ECOLOGY AND THE FLOWERING PROCESS 



41 



20 25 



TEMPERATURE IN »C 



Figure 3-8 

 Flowering response to different day-lengths (24-hr cycles) of six varieties 

 of pigweed {Chetwpodium album), collected from various locations in 

 North America as follows: 



A 62° 46' N; Yukon D 50M0' N; Saskatchewan 



B 60° 52' N; Yukon E 49° 58' N; Manitoba 



C 60° 47' N; Yukon F 34° 20' N; California 



For a given variety, flowering occurred at a given temperature on the 



two day-lengths indicated for the variety and at all day-lengths between 



the two indicated day-lengths. All even numbered day-lengths were 



tested. Except for the two points marked with an asterisk, all plants died 



on day-lengths shorter than those indicated by the lower curves. Some 



sets of plants on day-lengths shorter than those marked with the asterisk 



remained alive but vegetative. There was considerable variation among 



treatments in the quantitative aspects of the flowering response. This 



could not be shown in the figure. Curves drawn from data supplied by 



Bruce G. Cumming. 



at 30°C variety A acts as a quantitative long-day plant (not shown in 

 the figure). In a sense all varieties but A are absolute short-day plants 

 at 20° and 25°C, but the picture is complicated because they also fail 

 to flower on short day-lengths and might thus be considered day- 

 intermediate. Most of them died on short day-lengths, however, but 

 at least two remained alive but vegetative and could thus be considered 

 true intermediates (especially variety F at 15°C). Work of this sort 

 illustrates the points often brought out in the last chapter, that any 

 present classification system can only be considered as highly 

 tentative until many more data have been accumulated. 



