16 THE FLOWERING PROCESS 



than a minimum. Thus knowing that a plant flowers when days are 

 "short" or "long" does not indicate its response type. Both cocklebur 

 and henbane flower on 14-hr days, but cocklebur is a short-day plant 

 while henbane is a long-day plant. Cocklebur flowers when the days 

 are shorter than about IS-Jhr, and henbane flowers when the days 

 are longer than about 1 1 hr. 



Outline of The Three Dimensions 



Since the temperature response precedes the day-length response 

 as a rule, there would be logical grounds for listing the temperature 

 response first in the following classification. The system would thus 

 be more natural. Yet I have arbitrarily decided to base the system on 

 the day-length response, primarily because this was the first classifi- 

 cation of response as established by Garner and Allard, and also 

 because the day-length response is emphasized in this book. Further- 

 more, the temperature response is a bit difficult to use in a classifica- 

 tion because one should really distinguish between delayed and direct 

 temperature efl'ects on flowering, and this could nearly double the 

 categories. The present system, then, like Chouard's, is far from 

 complete. It takes into account more kinds of response, but a number 

 of important responses, which are probably physiologically quite 

 different, are grouped together. A truly comprehensive classification 

 will have to await many more studies, probably using controlled 

 conditions. 



It has been suggested that multiplication of the response types can 

 only lead to confusion. This may be true, but any search for truth 

 seems to lead to such a division and multiplication of types. It is 

 merely a description of our present understanding of nature. The 

 impressive thing is that the few basic types of Garner and Allard can 

 still provide a framework for the many details which keep coming 

 to our attention. 



I. Photoperiodism: The Flowering Response to Day-Length 

 (duration of photoperiod or dark-period) 



• 1. Day-neutral plants. No flowering response to day-length. 

 2. Flowering response to a single day-length. 



A. Quantitative response to day-length (promotes, but not 

 essential). 



