44 THE FLOWERING PROCESS 



Certainly the picture is not so bleak. The plasticity of response 

 type mentioned in the above paragraph must be one answer. For 

 example, the plants studied by Mooney were all of the same taxono- 

 mic species, and yet they had become physiologically adapted to their 

 given location. If we assume that they gradually migrated to their 

 present location, they must have become physiologically adapted (by 

 natural selection ?) along the way. We can see in the Appendix, for 

 example, that a given species may have many different response types, 

 as for example with the chrysanthemum. Furthermore, a particular 

 day-length requirement may not stop the plant from migrating, even 

 though the requirement itself is not changed. Short-day plants which 

 migrate southward will still encounter short-days, but at a slightly 

 different time of year (somewhat earlier in late summer, for example). 

 The new time of flowering may fall at a time of the year which is not 

 particularly bad for the plant, and thus it will survive. Actually it is 

 quite common to find a given species flowering at one time of year 

 at one location and at a diff'erent time of year at another location. 



The advantages of response to environment are also quite obvious. 

 As mentioned above, a plant which requires an extended low tempera- 

 ture treatment for flowering will not burst into bloom during a warm 

 spell in the late fall. Certainly a rather restrictive day-length require- 

 ment would allow a plant to fill a particular niche in time. Anyone 

 who studies a given biotic community throughout a season will 

 observe that there is a rather orderly sequence of flowering in the 

 different species which make up the plant community. Certain 

 species are early flowerers, others flower in mid-season, and some 

 flower late in the year. Thus each one seems to fill its niche in 

 seasonal time. If this does nothing else, it should at least serve to 

 keep the honey bees on a rather sensible annual work schedule. 



Perhaps the most distinct advantage of a rather critical day-length 

 requirement is that such a requirement insures that all members of 

 the population will be in bloom at about the same time. This is quite 

 essential for efficient cross pollination among members of the popu- 

 lation. Such intermingling of the germ plasm within a population 

 seems to be distinctly advantageous in nature, although no attempt 

 is made here to discuss the reasons for this. 



It is easy to see how accurate day-length requirements might be 

 developed within a natural population, since there is such a selection 

 pressure against flowering out of turn. A plant which requires 



