Vernalization and Photoperiodism — 46 — A Symposium 



macroscopic development, certainly not in equal numbers (Murneek, 

 1939). The histological analysis by Borthwick and Parker (1938) 

 suggests this, and their further studies on the effects of the photoperiod on 

 development of the Biloxi soybean (Parker and Borthwick, 1939) dem- 

 onstrate that when plants, with initiated floral primordia, were transferred 

 to long, 16-18 hour photoperiods, no flowers opened, and when photoperiods 

 above 13 hours were given no fruit was formed. Nielsen (1942) found 

 that even 10 cycles of short photoperiods resulted in a high percentage of 

 degenerated microspores in the Biloxi soybean. 



In view of these facts, and the thought-provoking statement by Gregory 

 (1936) that "the problem of photoperiodism may be considered not as con- 

 cerning conditions leading to flower formation but as concerning failure to 

 flower," it would seem to be desirable that in studies of photoperiodism, 

 as it affects sexual reproduction and metabolism, the whole reproductive 

 cycle should be followed in detail both by observation and histologically. 

 As far as the writer is aware this procedure was started for the first time 

 with the soybean, var. Biloxi (Murneek and Gomez, 1936) and has sub- 

 sequently been used successfully by other investigators (Hamner and 

 Bonner, 1938; Borthwick and Parker, 1938; Snyder, 1940; Mann, 

 1940, etc.). 



Selection and breeding of plants for adaptability to localities of certain 

 photoperiods has been in progress for a number of years. The testing of 

 species, varieties and strains was started by Garner and Allard (1920) 

 and has been continued by numerous other investigators. Unconsciously 

 growers have been doing it for a very long time, especially in comparative 

 tests of types and varieties of various economic plants. In this connection 

 emphasis should be placed on the importance of conducting selection by ex- 

 posing plants to near the critical length of day for flower initiation and de- 

 velopment, for there will be revealed the greatest degree of variation in 

 time of flowering (Allard and Zaumeyer, 1944). 



Phenotypic adaptation is adaptation of the individual, but there is also 

 genotypic adaptation of successive generations. In adaptability to a locality, 

 therefore, not only the environment but also the endogenous rhythm of the 

 selected plants must be taken into account. These do not always coincide. 

 Though we do not know much about the specific mechanism of this "inner 

 rhythm," it probably originated as a result of natural selection of random 

 mutations, but segregation and recombination or other gene mechanisms 

 may be operative (Lubimenko, 1939; BtJNNiNG, 1943). Breeding plants 

 for photoperiodic adaptability has been successful in several instances 

 (MuNERATi, 1931; Abegg, 1936; Owen et al., 1940; Goodwin, 1944, 

 etc.).* 



Technique and Application : — The original technique of treating 

 plants for photoperiodism, as used by Garner and Allard and others, was 

 to grow them in any convenient containers and either shorten the natural 

 photoperiod by placing them during part of the day under an opaque cover 



♦ On the basis of behavior analysis of F2 plants, obtained from hybridization of Maryland mam- 

 moth (short-day) and Java (day-neutral) varieties of tobacco, A. Lang (Special Supplement 3, 

 pp. 175-183) has reached the conclusion that the short-day character in Maryland mammoth tobacco is 

 recessive to the dominant day-neutral character and monofactorial in inheritance. This characteristic 

 response to a short photoperiod is probably the result of quite frequently occurring gene mutations in 

 this and many other races of tobaccos. 



