ECOLOGY AND THE FLOWERING PROCESS 43 



forces of evolution might account for the many response types and 

 differences within response type. Obviously the modification becomes 

 quite extreme in some cases. 



At present, we are not even sure whether day-length or temperature 

 dependence might be considered primitive or advanced taxonomi- 

 cally. Surely with the inconsistencies mentioned above, it could 

 never have any very significant meaning in this respect. It has been 

 suggested that the day-neutral non-temperature sensitive condition 

 is most primitive and might be found in some of the ancestral 

 stocks, such as the buttercups, and that day-length and temperature 

 sensitive species occur in the higher plant families, but in my attempts 

 to correlate taxonomic relationship with flowering response type, I 

 was unable to find any evidence for this viewpoint. 



Natural Selection and The Flowering Process 



Natural selection is a powerful factor in populations of plants and 

 animals. Thus any physiological or morphological feature of an 

 organism might conceivably have survival value. On this basis we 

 often ask ourselves, *'What is the advantage of a given feature?" 

 What, for example, is the advantage of being dependent upon some 

 change in the environment for the initiation of flowers ? How would 

 such a response tend to insure survival ? 



It is rather easy to think of a distinct disadvantage. If a plant 

 requires a certain day-length to initiate flowers, and only slight 

 deviations from this day-length will either restrict the plant from 

 flowering or cause it to flower at a season when it might be destroyed 

 by frost, then the plant would seem to be restricted to a rather narrow 

 distributional range. If it moves farther north or south, conditions 

 may be unsuitable for growth and completion of the life cycle. 

 Photoperiodism, then, might be a distinct disadvantage in that it 

 restricts a species to a certain latitudinal range. How can it migrate 

 and fill niches in the biotic community ? What happens if a climate 

 changes, but day-length does not ? Throughout all the plant kingdom 

 we encounter a great variety of highly ingenious dispersion 

 mechanisms, apparently designed to allow a species to move freely 

 on the surface of our planet. Yet a day-length requirement might 

 defeat this end. 



