PHOTOSYNTHESIS 107 



127. The work of leaves. A leaf has four principal functions: 



1. Photosynthesis. 3. Assimilation. 



2. Respiration. 4. Transpiration. 



128. Photosynthesis. All green leaves, when in healthy con- 

 dition, at suitable temperatures and with sufficient illumination 

 can produce carbohydrates (starch or sugars} from carbon dioxide 

 and water. 



This process is of the greatest importance, since directly or 

 indirectly all plants and animals depend upon it for their food 

 supply. The manufacture by the plant of carbohydrates from 

 the raw materials is known as photosynthesis (from two words 

 meaning light and putting together). It is often called fixation 

 of carbon or assimilation of carbon. Photosynthesis is per- 

 formed by the chloroplasts, especially in the palisade cells, and 

 goes on imperfectly or not at all in plants or parts of plants, 

 as in certain parasites and other forms, in which no chlorophyll 

 exists (Chapters xxn, xxx). 



129. Chemical formula for photosynthesis. The details of 

 the photosynthetic process are not wholly known, and it is not 

 at all likely that in starch-producing plants starch is the first 

 substance formed from carbon dioxide and water, but it is one 

 of the early products of the action of the chloroplasts and is 

 the easiest to detect by chemical tests applied to the leaf. In 

 some plants, as the onion, the products of photosynthesis are 

 all stored in the form of sugar. 



If the chloroplast produced starch as the direct result of 

 combining carbon dioxide and water, the chemical equation for 

 the process would in its simplest form be J : 



Six molecules 1 f Five mole- ] f One mole- 1 f gix moleculeg 

 of carbon f + -\ cules of i- = -i cule of ^ + -I OXV o- en 

 dioxide water starch ^ 



6CO 2 + 5H 2 O = C 6 H 10 O 5 + 6O 2 



1 Really some multiple of C 6 H 10 S probably more nearly expresses the 

 composition of starch than the simple formula given. It is certain that the 

 photosynthetic process is much more complicated than a mere combination 

 of carbon dioxide with water to form either starch or sugar. 



