CHAPTER XIV 



FOOD MANUFACTURE 



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II. FACTORS INFLUENCING THE RATE OF PHOTOSYNTHESIS 



The basic facts about photosynthesis discussed in the preceding chapter 

 do not exhaust the subject; neither do they satisfy anyone who wants to 

 understand the fundamental processes of his natural environment. The 

 answers to many questions about plants involve facts concerning the 

 chain of chemical reactions that occur in photosynthesis, and the influ- 

 ence of the various factors of the environment. The value of certain prac- 

 tices in the handling of plants and the interpretation of plant phenomena 

 in nature depend also upon a knowledge of these relations. Many ques- 

 tions concerning these relations cannot be answered todav, but addi- 

 tional facts that are helpful in answering some of them will now be 

 discussed. 



Light the energy of photosynthesis. Under natural conditions the en- 

 ergy of photosynthesis is sunlight. Heat cannot be substituted for light 

 in this process. No sugar is made from CO- and H-O bv green plants 

 deprived of light, regardless of the temperature to which they are 

 exposed. Radiant energy that we perceive as light is referred to as the 



VISIBLE SPECTRUM 



ULTRA-VIOLET 



3>0 430 470 600 



560 eoo 



Fig. 53. The spectrum of radiant energy. One millimicron (m^) = one-millionth 



of a millimeter. 



visible spectrum (Fig. 53). This is the radiant energy that is effective in 

 photosynthesis. It can initiate chemical change by displacing or activat- 

 ing the outermost electrons of atoms. The effects of radiant energy of 

 longer wave lengths, such as infra-red, are apparently limited to the 

 movement of atoms and molecules. Photosynthesis is not known to occur 

 when the plant is exposed only to infra-red radiation. 



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