42 THE FLOWERING PROCESS 



2. Response Type and Plant Classification 



A legitimate question concerns the problem of photoperiodism 

 and relationships within the plant kingdom. Are the response types 

 each confined to certain taxonomic groups? Flowering response 

 type symbols of the plants listed in the appendix were placed, along 

 with the number representing the plant in question, on a chart of 

 taxonomic relationships as envisaged by Lyman Benson, a taxono- 

 mist at Pomona College in California. It was very readily apparent 

 that no simple relationship exists between plant classification and 

 flowering response type. Virtually all of the response types, for 

 example, may be found within one family, the Compositae. Further- 

 more, a given response type may be found in species occurring in 

 many of the famiUes of flowering plants, and some of these families 

 may be on opposite ends of the classification scheme. In a few cases, 

 a given response type seems to be rather predominant within a family, 

 as for example the dual day-length requirements of many grasses, 

 but in such cases it is not certain whether the relationship actually 

 exists or whether we have simply not sampled the family widely 

 enough to recognize the diversity of response types. 



3. Response Type and Evolution 



What does all this mean ? Is short-day response in widely separated 

 families, for example, a case of parallel evolution in which the same 

 end point is arrived at from many different beginnings ? The number 

 of "convergences" make this seem rather unlikely. Two possible 

 explanations for such an observation come to mind. First, it is quite 

 possible that our categories do not represent single physiological 

 mechanisms. Perhaps the processes which cause one species to be a 

 short-day plant diff'er somewhat from those which make another 

 species a short-day plant. In such a case the convergence would be 

 apparent but not real in the physiological sense. There are examples 

 which can be interpreted on this basis, as mentioned in the last 

 chapter. Second, there are good reasons to believe that there is a 

 fundamental mechanism common to virtually all higher plants, 

 involving the pigment system (Chapter 7) and perhaps the flowering 

 hormone itself (Chapter 9). This mechanism might be a highly 

 plastic one, rather easily changed by mutations, crosses, etc. The 

 response might become more speciaUzed (e.g. day-length dependent) 

 or it may become less so (day-neutral). Such modification by the 



