ECOLOGY AND THE FLOWERING PROCESS 39 



In Chapter 8 we will discuss the problems connected with time 

 measurement, but as a preview it can be stated that it is very difficult 

 to imagine any mechanism which will account for an accurate time 

 measurement, yet be relatively insensitive to changes in temperature. 

 No one has yet proposed a scheme involving known physical processes 

 of plants such as diffusion or osmosis, which will adequately account 

 for time measurement. If it is a chemical process it must be tempera- 

 ture compensated in some very interesting way. Changes in tempera- 

 ture of 10°C are not at all uncommon in nature, yet such a change 

 will typically change the rate of a chemical reaction by a factor of 2. 

 Since temperature often varies this much at a given season from year 

 to year, this would obviously completely destroy any sort of season 

 detection by measurement of day. We can, then, look for some sort 

 of temperature-insensitive, time-measuring process in the plants 

 which are sensitive to length of day or night. 



The Many Response Types in Nature 



1 . Response Type and Plant Distribution 



In the last chapter, many different plant response types were 

 outlined, based upon environmental changes in light and tempera- 

 ture. How would we expect these to be related to plants in natural 

 habitats ? Obviously as one goes farther north the days in the middle 

 of the summer are longer, and one might expect to find more long-day 

 plants. Indeed plants which require long-days for flowering are more 

 common in the north. At the equator, length of day is not changed 

 throughout the year, and day-neutral plants should be common. 

 Short-day plants should be those which bloom in the spring or late 

 summer or fall, most likely in temperate cUmates where early spring 

 and fall are not too cold. Such is the case. Garner and Allard made 

 such observations in the years just following their initial discovery. 



As pointed out earlier, however, the actual day-length is not the 

 important criterion in determination of day-length response type. 

 The criterion is rather, whether the plant flowers in response to 

 increasing or to decreasing day (or night) lengths. Thus short-day 

 plants, for example, can and do flower at suitable seasons in either 

 the tropics or the far north. 



