Introductory. 15 



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 considerable time 

 with very little of either, and is far less susceptible to 

 external influences than in its active state. The proto- 

 plasm contained in plants during their rest period (171), 

 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 reser^'e 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 proto- 

 plasm from its crude food materials (59). 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 

 accomx^anied 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 

 (Pig. 1), or by the formation of new cells \Nithin older 

 ones — the young cells thus formed attaining full size by 

 subsequent enlargement. 



* Material is said to be " air-dry " wtien it is as dry as it will become by 

 exposure to the air at ordinary temperatures. 



