ECOLOGY AND THE FLOWERING PROCESS 35 



to the longer nights preceding the summer solstice because tempera- 

 tures are too low. When it does respond, about the time of the 

 longest day of the year, it is induced only minimally, and thus only 

 the lateral buds produce flowers, while the terminal one continues 

 to grow vegetatively for quite some time. If the terminal bud forms 

 a flower (as finally happens when the nights become somewhat longer) 

 then growth ceases At least it should be quite clear that short-day 

 plants do not always need short days in the usual sense of the word 

 if they are to become reproductive. 



5. Natural Photoperiods and Flowering 



The time at which the sun will cross the horizon at sunrise or at 

 sunset can be predicted for any location with the high precision of 

 astronomy. Figure 3-5 shows the relationship between day-length 

 and time of year at four diff'erent latitudes. This is, so to speak, a 

 relationship that the plant can count on. Surely there would be no 

 better way in nature to ascertain the time of the year than to measure 

 the length of day at a given latitude. Of course, as indicated above, 

 complications arise from the duration of twilight and dusk, especially 

 as these are influenced by clouds, mountains, etc., but because of 

 the low intensities required for response, the complications are 

 probably rather minor. As shown in Fig. 3-4, clouds do not influence 

 twilight intensities much when it is nearly dark. The same is true of 

 mountains or other obstructions. 



If time of year is ascertained by measurement of day-length, the 

 precision with which length of day is measured will determine the 

 accuracy of measurement of season. This accuracy is strongly 

 influenced by the time of year, because the diff"erence in length of 

 day between any two successive days is not constant throughout the 

 year, but varies according to the curve shown in Fig. 3-6, which may 

 be derived directly from Fig. 3-5. Figure 3-6 shows that the rate of 

 change in day-length is least^ near the summer and winter solstices 

 when the days are approaching their longest and their shortest 

 durations. For more than half the year, however, from February to 

 the middle of May and from August to the first part of November, 

 the rate of change in day-length is very high and very constant (about 

 2.6 min per day at 40° latitude). Thus a plant must measure time 



3 Near the solstices the rate of change (the acceleration and deceleration) is 

 very great, but we have no evidence that plants might detect this. 



