Photoperiodism and Vegetative Growth • 27 



has been studied extensively in connection with the possible rhyth- 

 mic components of photoperiodism (see Chapter Three). The paper 

 by Blaney and Hamner, previously cited, also contains data on the 

 interactions of temperature with the various light-dark cycles used. 

 A simpler example of such work is a paper by Schwemmle (1957) 

 reporting the effects on the SDP Kalanchoe blossfeldiana of brief 

 exposures to 30° C during various portions of 12-hour dark periods 

 alternated with 12-hour light periods (inductive for Kalanchoe), 

 the temperature otherwise being 20°. Such exposures promoted 

 flowering significantly when given for the first three hours of each 

 dark period, but inhibited it completely when given for the last 

 three hours. Full 12-hour exposures to 30° during the night also 

 inhibited completely. 



One of the most striking temperature effects reported recently 

 deals again with Xanthium, which will apparently flower on a 

 16 hours light-8 hours darkness schedule, completely noninductive 

 at 23° C, if the first 8 hours of each light period are given at 4°. 

 Low temperatures during the second half of each light period, or 

 during the dark period itself, do not cause flowering, nor does 

 flowering occur on continuous light with any alternation of tem- 

 peratures used (Nitsch and Went, 1959); see Fig. 2-2. The SDP 

 Pharbitis can be brought to flower even under continuous light 

 by low-temperature treatments (Ogawa, 1960). 



On the basis of some experiments with Hyoscyamus and the 

 SDP Chenopodium, as well as other results in the literature, 

 Schwemmle (1960) has suggested in a brief paper that, in a physio- 

 logical sense, high temperatures may be equivalent to light and 

 low temperatures to darkness in their effects on photoperiodism. 

 Whether this generalization will withstand critical examination 

 remains to be seen. So far, all that can be said with certainty is 

 that high or low temperatures can modify both dark and light 

 processes in photoperiodism in a manner varying widely with the 

 temperatures, species, specific cycle, and portion of light or dark 

 period chosen. 



PHOTOPERIODISM AND VEGETATIVE GROWTH 



Structures and processes of all kinds can be affected by photo- 

 periodism, and such results are widespread in the literature, 



