476 Plant Physiology 



the utilization of large numbers, in order that there may be 

 more chance for variation ; and in the detection and iso- 

 lation of the unusual individual or type. 



HEREDITY 



In the production of plants there are two primary re- 

 quirements, there must be (1) the seed or propagative 

 parts, and (2) certain favorable conditions for growth 

 and reproduction. The one is a biological mechanism 

 which has behind it ages of ancestors determining specifi- 

 cally or racially what type of plant there shall be ; the other 

 is a complex of physical and chemical factors conditioning 

 what kind of individual there shall be. The embryo 

 plant possesses its particular hereditary possibilities, and 

 it is encompassed by an environment which sustains it or 

 subjects it. Heredity and environment are therefore 

 forces closely linked together in biological investigation. 

 Environment is important in molding heredity, and 

 heredity constantly affects the method of response to 

 environment. All biologists agree that either structural 

 or functional adjustments to environment may ultimately 

 become hereditary ; but a chief tenet of Weismannism is 

 that no change is hereditary which does not affect the 

 germ cells. 



There is at present great activity in the study of hered- 

 ity, a manner of cell behavior which we may now tenta- 

 tively define as being concerned with the transmission 

 through successive generations of racial and individual 

 characters. A fundamental study of transmission is 

 properly termed Genetic Physiology, or simply Genetics. 



