RESEARCH METHODS IN 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 sup 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 is 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 tune 

 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 oi 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 



