CLIMATIC FACTORS : RADIANT ENERGY 129 



LIGHT 



That portion of the sun's radiant energy which forms the vis- 

 ible spectrum and which we commonly term "light" strikes the 

 earth in quantities far in excess of the apparent needs of plants. 

 Although green plants, with very few exceptions, are the only 

 organisms that can directly convert this energy to their own use, 

 they actually change to potential energy only about one percent 

 of the light energy they receive. It has been estimated that, of the 

 total solar energy falling upon a given field of corn during a grow- 

 ing season, only 0.13 percent can be "stored" as potential energy. 

 However, this also suggests that, to function normally, plants 

 require much more light energy than they actually use. Not all 

 wave lengths are equally usable. Green light is reflected or trans- 

 mitted, while the longer wave lengths, in the red end of the spec- 

 trum, are much more effective in photosynthesis than are the 

 shorter lengths of yellow and blue. Not all species are equally 

 efficient under equal illumination. Some require certain intensities 

 and some need certain lengths of day or season to function nor- 

 mally. To add to the difficulties of interpreting plant-light relation- 

 ships, it is not always possible to distinguish between light effects 

 and those of total insolation, which include heat and its influence 

 on physiological processes. 



Light Measurements.— Ecological studies of light should not be 

 casually undertaken in spite of the apparent simplicity of making 

 measurements with modern instruments. As suggested above, plant 

 responses and light values rarely bear a simple and direct relation- 

 ship to each other. Whether or not these relationships can be in- 

 terpreted may depend upon proper planning before making meas- 

 urements. In addition, there are problems related to obtaining 

 measurements for ecological purposes that must be considered. 



Chemical, illuminating, electrical, and heating effects of light 

 are measurable, and for each a different type of instrument is 

 used. 233 Field ecologists have largely abandoned the first two ap- 

 proaches in favor of electrical measurements because of the recent 

 perfection of compact, sturdy photoelectric apparatus with which 

 accurate and rapid determinations can be made. These instruments 

 are sensitive to approximately the same portion of the spectrum as 

 is the human eye. Since they are selective instruments, there may 



