110 APPLIED BIOLOGY 



But all cells do not store starch. Sugar, from which our 

 ordinary granulated sugar is obtained, is stored in large quan- 

 tities in the cells of sugar-beets and in sugar-cane. Some- 

 times the sugar which enters cells is changed to oil, as in 

 nuts ; and when the plant needs the oil elsewhere it is changed 

 (sometimes to sugar again) and osmoses out of the cell. 

 In still other cases the sugar from the leaves enters cells and 

 is used in combination with nitrogen and other elements from 

 the soil to form protein substances. 



Enzymes. All these remarkable changes which take 

 place when foods are stored in plant cells have long puzzled 

 chemists. No one yet understands fully how the plant cells 

 are able to make these changes; but it is known beyond 

 doubt that these changes do occur regularly in the life of 

 plants, and further it is known that many peculiar substances 

 called enzymes (e.g., diastase) are present in plant cells, and 

 that in some way not understood and not yet imitated in 

 the chemist's laboratory these enzymes can change sugar into 

 starch, oil, or albuminous foods, or these back into sugar. 

 A peculiarity of enzymes is that they can change other sub- 

 stances without undergoing change themselves, and that a 

 small quantity of enzyme can produce a large amount of 

 change. The most familiar example of an enzyme is pepsin, 

 which in the stomach of animals digests protein foods. 



Whatever the forms in which foods may be stored, it 

 appears that they are commonly in the form of sugar when 

 passing (by osmosis) into or out of any living cell. 



105. The Oxygen-Supply of Plants : Respiration. It 

 has been pointed out that plants breathe or respire and have 

 the same effect upon air which animals have; namely, 

 they absorb oxygen from the air and give off carbon dioxide 

 (062). The term respiration is in both plants and animals 

 usually applied to this combined process ; but it is simpler 

 to consider first the obtaining of oxygen. 



In a plant without chlorophyll (e.g., a mushroom), oxygen 



