THE EVOLUTION OF THE PLANT BODY 



25 



acterized by a constriction in the middle; in the cytoplasm are 

 two large green chloroplasts, important parts of the desmid 

 protoplasm. Frequently vacuoles are present. As the desmid 

 plant floats about in the sunlit water, it is surrounded by an 

 environment which furnishes all of the essentials for its existence. 

 One of the raw materials for photosynthesis is always present, 

 since water permeates the entire cell due to its ability to pass 

 through the cell wall. Carried with the water into the protoplasm 



Fig. 8. — Desmids are minute plants living among leaves and stems of submerged 

 vegetation or floating in surface waters of ponds and lakes. 



is dissolved air, containing the carbon dioxide also needed for 

 photosynthesis-^as well as the oxygen required for respiration. 

 Thus all the inorganic materials needed for food manufacture are 

 continuously available. The chloroplasts, intercepting the sunlight 

 which penetrates into the water, effectively aid cell protoplasm in 

 the decomposition of carbon dioxide and water, and their recom- 

 bination in the form of glucose. Some of this sugar passes in solu- 

 tion to various parts of the cell where it may be used as fuel during 

 respiration or may be transformed into starch, cellulose or fat. 

 Dissolved nitrates and other necessary salts pass into the cell in 

 the same fashion as the carbon dioxide. With the addition of the 



