94 Introductory Biology 



the gradual developmental changes (evolution) constantly undergone by 

 living organisms (evo-lu' shun) (L. evolvere, to unroll or change). 



There are certain nonliving, nonprotoplasmic chemical substances such 

 as giant molecules of proteins which also possess autosynthesis. However, 

 these molecules of protein are much simpler than those in living proto- 

 plasm. There seem to be autosynthetic phenomena in both living and 

 nonliving substances, but in each case the product formed is specific and 

 unique for that substance. 



The ability of a catalyst to synthesize more of its own substance (create 

 more molecules of its own kind ) , thus gradually increasing the speed of 

 the chemical action, may be considered as autocatalysis (aw to ka -tal'i 

 sis) (Gr. autos, self; kata, down; lysis, loosing). Sometimes this term 

 seems to be used as a synonym of autosynthesis. The disintegration of 

 cells or tissues by the action of autogenous enzymes (aw -toj' e nus) (Gr. 

 autos, self; genesis, origin) which they produce may be considered as 

 autolysis. Life seems to be the result of the interactions and counterac- 

 tions of these and many other factors, some of which are not yet known. 



IV. GROWTH, ASSIMILATION, AND DIFFERENTIATION 



True growth is characteristic of all living organisms and consists of 

 anabolism, in which the protoplasmic substances synthesize more of their 

 kind from materials which are unlike the protoplasm, thus increasing 

 their bulk. True growth in living organisms may be the result of the 

 increase in the size of cells, an increase in the number of cells, or a com- 

 bination of these two. All living organisms have limits beyond which 

 they cannot, or do not, grow. Certain organisms do not seem to grow 

 much after the adult stage, while others continue growth for a longer 

 period of time. The particular size of each organism is probably due to 

 its inherent hereditary materials and the influence of environmental fac- 

 tors both outside and inside the organism. The process of building 

 smaller particles of chemicals into larger particles of protoplasm which 

 differ from the original particles (food) is known as assimilation 

 (a sim i -la' shun) (L. ad, to; similis, alike) . 



In the nonliving world there are phenomena which approximate 

 growth somewhat. A crystal of table salt (NaCl) placed in a super- 

 saturated solution of table salt (maximum amount of salt which will 

 normally dissolve in hot water and which will release some of the salt 

 when the solution cools) will add to its surface some of the dissolved 

 salt, thereby forming a larger and larger crystal which maintains a fairly 



