136 ENVIRONMENTAL CONDITIONS. 



through its physiological effect upon our eyes, the range of any given 

 light mixture is usually thought of in terms of the spectral colors, which 

 are simply names of certain physiological responses of the human 

 organism to light of various wave-lengths and intensities. Thus, we 

 may state that a certain Ught mixture ranges from red to green, for 

 example, is particularly intense in the green, etc. It is highly desirable, 

 however, that biological measurements of light quaUty be made in 

 terms of wave-lengths, for such definition does not depend upon the 

 eye.^ 



The only practical way to describe Hght conditions that is so far 

 available is arbitrarily to divide the spectral range of wave-lengths 

 into smaller ranges, and to state the intensities of these smaller ranges 

 in terms of their respective equivalent heat intensities. Certain of 

 the well-known Fraunhofer lines of the sun's spectrum may con- 

 veniently be used in this arbitrary subdivision. 



2. EFFECT OF LIGHT UPON PLANTS. 



We have already seen that radiant heat is effective upon plants in 

 controlling their temperature. Light also has the same effect, in so 

 far as it is absorbed by the plant-body and converted into molecular 

 vibrations of thermal nature. Where this effect is alone to be con- 

 sidered it is not directly necessary to analyze the impinging waves into 

 their spectral groups or component ranges of wave-lengths; it is only 

 requisite to determine the total heating effect produced by the ab- 

 sorbed portion of the total impinging radiation. Since, however, any 

 given surface, as of a plant, absorbs the different wave-lengths in 

 different amounts, the study of hght quahties may become essential 

 even in this connection. The heating effect of hght upon plants is 

 only incidental in our present discussion; absorbed radiation is seldom 

 if ever of primary importance in determining the temperature of plant 

 parts. 



A second kind of effect produced by Hght upon plants is a morpho- 

 genic one and does not seem to depend upon the heating of the tissues. 

 This is, in all probability, a photochemical effect, but the very diffi- 

 cult question thus raised still awaits investigation. Here we need to 

 consider, perhaps, the retardation of growth in length, apparently 

 due to the action of light upon most cylindrical plant-parts and the 

 corresponding acceleration of enlargement in most dorsiventral organs. 

 It is usually supposed that ordinarily plants would not assimae their 

 usual form without this action of light; the characteristic, much 

 elongated stems and greatly dwarfed leaves of etiolated plants are an 

 example of the effect of lack of light, though the moisture relation 



1 Watson and Yerkes's valuable monograph on light measurement for biological purposes 

 should be referred to in this connection: Watson, J. B., and R. M. Yerkes, Methods of studying 

 vision in animals, Behavior Monographs, Serial No. 2, 1910. See also: Pulling, H. E., Sunlight 

 and its measurement, Plant World 22: 151-171, 187-209, 1919. 



