Vernalization: Cold Treatments and Flowering • 57 



timally to long days. Thus the site of vernalization is in the meri- 

 stem itself, and the results of vernalization are somehow maintained 

 throughout the development of the plant derived from the few 

 cells originally exposed. The technique of vernalizing isolated 

 embryos also made it possible to show that vernalization requires 

 a carbohydrate source, presumably as an energy supply for the 

 process involved. Rapid flowering takes place only if the embryos 

 are cold-treated on a medium containing sucrose, although sub- 

 sequent vegetative growth is excellent even if the medium consists 

 of mineral salts alone (Gregory and DeRopp, 1938). 



Oxygen is also required during vernalization, confirming the 

 suggestion that the process requires a considerable amount of 

 energy. For example, Gregory and Purvis (1938b) found that germi- 

 nating seeds held at 1° C for 9 weeks would eventually produce 

 inflorescences after the eighth leaf if the cold treatment was given 

 in air, but only after the twenty-third, as in the unvernalized 

 controls, if the treatment was in nitrogen. As little as 1/500 of the 

 normal air concentration of oxygen allowed some vernalization to 

 take place, but not the maximum effect. 



Before proceeding further, one should bear in mind that 

 confusion occasionally arises between vernalization and the favor- 

 able effects of chilling on seed germination in many species. The 

 former has relatively specific effects, inductive in the sense that they 

 lead to subsequent changes in the flowering response of the plants. 

 Mere cold treatment to hasten germination is not necessarily ver- 

 nalization. It may indeed result in earlier flowering, but the use of 

 developmental criteria (number of leaves before the inflorescence, 

 for example) can usually indicate whether a genuine hastening of 

 flowering relative to vegetative growth has occurred. 



Vernalization in other plants 



The flowering not only of winter cereal strains, but of many 

 other plants, can be hastened by vernalization. Certain varieties of 

 peas, Pisum sativum, can be made to produce their first flower at 

 an earlier node. In the variety Zelka, the eighteenth or nineteenth 

 nodes are the first to bear flowers if germination and growth take 

 place at about 20° C, but if the germinating seeds are kept at 7° 

 for 30 days before planting, flowers occur beginning with the 

 fourteenth or fifteenth nodes. The physiological stage susceptible 



