ZOOLOGY 



23. 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 



FIG. i. 



ANIMAL SUBSTANCES - 



REPRODUCTION 



OXIDATION 



SUN 



I. ENERGY 

 HEAT . 

 MOTION 

 THOUGHT 



WAS! 



\ \ 



NITRITES NITRATES 

 CARBON DIOXID 

 ^-O WATER 



FIG. i. A diagram indicating the general cycle of events in the building up of inorganic sub- 

 stances into living matter. Sunlight enables the chlorophyll of plants to build up water and carbon- 

 dioxide (photosynthesis) into carbohydrates. Plants may use these together with nitrogenous com- 

 pounds to form plant protoplasm and other proteids. Animals may by assimilation utilize these for 

 growth, reproduction, and in doing work of various kinds. Work results in waste. Plants may 

 again use these wastes. 



Questions on the figure. What are the three main courses open to plant sub- 

 stances? In what sense are animals dependent on plants? For what? How 

 important is photosynthesis in life? What is the real source of the energy of 

 organisms? In what sense is the term cycle appropriate here? 



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 



