44 MANUFACTURE OF FOODS AND FUELS 



may be added, forming still more complex carbon com- 

 pounds, as we shall presently see. Although the eminent 

 plant physiologist Sachs long ago spoke of starch as the 

 " first visible product of assimilation," we know now that 

 his famous dictum was only approximately correct. 

 Sugar is the fundamental product of photosynthesis, and 

 because of this fact photosynthesis is *' the most import- 

 ant chemical reaction in nature," for all food is built up 

 from sugar. 



Fuels. — The fuels no less than the foods are also of 

 vegetable origin and are built up from the sugar made 

 in photosynthesis. Wood, for example, is obviously so. 

 Coal is well known to be the changed remains of wood 

 and other parts of plants. It is supposed that oil and 

 natural gas have also been derived from the cell-deposits 

 of accumulated plant-remains, in some cases from the 

 oil produced by diatoms. Alcohol is obtained (see Chap. 

 XXIII) by the fermentation of sugar, or of sugar deriva- 

 tives reconverted into sugar. The study, therefore, of 

 photosynthesis is of the utmost importance from the 

 economic point of view, since man's comfort as well as 

 his nourishment are absolutely dependent upon this re- 

 action. Everything that man uses which is not mineral 

 comes directly or indirectly from the combhiation of car- 

 bon dioxid and water into sugar in the green parts of 

 plants in warmth and light. With the exception of wind 

 and water in motion, or the power obtained therefrom, 

 man uses these same fuels as the sources of all the energy 

 which he requires. Indeed, under certain market condi- 

 tions it is more profitable for the farmer to burn corn 

 than coal. 



