ROLE OF LIGHT IN NATURAL AND ARTIFICIAL REFORESTATION 237 



ers have overlooked the greatest fault of this method and have used 

 it almost exclusively. The importance of photo-thermal measurements 

 was mildly emphasized by Dr. Livingston ^^ in introducing his radio- 

 actinometer some five years ago, and has doubtless been recognized 

 by laboratory scientists for a much longer time. But I do not know 

 of a single instance in which the question of tolerance of forest trees 

 has been considered on the basis of the physical, rather than the chemi- 

 cal ettects of light, and I seriously doubt whether ecologists in other 

 lines have made much use of this conception. 



To bring out the importance, to foresters at least, of considering 

 the role of light on a physical basis, there are two points which I wish 

 to emphasize. First, without any argument as to the function which 

 transpiration may have beyond its incidental occurrence with photo- 

 synthesis and respiration, one should recall that, of the total radiant 

 energy incident upon the leaf, only about 5 per cent, according to 

 Clements. ^^ is used in photosynthesis, and that at the most, all of the 

 activities which seem to require chemical rays probably do not utilize 

 more than 10 per cent of the heat value of insolation. The remaining 

 90 per cent of the power of radiant energy is transformed into heat, 

 which raises the temperature of the plant to a point suitable for all 

 physiological activity, and beyond this is utilized almost exclusively 

 in changing water to water vapor. In either of its thermal effects, 

 radiant energy becomes inseparable from heat which may be obtained 

 by conduction from the air. 



The second point is this: 



The fallacy in the present method of measuring radiant energy by 

 photometric means, does not, as many writers have urged, consist 

 primarily in the existence of variable proportions of chemically active 

 and heating rays in different spectra, but consists in the fact that the 

 effectiveness of rays for heating purposes is proportionate to their 

 intensity only in a vacuum or in still air. It is demonstrable with the 

 simplest of physical apparatus (<?. g., a blackened thermometer bulb) 

 that the temperature of a body which absorbs, radiant energy uses in 

 still air to a point where radiant from the body is exactly equal to 

 absorption. The slighest disturbance of the air around the body upsets 

 this equilibrium and by increasing conduction, lowers the temperature. 



" Livingston, B. E. "A Radio-atmometer for Comparing Light Intensities. 

 Plant World, XIV, 4, 19 IL 



12 Clements, F. E." Plant Physiology and Ecology." New York, 1907. 



