60 • Temperature and Flowering 



nalized controls in this experiment produced about 4.7 tillers 

 (lateral branches from the base) per plant, and a flowering "score" 

 of 19. The "score" is an arbitrary scale adopted to indicate the 

 intensity and earliness of flowering. Vernalized seed held dry for 

 one day only (which has essentially no effect) gave a score of 51 

 and about 2.7 tillers per plant— vernalization typically decreases the 

 number of tillers. Seed devernalized by being dry for 20 weeks, 

 however, gave a score of 20 and about 13.7 tillers per plant; the 

 promotion of flowering was completely reversed, but the number 

 of tillers was much higher than in either vernalized or unvernalized 

 plants. Thus devernalization here is not a simple reversal of vernal- 

 ization but a conversion of its effects to a different physiological 

 expression. Like vernalization itself, it is proportional, within limits, 

 to the duration of exposure to the condition bringing it about. 



Even spring Petkus rye, which may be regarded as already 

 genetically vernalized, can be devernalized to some extent. The 

 leaf number preceding flowering (in long days) is increased from 

 an average of 6.8 to 8.3 by a three-week germination period under 

 anaerobic conditions, and this effect is removed by a subsequent 

 three-week vernalization treatment (Gregory and Purvis, 1938b). 



The devernalization of vernalized biennial Hyoscyamus is 

 brought about by relatively high temperatures. Vernalized plants 

 may be kept under short-day conditions for at least several weeks 

 at about 23° C and still retain their capacity to respond as LDP. 

 Ten days at about 38° will completely remove this capacity, if 

 started immediately after the vernalization treatment; if even a 

 lew days of moderate temperature intervene between vernalization 

 and the high temperature, however, the vernalized condition 

 becomes stabilized and can no longer be removed (Lang and 

 Melchers, 1947). In general, studies of various plants indicate that 

 the more complete the original vernalization and the greater the 

 length of the treatment, the more difficult devernalization becomes. 

 Revernalization after devernalization is also possible in certain 

 plants. 



As the only perennial studied in any detail, Chrysanthemum 

 again appears unusual in that devernalization is not brought about 

 by high temperatures alone, but requires several weeks of low light 

 intensity (or darkness) as well as temperatures of 23° to 28° C. 

 The mechanism of this effect is unknown. It is not due simply to 



