132 PLANTS AND MAN 



the plant to another. It is used as a storage food. During photo- 

 synthetic activity, the excess glucose in a cell is stored temporarily 

 as starch grains in the chloroplasts; this removes surplus sugar 

 from the cell sap and eliminates possible saturation which would 

 interfere with further sugar manufacture. At night, the starch 

 of the chloroplasts is transformed back into glucose, through the 

 digestive activity of enzymes, and can then pass to regions of 

 permanent storage. In storage parts of plants, principally fleshy 

 roots, stems, and seeds, are special colorless plastids (leuco- 

 PLASTs) which deposit the starch grains from the glucose in 

 solution. 80% of the dry weight of a potato tuber is starch, which 



Fig. 83. — Most of a potato tuber is made up of cells filled with starch grains. 



can be seen microscopically as granules gorging the cells (fig. 83). 

 Most seeds average 60% starch. Starchy foods used by man 

 include for the most part underground stems, fleshy roots and 

 seeds. When eaten by animals or man, the starch has to be 

 changed back into glucose, by the process of digestion, before it 

 can serve as an energy food. 



Another common carbohydrate derived from glucose is 

 CELLULOSE (C6Hio05)n; this is not used by plants as a source of 

 energy, but as a structural material in its cell walls, out of which 

 to build its skeletal system. Wood is cellulose plus another com- 

 pound, lignin — found useful by man even though indigestible. 

 Practically pure cellulose is found in cotton fibers, which are 

 therefore the chief commercial source of the cellulose used in 

 industry. Cellulose is the most insoluble of all the carbohydrates; 



