16 • Photoperiodism: An Outline 



of day-neutrality from an initial qualitative or quantitative long- or 

 short-day response. 



A particularly clear example of the last sort of behavior is 

 shown by a variety of sunflower, Helianthus annum, recently 

 studied by Dyer et al. (1959). Seedlings raised under 12-hour 

 daylengths all showed inflorescences after 40 days, while seed- 

 lings raised under 16-hour daylengths showed no detectable flower 

 primordia at the time. Over 90 percent flowering occurred on 

 both 12- and 16-hour photo periods in experiments carried to 130 

 days, however, and even 20-hour photoperiods gave over 70 percent 

 flowering. In other words, young plants had a qualitative short-day 

 response with a critical daylength between 12 and 16 hours, but 

 older plants were either day-neutral or showed a weak quantitative 

 short-day response. 



While this brief list by no means exhausts the ways in which 

 photoperiodic responses may differ within the overall classification, 

 and examples will appear frequently in what follows, there do 

 appear to be limits on such variation. Although varieties of the 

 same species often differ in critical daylength and frequently show 

 a range from day-neutrality to a qualitative long- or short-day 

 requirement, the writer knows of no species with both LDP and 

 SDP varieties; it is even relatively unusual to find both types 

 within a single genus. The range of variation that can be caused 

 by age or environmental conditions is also apparently limited in 

 the same way as that within a species; that is, no experimental 

 treatment yet found will convert an LDP to an SDP, or vice versa. 

 Such an effect would obviously be very valuable for studies of the 

 mechanism involved. Aside from these generalizations, however, 

 the responses of species and varieties within a given class are 

 extremely various, and there is no evident correlation between 

 photoperiodic response classes and any taxonomic or ecological 

 category. Thus, although much of this discussion will proceed by 

 considering some of the results from a few well-studied plants, let 

 the reader beware: the country is large, and the map, so far, is 

 small. For many variations and modifications in photoperiodic 

 response that have not been studied systematically, see Chouard 

 (1957). 



