Vernalization and Photoperiodism — 52 — A Symposium 



gards photoperiodic induction of short-day plants. Long (1939) found 

 that at least 3 long dark periods must be given consecutively to initiate 

 flowers in Biloxi soybeans and that variations in temperature afifect con- 

 siderably the length of the critical dark period. In Xanthium, likewise, the 

 response is largely a reaction to the dark period (Hamner and Bonner, 

 1938; Mann, 1940; Hamner, 1940). While in both the light and dark 

 phases of the inductive cycle, temperature and probably other environ- 

 mental factors may modify or even inhibit the response of plants to induc- 

 tion, with increasing number of photoperiods the temperature effect usu- 

 ally becomes less noticeable. Moreover, there is the possibility that a cer- 

 tain length of night, like that of day, may be conducive to initiation of 

 floral primordia, while a different dark period may be better for further de- 

 velopment of flowers or for their function (Purvis, 1934). 



Recent detailed investigations of the relationship of the light and dark 

 periods of the inductive cycles, previously referred to, have been sum- 

 marized critically by Hamner (1942, 1944). The conclusion is drawn 

 that in short-day plants photoperiodic induction consists most probably of 

 an inductive cycle requiring photoperiods of a minimum duration and in- 

 tensity and dark periods of minimum duration. In agreement with this 

 conception appear to be the findings of Moshkov (1939, 1940) and 

 Cajlachjan (1941), who, in their studies on several short-day plants, 

 also observed that an alternation of light and darkness was a prerequisite 

 for inception of flowering in this group. In Perilla ocymoides, for exam- 

 ple, 9 hours of darkness and 9 hours of light was found the minimum 

 diurnal requirement for flowering. Rasumov's (1941) experimental evi- 

 dence has led him to believe that in short-day plants initiation and growth 

 of reproductive organs takes place during the dark period but development 

 in long-day plants occurs chiefly in light, which probably do not require 

 for this function an alternation in light and dark periods. Several years 

 previously to the above investigations Lysenko (1931, 1932) suggested 

 that long-day plants require light for completion of sexual reproduction 

 (his second or light phase of development) while short-day plants require 

 darkness for it. But since such plants cannot exist for any length of time 

 in continuous darkness because of absence of photosynthesis, alternating 

 periods of light and darkness are essential. 



Present information does not seem to indicate that long-day plants re- 

 quire for flower-induction a cyclic change of light and dark periods, for 

 they have been made to bloom in continuous light. Hamner (1944) is of 

 the opinion that in nature flowering in both groups of plants is determined 

 by the length of night: "In short-day plants flowering seems to be stimu- 

 lated by long dark periods while in long-day plants flowering tends to be 

 inhibited by long dark periods." This is in conformity with Blackman's 

 (1936) suggestion that the length of the night was chiefly responsible for 

 photoperiodic induction of flowering. 



Light intensity, to some extent, is a factor in this process. In artificial 

 light Biloxi soybeans were found to initiate flowers at intensities of 100 

 f.c. but not below it (Borthwick and Parker, 1938b). When daylight 

 was extended with Mazda light, initiation did occur if the supplemen- 

 tary light was less than 0.5 f.c. Biloxi soybeans did not form flowers 

 when kept during the short (8 hour) photoperiods at light intensities of 



