CHAPTER XIII 

 FOOD MANUFACTURE 



I. THE SYNTHESIS OF SUGAR— PHOTOSYNTHESIS 



Some of the significant facts that finally enabled botanists to arrive at 

 an understanding of the food of green plants, and a realization that this 

 food is actually made within the plant were briefly mentioned in the 

 preceding chapter. Much time and exact experimentation were required 

 to discover the simpler material and energy transformations that occur in 

 food manufacture, and the various conditions both inside and outside 

 the plant that affect these transformations. It is now common knowledge 

 among botanists that sugar is the first kind of food made by green plants, 

 . and that all other kinds of food are made by chemical alterations of this 

 sugar. Furthemiore, the facts discussed in the next few chapters will 

 help us see that the material make-up of all living organisms is de- 

 pendent upon chemical derivatives of the sugar made by plants; that the 

 potential energy in sugar is the primarv source of the chemically bound 

 energy supply of all organisms; and that most of the energy that man 

 transfonns by various means into heat, light, electricity, and mechanical 

 energy may be traced back through various transformations to the 

 potential energv of sugar. In this chapter we are concerned primarily 

 with the making of sugar in green plants. 



Carbon dioxide and oxygen in relation to green plants. Toward the 

 close of the 18th century oxygen and carbon dioxide were clearly recog- 

 nized and named bv the French chemist, Lavoisier. Following this ad- 

 vance in chemistry, a number of carefully conducted experiments by 

 different investigators finally led to the discovery of certain fundamental 

 relations between these two gases and living organisms. When the green 

 parts of plants were exposed to light, the amount of oxygen in the sur- 

 rounding air increased and the amount of carbon dioxide decreased. 

 During the hours of darkness the converse occurred. When the roots or 

 other non -green parts of a plant were substituted for the green tissues 

 in the experiments, the amount of oxygen in the surrounding air de- 



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