INTRODUCTION 7 



13. Protoplasm. — Living cells consist of a transparent, 

 jelly-like substance called protoplasm, which manifests 

 the various phenomena of life. Protoplasm 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 protoplasm contained in 

 plants during their rest period (170), in mature air-dry 

 seeds, and in the lower animals during their torpid condi- 

 tion is in the dormant state. Material is said to be " air- 

 dry " when it is as dry as it will become by exposure to 

 the air at ordinary temperatures. 



14. Reserve food. — Active protoplasm may absorb 

 nourishment in excess of immediate requirements and hold 

 it as reserve food. In plants, much of 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 (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 older ones — the 

 young cells thus formed attaining full size by subsequent 

 enlargement. 



16. Reproduction is the increase in number of living 

 beings. It is one of the properties of protoplasm and is 



