 
must be pointed out that the leaf itself is a stationary wet surface- 
and evaporation from it varies with every change of wind velocity. 
If then one wishes for information about transpiration and not 
merely, as in meteorology, to know the actual relative humidity of 
the air away from the plant it would seem to be wiser to use the 
old fashioned stationary wet bulb thermometer. Errors due to 
variable wind would then be similar on both, instead of it being, 
necessary to measure the rate of the wind and to attempt to allow 
for it 
From this parallel between evaporation from a stationary 
surface and from a leaf, one is lead to wonder whether direct 
measurements of the rate of evaporation from a wet surface (by an 
atmometer) would not be more luminous biologically. Even if Dr. 
Clements has considered this point and decided against it, we think 
it would have been helpful to have discussed it in his pages. There 
is, however, a regrettable scarcity of discussion on debatable points 
throughout many parts of the book. 
The third important factor in modifying function and structure 
is light. In natural habitats this is a troublesome factor because 
in the form of solar radiation it is bound up with heat radiation 
(apart from air temperature) and generally with smaller relative 
humidity. The leaves in direct insolation may be heated up to 
even 10"C above the air temperature and this may well produce 
disturbing effects on leaf-function which have not yet been 
sufficiently allowed for. 
The intensity of the light in the habitat is measured by a 
useful form of disc photometer containing “ solio ” silver paper. 
The intensity of darkening of this on exposure is compared with 
standard tints and thus the illumination (only taken about noon) is 
expresed in fractions, the cloudless noontide sunlight at the 
summer solstice, being taken as unity. Dr. Clements gives a way 
of calculating the position of the sun and so the intensity of 
sunlight at any hour of the year. In his table of intensities 
however he neglects to state whether the calculation is for the sun 
alone or for the “total light,” i.e the sun plus the diffuse light; and 
also whether it refers to the illumination of a surface always at 
right angles to the rays of the sun as it moves, or to a surface fixed 
approximately horizontal, as are most leaves. From inspection, the 
calculation seems to be intended for the former while the latter 
would appear the more directly biologically useful 
