108 STRUCTURE AND FUNCTIONS OF LEAVES 



If glucose (grape sugar) were the first product, the simplest 

 equation would be : 



6 CO 2 + 6 H 2 O = C 6 H 12 O 6 (one molecule of glucose) + 6 O 2 . 1 



It should be noticed that each of the processes above formu- 

 lated results in the disappearance of six molecules of carbon 

 dioxide and the production of six molecules of oxygen as a 

 waste product. 



These facts, namely, that in the green parts of plants exposed 

 to sunshine carbon dioxide is consumed and oxygen liberated, 

 form the foundation of our knowledge of photosynthesis. The 

 first step in the study of the subject was taken by Joseph 

 Priestley in 1771, by his discovering that air in which candles 

 had been burned until they went out could be restored to 

 something like its original condition by leaving in it for some 

 time vigorous leafy sprigs of mint. 



130. External conditions for photosynthesis. Photosynthesis 

 can only occur : 



1. When the plant is supplied with air containing carbon dioxide. 



2. When the temperature is neither too high nor too low. 



3. When the illumination is sufficient. 



Ordinary air contains about one twenty-fifth of one per cent 

 of its bulk of carbon dioxide. An increase of this amount up 

 to four per cent, or one hundred times the normal quantity, in- 

 creases photosynthesis, but a larger proportion usually at length 

 proves injurious to the health of the plant. 



Some arctic and alpine plants can perform the work of mak- 

 ing carbohydrates at temperatures as low as the freezing point 

 of water, but plants of warmer climates require a higher tem- 

 perature. The rate of photosynthesis usually increases with 

 rise of temperature up to about 77 F. (25 C.), after which 

 it decreases. 



Photosynthesis may go on very feebly, even in compara- 

 tive darkness, but the light of the interior of ordinary rooms is 



1 See Peirce, Plant Physiology, pp. 58-66. 



