4 



Photoperiodism and Temperature ■ 25 



In several SDP, on the other hand, noninductive cycles have 

 a clearly inhibiting action on induction. Schwabe (1959) has shown 

 that for Perilla ocymoides, Chenopodium amaranticolor, and Biloxi 

 soybean, noninductive cycles intercalated between inductive cycles 

 positively inhibit the effects of the latter. Each long-day cycle, in 

 fact, appears capable of counteracting the effect of two short-day 

 cycles. A long-day cycle probably acts by annulling the effectiveness 

 of the short days immediately following it, rather than by destroy- 

 ing the effect of the short days preceding it. Such a conclusion agrees 

 with the results of Harder and Biinsow (1954) who had found that 

 the number of flowers formed by Kalanchoe blossjeldiana after a 

 given number of short-day cycles was inversely related to the 

 daylength used in the noninductive cycles on which the plants were 

 kept previous to short-day treatment. However, Carr (1955) obtained 

 fractional induction in a number of SDP, including some of the 

 same plants used by Schwabe, above. Carr also cites other results 

 that oppose the generalization that only LDP exhibit the phe- 

 nomenon, holding instead that it shows no particular correlation 

 with response type but rather is an individual species characteristic. 



Possibly the ability of Xanthium and a few other SDP to 

 flower in response to one short-day cycle is due to the lack, or 

 weaker operation, of inhibitory long-day effects. Even in Xanthium, 

 of course, flowering intensity increases proportionately to the 

 number of short-day cycles over a considerable range (see Chapter 

 Five) so that the phenomenon may be quite general. 



PHOTOPERIODISM AND TEMPERATURE 



Temperature enters into the physiology of flowering in numerous 

 ways, many of which will be considered later. A few interactions of 

 temperature with photoperiodism will be mentioned now, but with 

 the cautionary note that the results of such studies tend to defy 

 generalization more completely than any other aspect of the field. 

 For a major treatment of the effects of temperature on plant 

 growth, see Went (1957). 



Temperatures differing slightly from one another may strongly 

 modify the effects of daylength on flower initiation. For example, 

 Roberts and Struckmeyer (1938) found that both Maryland Mam- 

 moth tobacco and Jimson weed, Datura stramonium, were SDP only 



