GROWTH 9 



plant them, and use growth as a test of 

 Hfe. 



If we are to employ growth in this man- 

 ner it is important to have a clear under- 

 standing of what it means. Growth is 

 often thought of as comprising the whole 

 development of the organism. Ordina- 

 rily the life cycle of an animal or plant 

 begins with a single cell, which by 

 repeated division produces a mass of 

 cells. The form of the organism then 

 changes, and its parts become differen- 

 tiated so as to perform different func- 

 tions. 



The question now arises, What is 

 essential to the conception of growth? 

 A simple illustration will make it clear 

 that growth may go on without cell divi- 

 sion, change of form or color, differentia- 

 tion, or assimilation of food. A small, 

 spherical, green cell, desiccated by the 

 drying up of a pool in which it has lived, 

 and blown about by the wind, eventually 

 falls into water. Such a cell often re- 

 mains alive and when it again finds it- 



