CHAPTER 2 



THE MANY RESPONSE TYPES 



A STRIKING feature of the initial work of Garner and Allard was that 

 the principles which they discovered seemed to unify and simplify 

 our understanding of nature. The flowering plants could be classified 

 into one of three simple groups : day-neutral plants, short-day plants, 

 and long-day plants. The simplifying beauty of these discoveries 

 still remains, and the classification system which follows will be based 

 primarily upon that of Garner and Allard. Right now, however, we 

 are discovering that the details of flowering response are very diff'erent 

 among the species of flowering plants. Indeed, it now appears 

 possible that no two species will be found to operate exactly alike. 

 For that matter, it is a common thing to find diff'erences in flowering 

 response between varieties within a species or even individuals within 

 a variety. Response to the environment is such a complex thing that 

 only slight differences in genetic make-up seem to influence it. 



The three day-length responses of Garner and Allard have become 

 complicated. To begin with it is now known that plants do not 

 necessarily respond in an all-or-none way to day-length. Many 

 plants are known which will flower on any day-length but which 

 flower earlier on short-days. Other plants will flower on any day- 

 length but will flower earlier on long-days. Such a promotion in 

 flowering by a given day-length is referred to as a quantitative 

 response, and such plants may be termed quantitative short-day or 

 long-day plants. Qualitative short-day or long-day plants are those 

 which have an absolute requirement for a specific day-length, so 

 far as their flowering is concerned. We will often refer to the qualita- 

 tive type of response as absolute, a term phonetically unrelated to 

 quantitative and therefore not easily confused with it. 



In addition to this quantitative-absolute complication, plants have 

 been discovered which require neither long-days nor short-days but 

 some day-length intermediate between these two. Thus if the days 

 are either too short or too long, the plant remains vegetative, but 



