50 THE FLOWERING PROCESS 



course hundreds of other names appear in the literature of vernaliza- 

 tion, but those mentioned above seem to recur most frequently or 

 significantly. 



Vernalization (12, 23, 24, 31) 



Because of space limitation and the present complexity of the topic, 

 the facts about vernalization will be summarized under eight headings. 

 Of course, this is a rather drastic over-simplification, but the classical 

 work of Melchers and of Gregory and their associates can now be 

 outUned in a rather straightforward manner. Some of the newer 

 work with other plants is also mentioned, although it does not always 

 fall easily into this pattern. This is probably to be expected, and it 

 does not invalidate the facts which were already available. 



1. The Many Response Types 



Melchers and Lang and other German workers studied two races 

 of the henbane, Hyoscyamus niger. One race is an annual, flowering 

 the summer following the spring in which it germinates. The other 

 race is a biennial, which germinates one spring, grows as a rosette 

 during summer, winters over, and then flowers the following summer. 

 It had been shown as early as 1904 by C. Correns, the early German 

 geneticist, that these two races diff'er from each other by only one 

 gene. Both are long-day plants, but the annual has no cold require- 

 ment, while the biennial will remain vegetative indefinitely, Hterally 

 for years, unless it has been exposed to a few weeks of very low 

 temperatures. In the biennial race the cold requirement is absolute, 

 and the long-day requirement is absolute for both races. 



Purvis and Gregory studied primarily a spring and a winter race 

 of Petkus rye, Secale cereale. These are both considered annuals, 

 but one is a winter annual and the other is a spring annual. Both 

 require long days, and both will flower without cold, but the spring 

 variety flowers in 7^ weeks, while the winter variety requires 15 weeks. 

 Cold treatment reduces flowering time in the winter variety to 7^ 

 weeks. The promotion by low temperatures can be replaced by a 

 short-day induction, but this is not the case with henbane. 



Of course, many other species have been used in studies on the 

 mechanisms of vernalization. A number of these are listed in the 

 appendix. They include such unexpected types as small summer 

 annuals which germinate and flower within a few weeks and the 



