VIII. RELATIONS OF PLANTS TO LIGHT 



168. Nature and Derivation of Light. - The term light may be 

 applied to all waves of radiant energy included in the spectrum 

 between the infra-red rays with a length of .760 /", and the supra 

 violet with a length of .397/*. The light of chief importance 

 to vegetation comes from the sun with a fairly constant steadi- 

 ness. The movements of the earth however, are such that the 

 intensity of the rays varies through a wide range. The earth is 

 nearer the sun in the summer of the southern hemisphere, and 

 hence plants of that region are exposed to a greater intensity 

 than, those of the northern hemisphere during the vegetative 

 season. The inclination of the axis of the earth changes the 

 angle at which the rays strike the surface, thus producing varia- 

 tions in light and temperature, constituting the principal factors in 

 the different seasons. Furthermore the daily rotation of the 

 earth produces a constant change in the angle at which the rays 

 strike the surface of the earth and the plants growing upon it, 

 with the result that the exposure to light varies from darkness to 

 the full intensity of the rays, and back to zero in the course of 

 24 hours, except, of course, in extremely high latitudes where 

 peculiar conditions prevail. l 



Local variations in the intensity of light are induced by topo- 

 graphical and meteorological conditions. Light from artificial 

 sources, such as that emitted from flames, phosphorescent sub- 

 stances, the electric arc and incandescent filament, exhibits diver- 

 gences from the sunlight in the relative intensity of the various 

 portions of the spectrum, a fact that must be taken into account 

 in all experimentation. 



1 Wiesner, J. Untersuchungen ueber den Lichtgenuss der Pflanzen im Arktischen 

 Gebiete. Sitzungsber. d. k. Akad. d. Wiss. Wien. 109 : May, 1900. 



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