RESEARCH METHODS IX STUDY OF FOREST ENVIRONMENT. 41 
On the other hand, if the chloroplasts find themselves in a medium 
of cell sap which is cold, it is perfectly evident that the energy which 
has been concentrated in them through the absorption of a special 
assortment of rays may be dissipated to the surrounding medium 
by the simple process of conduction. The rate of conduction will 
decrease directly as the temperature of the cell sap approaches that 
of the plastids. It is thus seen that both radiant energy and heat of 
the air. which may serve to warm the leaf as a whole, do have an 
influence on photosynthesis; and that for a given intensity of sun- 
light there must be a leaf temperature below which photosynthesis 
can not be effected, because of the dissipation of the energy in the 
plastids. This leaf temperature will depend on every atmospheric 
condition, including the air temperature. The most important factor 
tending to keep the leaf temperature below the air temperature, is 
the use of any available heat in the water-vaporizing process of 
transpiration. This consumes a very large proportion of all the 
heat obtainable from all sources. The loss of water and consump- 
tion of energy is, presumably, to be looked upon as an unavoidable 
consequence of the need for stomata to admit carbon dioxide. 
The Nature of Sunlight. 
Biologists must enter upon the measurement of radiant energy, 
or even upon a discussion of the subject, with the greatest hesitancy, 
realizing (1) that the physicists' conception of energy is, at this 
writing, undergoing a change almost daily; (2) that investigations 
of the solar constant and of sky radiation have made enormous 
strides during the last two or three decades, creating a vast array 
of equipment none of which are of proven value, and leaving the 
whole situation in a state of flux; and (3) that these investigations 
have shown beyond question the constantly changing quality of sun- 
light, due both to variations in the sun itself and to absorption in the 
earth's atmosphere. Realizing these things, it must be admitted 
that the past investigations of light in connection with forestry and 
other biological subjects are, practically without exception, obsolete 
and of no assistance in looking into the problems of the future. 
It can not be attempted in this discussion to predict the line of 
endeavor for future investigators in light, Plainly it is a problem 
for specialists only. A few of the most fundamental facts or prin- 
ciples which, it seems, must govern the method of attack at this time, 
may, however, be pointed out. 
1. As to the character of sunlight, probably the most important 
point to be borne in mind is that it is an extremely variable quantity, 
both as regards its whole energy and its constitution of various wave 
lengths. Setting aside for the present the fact that the emanations 
from the sun vary periodically in total intensity and also in wave 
