ALIMENTATION AND DIGESTION 95 



work of cells as chemical factories, which we studied in 

 Chapter IV. The cells of the green parts of plants, espe- 

 cially of the leaves, take in carbon dioxide from the air and 

 water from the soil, and from these plant foods, with the 

 aid of sunlight, manufacture sugar, which is transported in 

 the sap from one part of the plant to the other and is used 

 as a source of power for plant work. The excess of sugar 

 is converted by certain cells into starch and is stored in the 

 form of small granules in the cytoplasm for future use. 

 A potato or a grain of wheat consists of cells loaded with 

 these starch granules. When the plant is not manufacturing 

 sugar directly from carbon dioxide and water, its cells again 

 transform the starch granules into sugar. The presence of 

 sugar in sugar beets, apples, pears, and peaches and in the sap 

 of sugar maples are familiar examples of this manufacture 

 and transport of sugar by plants. 



It will be noticed that only green plants have this power 

 of manufacturing carbohydrates from carbon dioxide and 

 water; hence we do not find large quantities of sugar and 

 starch in mushrooms and other fungi. The cells of green 

 plants, in short, are the starch factories of the world, the 

 factories from which we purchase our supplies of starch be- 

 ing only refineries, that is, places where starch is separated 

 from other constituents of plant cells. 



All plants, however, possess the power of manufacturing 

 proteins from carbohydrates and certain salts, which salts 

 they get from the soil. The carbohydrates furnish carbon, 

 hydrogen, and some of the oxygen, while the salts furnish 

 nitrogen, sulphur, phosphorus, etc. One great difference 

 between plants and animals is this power of protein manu- 

 facture by the cells from material which is not protein. The 

 animal cell can manufacture protein only from protein itself 

 or from certain decomposition products of protein. 



7. The group of fats. Fats are familiar to us in such 

 forms as butter, lard, olive oil, and the fat of meat. Like 



