28 • Photoperiodism: An Outline 



Fig. 2-2. Photoperiodic control of flowering in cocklebur {Xanthium pennsyl- 

 vanicum) as modified by low temperature. Growing points of plants of the same 

 age — with all except terminal leaves removed to show development — photo- 

 graphed after 13 days of the following treatments: (A) 8-hour days at 23° C 

 (flowering); (B) 16-hour days at 23° C (vegetative); (C) 16-hour days as in (B) 

 but with 4° during first 8 hours of each light period; (D) 24-hour (continuous) 

 days at 23° except 4° during 8 hours of each day. (Photographs from Nitsch and 

 Went [1959], by permission of the American Association for the Advancement 

 of Science and courtesy of Dr. J. P. Nitsch, Le Phytotron, Gif-sur-Yvette, France.) 



starting with Garner and Allard. Some of the characteristics 

 frequently under photoperiodic control even when flowering is not 

 are stem elongation, leaf shape and size, branching, pigmentation, 

 tuberization, and pubescence (see, for example, Nay lor, 1953). 

 Effects on these have been studied far less than the flowering 

 responses, but the data at hand suggest that they are less likely 

 to be inductive. That is, when the photoperiodic conditions are 

 changed, the new vegetative growth quickly reflects the new con- 

 ditions. This may even be true when the vegetative change would 

 normally be associated with a truly inductive effect on flowering. 

 In Murneek's work on Rudbeckia bicolor, for example, continuous 

 treatment with long days (longer than 12 hours) caused both 

 flowering and stem elongation. Exposure to only 25 long days still 

 brought about flowering, both normal and abnormal, but the plants 

 remained in a semirosette stage. 



Many papers on responses of all types make it difficult to decide 

 whether they are truly photoperiodic or not. Paradoxically, this 



