186 Light 



siderable variation exists in the upper limits of tolerance to the light 

 factor. Second, for those animals and plants that require light it must 

 be sufficient in intensity and in duration. The intensity of the light 

 must be above the threshold for the organism concerned, and the total 

 amount of light received during the period when it is needed must 

 be adequate. 



DISTRIBUTION OF LIGHT 



In discussing the distribution of light it is convenient to start with 

 sunlight as the chief source and trace the radiation as it passes through 

 the air and water environments. The magnitude of the solar radia- 

 tion as it reaches the outer atmosphere referred to as the solar con- 

 stant has the value of about 1.9 g-cal/cm^/min. At sea level the in- 

 tensity of solar radiation averages about 1.5 g-cal/cm'/min. If the 

 radiation received from the sun were evenly distributed over the sur- 

 face of the globe, it would be sufficient to melt a layer of ice 35 m 

 thick during the course of a year. At a latitude of 44° N the energy 

 received on the earth's surface from the sun is equivalent to the light 

 that would be produced by hanging a 250-watt lamp over each square 

 meter of the ground. 



We know very well, however, that the radiation from the sun is not 

 evenly distributed either in time or in space. We wish to inquire 

 what its variations both in quality and in quantity are. Light changes 

 in spectral distribution and in angular distribution. The light factor 

 also varies in intensity and in duration, resulting in differences in the 

 total amount of light falling on each unit of surface for each unit of 

 time, such as a day or a month. The changes in the light factor in 

 these respects will be examined as they affect the terrestrial and the 

 aquatic environments. 



Light on Land 



Spectral Composition. The spectral distribution of light as it 

 reaches the earth's surface is shown in Fig. 5.3. Authorities differ as 

 to the exact wavelength limits to be assigned to the different portions 

 of the spectrum. Roughly speaking, radiation of wavelength longer 

 than 7600 Angstrom units is considered to be infrared, and that of 

 wavelength shorter than about 3600 A is designated as ultraviolet. 

 Almost one-half of the total emission of the sun is infrared radiation, 

 and almost one-half is visible light. These proportions remain ap- 

 proximately the same at the earth's surface, regardless of the total 

 intensity of sunlight. The ultraviolet component, however, is always 



