162 THE LOWEST ORGANISMS AND THE CELL 



formation outside of the protoplast and is not a living part 

 of the plant cell. Many lower plants form reproductive cells 

 (zoospores, gametes, etc.), which for some time are without a 

 cellulose wall,- and in this condition are motile and behave like 

 animal cells. However, the cell walls and the chromatophores 

 are responsible for the most conspicuous differences between 

 plants and animals, as is noted in Sec. 202. 



196. Photosynthesis. 1 Chromatophores and chloroplasts in 

 the presence of sunlight are able to manufacture from water 

 and the simple gas carbon dioxide certain complex organic 

 foods of which starch is generally the first visible product. 

 This process is called photosynthesis, which signifies a putting 

 together by light. The chemical formula for carbon dioxide is 

 C0 2 , for water H 2 O, and for starch C 6 H 10 5 . The chemistry of 

 the manufacture of starch cannot be truthfully shown by a 

 simple equation, for starch is not formed directly from carbon 

 dioxide and water, but by several steps through invisible sub- 

 stances that have not been isolated and therefore have never 

 been studied. The chemical processes in these steps are not 

 well understood. The final results may be roughly expressed 



as follows : 



6C0 2 + 5 H 2 = C 6 H 10 5 + 6 2 . 



This shows why free oxygen is formed during the processes of 

 photosynthesis. In some plants starch is never manufactured, 

 but instead sugars, which are substances closely related to 

 starch, some of them having the formula C 6 H 12 6 . The sugars 

 are in solution and invisible. Oil is formed in some plants, as 

 in the green felt ( Vaucheria), diatoms, etc., in place of starch as 

 the first visible product of photosynthesis. 



Many chromatophores have well-defined denser regions called 

 pyrenoids, which are the centers of starch formation, as is 

 well illustrated in the pond scum (Fig. 168, (7). Chloroplasts 



1 The subject of photosynthesis is treated in greater detail in connection 

 with the structure and functions of leaves (Chapter xn), especially in 

 Sees. 127-132. 



