Introductory. 15 



an independent structure. As a rule, each cell is sur- 

 rounded by its own closed cell-wall. 



13. Protoplasm (pro'-to-plasm). Living cells consist 

 of a transparent, jelly-like substance called protoplasm, 

 which manifests the various phenomena of life. Proto- 

 plasm may exist either in an active or dormant state. 

 In the active state it requires both nourishment and 

 oxygen ; in the dormant state it may exist for a consid- 

 erable time with very little of either, and is far less 

 susceptible to external influences than in its active state. 

 The protoplasm contained in plants during their rest 

 period (170), in mature air-dry* seeds, and in the 

 lower animals during their torpid condition, is in the 

 dormant state. 



14. Reserve Food. Active protoplasm may absorb 

 nourishment in excess of immediate requirements and 

 hold it as reserve food. In plants, this reserve food is 

 in the form of starch, sugar or oil ; in animals it is in 

 the form of fat. These substances are formed by the 

 protoplasm from its crude food materials (58). The. 

 reserve food enables the plant or animal to live through 

 limited periods of scarcity, and to meet the demands 

 necessitated by reproduction (16). 



15. Growth is the normal, permanent change in the 

 form of a living vegetable or animal body, and is 

 usually accompanied by increase in size. It may occur 

 either through expansion of cells already formed, or 

 through cell multiplication. The latter may take place 

 either by division of older cells into two or more smaller 

 cells (Fig. 1), or by the formation of new cells within 



* Material is said to be "air-dry" when it is as dry as it will 

 become by exposure to the air at ordinary temperatures. 



