92 ■ Floral Hormones and the Induced State 



general, and what their significance might be, is unknown. The fact 

 that various well-known respiratory poisons, including cyanide, 

 azide, and fluoride, may inhibit the dark period induction (Naka- 

 yama, 1958, on Pharbitis nil) does not afford any special insight 

 into the processes involved, but indicates simply that normal 

 respiration is required to support them. This is true also of ver- 

 nalization, at least on the basis of the oxygen-level and sugar-feeding 

 experiments mentioned in the previous chapter. 



There has been a series of investigations on the fixation of 

 carbon dioxide in darkness, particularly by Kalanchoe, since photo- 

 period influences its time-course and intensity in a manner sug- 

 gestive of the effect on flowering. In addition other work has shown 

 that exclusion of CO, during dark periods can reduce the induction 

 of several SDP. These results are reviewed by Kunitake et al. (1957), 

 who concluded from their own experiments with radioactive tracer 

 techniques that short-day induction of Kalanchoe affected not the 

 proportion of COo fixed in various compounds but only the total 

 amount. This conclusion, together with the fact that even this 

 change occurs relatively late in induction, affords no support for 

 the suggestion of a specific significance for dark C0 2 fixation in 

 the inductive process. 



The induced state in many plants has some of the character- 

 istics of infection with a virus, or some other self-replicating entity. 

 This is true both of photoperiodically induced Xanthium, in which 

 llorigen production appears to be autocatalytic, and, in a different 

 way, of vernalization in those plants in which the vernalized state 

 is maintained in all cells descended from those originally treated. 

 Unfortunately this stimulating hypothesis of flowering as a virus 

 disease has as yet no direct evidence in its favor. Changes in the 

 levels of both ribonucleic and desoxyribonucleic acids during and 

 following photoinduction have been observed (Gulich, 1960, and 

 bibliography therein), but all attempts to show qualitative dif- 

 ferences between the nucleic acids or proteins of induced and non- 

 induced plants have been unsuccessful (see Bonner and Liverman, 

 1953; Bonner, 1959b). However, some indirect evidence has been 

 obtained by the use of compounds believed to inhibit nucleic acid 

 synthesis. Hess (1959) found that 2-thiouracil given during the 

 vernalization of Streptocarpus could reduce or abolish flower 

 initiation without affecting vegetative growth; 5-fluorouracil is 



