THE INDUCTION OF FLOWERING IN Xanthium 

 pensylvaniciim UNDER LONG DAYS 



J. P. NITSCH 1 and F. W. WENT 2 



Laboratoire du Phytotron, Gif-sur-Yvette (Seine-et-Oise), France and 

 Earhart Plant Research Laboratory, California Institute of Technology, 



Pasadena 



Since ancient times, it has been well known that flowering can be 

 brought about by many diverse influences. In numerous cases, appar- 

 ently, it is a question of physiological age. When the plant has reached 

 a certain development, it initiates flower buds. Environmental and 

 nutritional factors, of course, may accelerate or delay this basic trend. 

 Recently, studies in the field of photoperiodism have focused attention 

 upon one mechanism: the regulation of flowering by the relative dura- 

 tion of day and night. According to this view, plants can be classified 

 into groups such as "short-day plants," "long-day plants," and "day- 

 neutral plants." The day-neutral plants constitute a very large group. 

 Among the species which do respond to photoperiodic induction, most 

 numerous are those which are not strictly photoperiodic. Other factors, 

 such as temperature, profoundly alter their behavior. Physiologists, 

 however, have tended to select one or two species known to be strictly 

 photoperiodic, forgetting that the plants they work with have been 

 picked out of hundreds of species and constitute, therefore, exceptions 

 rather than examples of a general behavior. The prototype of such 

 exceptions is the plant most commonly used in photoperiodic experi- 

 ments, namely the cocklebur, Xanthium pensylvonicum. 



Even in Xanthium, however, the photoperiodic mechanism is not the 

 only one that is conducive to flowering. This fact has been recently 

 reported by De Zeeuw (1957) who obtained flowering cockleburs 

 under long days by submitting the plants to cold treatments. In view 

 of the importance of such a claim, new experiments were started along 

 the same line. The results in brief follow. 



^ The work reported here was done in the Earhart Plant Research Laboratory 

 under Grant G4046 from the National Science Foundation, which made it 

 possible for one of us to go to Pasadena as a part of a more general investiga- 

 tion on the regulation of growth in plants. 



- Present address: Missouri Botanical Garden, St. Louis, Missouri. 



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