6 



Light 



The chief natural sources of Hght are sunhght, moonhght, starhght, 

 and the hght from luminescent organisms. In this chapter we shall 

 deal primarily with visible light, but since the sun's emission also in- 

 cludes ultraviolet and infrared radiation these components of sunlight 

 will also be considered. Practically all the energy of importance for 

 organisms under natural conditions is derived directly or indirectly 

 from the sun. It is true that man has learned to obtain energy from 

 other sources such as tides, and more recently from nuclear fission, 

 but man like other organisms depends primarily on the sun for his 

 main supply of energy. 



The radiation from the sun produces a direct heating effect, and 

 also produces photochemical transformations. After these transfor- 

 mations have been completed, the energy appears in the form of heat. 

 Accordingly, all the sun's energy eventually ends up as heat, but cer- 

 tain portions of the radiation from the sun take part in vital photo- 

 chemical processes before becoming heat. 



Light as an ecological factor is generally highly directional. It 

 differs in this respect from the temperature factor since heat often 

 reaches the living organism from many different directions at the 

 same time. Light is extremely variable. It changes over a tre- 

 mendous range, often very rapidly. Many organisms can respond to a 

 value of light that is only one ten-billionth of full sunlight. Although 

 we often think of temperature as varying widely on the earth's sur- 

 face, the magnitude of its variation is extremely small compared to 

 that of light. Light can change from a value near zero to its maxi- 

 mum within a few hours. 



Light is essential for most plants and animals, though some can do 

 without it. For the continued existence of organisms two require- 

 ments must be met. First, light must not be so strong as to cause 

 serious harm at any stage in the life history. We shall see that a con- 



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