THE LOW TEMPERATURE PROMOTION OF FLOWERING 51 



common pea which flowers somewhat sooner (at a lower node) if 

 germinating seeds are subjected to low temperature. A number of 

 vegetables may be promoted in their flowering by low temperature 

 treatment of the whole plants, but this could be a direct effect instead 

 of vernalization in the narrow sense. 



Most of the species used in vernalization studies also have a 

 requirement for long-days following the low temperature treatment. 

 The appendix shows that there are also a number of plants which 

 require low temperature but are completely day-neutral following the 

 low temperature treatment, and that certain varieties of chrysan- 

 themum require low temperature followed by short-days in order to 

 flower best. Nevertheless, no exception to the following generaliza- 

 tion has yet been found : plants which may be vernalized as seedlings 

 have a subsequent long-day requirement. 



The genetics of cold requirement have been studied in a preliminary 

 way. The situation is sometimes more complicated than the single 

 gene difference discovered by Correns in Hyoscyamus, yet usually it 

 is relatively simple. One of the more complex examples is wheat, in 

 which the vernalization requirement exhibits a quantitative inheri- 

 tance. The degree of cold requirement varies, and there are a number 

 of steps or intergradations between an absolute requirement and no 

 requirement at all. The vernalization requirement is commonly a 

 double recessive ; that is both genes for vernalization must be present 

 if the trait is to be expressed. Perhaps this indicates that vernalization 

 requirement is a defect in a gene which normally leads to flowering in 

 the absence of cold. Thus when two defective genes are present, some 

 sort of metabolism which takes place best at low temperatures is 

 required for flowering. Of course two positive acting (instead of 

 defective) genes could also account for the cold requirement. 



At any rate the vernalization response is an operation of certain 

 genes under certain environments. Thus it is not the flowering which 

 is inherited, but it is the gene, the potential to flower under the right 

 conditions. 



2. The Site of Perception of the Cold in Vernalization 



An early question asked which organ of the plant was involved in 

 vernalization. The trick was to cool only a certain portion of the 

 plant while leaving the rest at a normal temperature. It was discovered 

 with plants beyond the seedUng stage that cooling the leaves or the 



