chapter two 



Photoperiodism: 

 An Outline 



For obvious reasons, flowering has been studied largely in 

 plants in which it is controllable by environmental factors that 

 in turn are easily controlled by the plant physiologist. Chief 

 among such factors is the photoperiod, or daily length of illumina- 

 tion. Whether or not it eventually turns out to be as significant 

 for the flowering of most plants as it is for many that have been 

 studied, the following three general statements can be made with 



certainty. 



The phenomenon to be defined as photoperiodism is observed 

 not only among plants but in many animals as well, and is a wide- 

 spread mechanism in the seasonal regulation of biological processes, 

 particularly reproduction. Although it was first discovered through 

 its connection with flowering, photoperiodism controls other plant 

 processes also, even when it does not affect flowering. Finally, 

 part of the basic mechanism involved in plant photoperiodism 

 occurs in, and can modify the growth of, most higher plant cells 

 and tissues. 



DEFINITIONS OF PHOTOPERIODISM 



Photoperiodism has been variously defined as a response to 

 the daylength, photoperiod, or daily duration of illumination; as 

 a response to the relative lengths of day and night, or light and 

 darkness; or, in view of later information, as a response to the 



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