NUTRITION AND SEASONAL LIFE OF PLANTS 119 



the leaf with iodine, but it is finally transported back into the 

 other organs of the plant, either for immediate use or for storage. 



The process of photosynthesis is not fully understood, but it 

 is supposed to involve the union of carbon dioxide and water 

 to form carbonic acid (CH 2 O 3 ). Under the influence of sunlight 

 and chlorophyll the carbonic acid is reduced to form a com- 

 pound, possibly formaldehyde (CH 2 O), which is then multiplied 

 or condensed into sugar (C 6 H 12 O 6 ). A part of the original 

 molecule of carbonic acid is at the same time given off in the 

 form of free oxygen, which represents the excess of that gas not 

 needed for the building of sugar molecules. The reduction of 

 the carbonic acid is accomplished, in some manner which is not 

 fully understood, by the sunlight acting upon this substance 

 in the presence of the green pigment (chlorophyll) of the leaf 

 plastids. It is estimated that under ordinary circumstances this 

 decomposition would require the production of energy " equiv- 

 alent to 1300 of heat," and yet the green leaf, through the 

 agency of chlorophyll, is able to do this work without high 

 temperatures or elaborate machinery. 



The importance of photosynthesis to both plants and animals 

 can hardly be overestimated, since its first products, sugar and 

 starch, form the basic food for plant and animal nutrition. Not 

 only are these products the chief forms of reserve foods in the 

 special storage organs and cells of plants but they also function as 

 the most important material around which other kinds of organic 

 foods are constructed. Thus, the water stream from the roots 

 brings up soil salts and deposits them in the mesophyll cells of 

 the leaves. The nitrogen, sulphur, and phosphorus of these soil 

 salts is then combined with the sugar molecules formed by 

 photosynthesis into nitrogenous foods, such as the gluten of 

 wheat and other forms of protein food material. Without the 

 sugar furnished by photosynthesis this formation of available 

 protein food, upon which both plants and animals depend for 

 sustenance, would not be possible. We know also that the fats 

 and oils of such seeds as flax, hemp, and castor beans are derived 

 from sugar in some unknown manner, and that they are recon- 

 verted into sugar and starch during the germination of such 



