PROTOPLASM. 13 



oxygen, water, and carbon dioxide, in a larger measure than 

 that of animals, which must have complex organic foods. 



22. Growth and Reproduction. The result of assimila- 

 tion is the addition of new molecules of complex organic 

 matter among the molecules of the old. This produces growth. 

 It is to be defined as increase in mass. If this continues in- 

 definitely in excess of whatever may tend to destroy the pro- 

 toplasm, the increase in size may lead to the division of the 

 protoplasm. The parts may separate and lead an independent 

 existence. Such is reproduction. In its simplest form it is 

 merely growth beyond the limits of the individual. The cell 

 cannot continue to grow indefinitely. Its size is limited by the 

 necessity of physical support on the part of the soft protoplasm, 

 and by the relation between the outer surface, through which 

 the food must be taken, and the volume, which represents the 

 mass to be fed. The surface increases as the square of the 

 diameter, whereas the volume increases as the cube of the 



FIG. i. 



FIG. i. Streaming of Protoplasm in the Amoeba. The forward motion of the, 

 granules takes place more rapidly in the centre of the pseudopodium (/>). Those at 

 the margin fall behind those in the centre as the pseudopodium advances. 



Questions on the figure. Why may the amoeba readily change its 

 form? Do its internal parts preserve a constant relation to each other? 



diameter. It is apparent that the nourishing surface does 

 not increase as rapidly as the mass to be nourished, and in con- 

 sequence the time will come when the nourishment possible to 

 be absorbed will just nourish the volume, and growth must 

 cease. This condition may constitute an internal stimulus to 



