METHODS OF EXPERIMENTATION WITH COCRLEBUR 75 



2-0 

 1-0 



Before 



27 °C Day temperature 

 (16 hours) 



18 °C Day temperature' 

 (16 hours) 



/ 



,^'^efore 



After 



Before and after 



• \ \ r 



5 10 15 20 25 30 



5 10 15 20 25 



NIGHT TEMPERATURE IN °C 



Figure 5-3 

 Effects upon flowering of cocklebur of various day and night tempera- 

 tures applied before, after, or before and after a 16-hr inductive dark 

 period at 23 'C. The system of Floral Stages which indicates relative 

 flowering response is explained near the end of this chapter (see Fig. 5-8 

 and Table 5^). Experiments were carried out during the spring of 1962 

 in chambers such as that shown in Fig. 5-2. 



range of temperatures. Yet in recent growth chamber studies, we 

 have found that flowering is strongly modified in a quantitative way 

 by temperature treatment. For years we have experienced difficulties, 

 especially in winter, in exactly duplicating the results of two or more 

 "identical" experiments. The results of our temperature studies, 

 summarized in Fig. 5-3, give one possible explanation. Low night 

 temperatures and low day temperatures are bad for flowering of 

 cocklebur, whether they are given before the long dark period or 

 following it. Even though the temperature is maintained fairly high 

 in the greenhouse in the winter, plants may be radiating to the cold 

 glass above, so that the temperature of the leaf is lower than air 

 temperature alone might indicate. We have now obtained much 

 better results by maintaining temperatures quite high, day and night, 

 summer and winter, at high or low light intensities. This optimal 

 esponse to high temperatures agrees well with the idea that our 



