42 MANUFACTURE OF FOODS AND FUELS 



point, and then falls rapidly if the temperature continues 

 to rise. Excessively high temperatures are recorded in 

 connection with experiments on desert plants; but ordi- 

 narily, out of doors, as well in the tropics as in temperate 

 regions, the temperatures to which plants are subjected 

 are, after all, comparatively moderate. The effect of 

 temperature on photosynthesis must be conceived as a 

 chemical process influenced not only by the action of 

 heat upon the raw materials but by the action of heat 

 upon the living cells in which " the most important 

 chemical reaction in nature " takes place. The tempera- 

 tures which are most favorable for other protoplasmic 

 activities are fortunately also those highly favorable for 

 photosynthesis. This is the reason for the luxuriant 

 growth of the tropics, rather than because there is any 

 considerably greater amount of light or any superior 

 quality of the light in tropical regions. 



(5) Vaiiations in carbon dioxid are so slight and so 

 infrequent that they are of no practical importance in 

 agriculture ; but the very great dilution of carbon dioxid 

 in the air, in spite of its enormous volume, fixes the rate 

 at which food can be made. This can be shown by labora- 

 tory experiment, for increasing the proportion of carbon 

 dioxid in the air in which plants are grown will increase 

 the yield of sugar, especially if the light is also increased; 

 but the idea of adding to the carbon dioxid content of 

 the air on a commercial scale, as has been suggested more 

 than once, seems of no practical importance at present. 



The Photochemical Reactions in Photosynthesis. 

 — The general nature of the process called photosynthesis 

 may be expressed in the following equation : 



6 CO. + 6 H,0 = C«H,,Oe +6 0, 



Carbon Dioxid Water Sugar Oxygen 



This equation expresses the fact that out of six molecules 

 of carbon dioxid and six molecules of water, one molecule 



