THE LOW TEMPERATURE PROMOTION OF FLOWERING 53 



respond to the cold. This involves the technical difficulty of demon- 

 strating a promotion by cold even though the plant would flower 

 anyway within a short time. With this difficulty in mind, experi- 

 mental results can be interpreted to show that vernalization is 

 effective (to some extent at least) right up to the time the plant would 

 flower anyway. The biennials as a rule do not respond in the seedling 

 or moist seed stage. A rosette of leaves must be formed. Hyoscyamus 

 must be at least 10 to 30 days old before it will respond to the low 

 temperature treatment. Sugar beets normally respond when they are 

 2\ to 3 months old, but the time can be shortened considerably if 

 plants are exposed to continuous light during the low temperature 

 treatments. In some plants (beet, henbane) sensitivity to vernalization 

 increases at least for a time, as plants get older. In plants that respond 

 as seedlings, however, the sensitivity may decrease as the plant ages. 

 The topic of "ripeness to flower" is discussed again in Chapter 6. 

 In order for the cold to be perceived, the cells at the apical meristem 

 can not be metabolically inactive. Completely dry seed will not 

 respond, although seeds which have imbibed water only slightly are 

 sensitive, even though they have absorbed insufficient water for 

 active germination (30 % to 40 % produces ample sensitivity in wheat 

 seeds). It would be interesting to know if this requirement for some 

 water is related to an initiation of cell division, but so far such 

 information is not available. Following vernalization of this slightly 

 moist seed, Lysenko showed that it may be dried out again, stored, 

 and mechanically sown. Thus the results of vernalization are 

 relatively stable, once the process has taken place. 



4. Effective Temperatures and Times 



In rather early experiments, vernalization was carried to a maximum, 

 and it was found that the temperature optimum was very broad. As 

 shown in Fig. 4-1, temperatures from 1 to 7°C, are almost equally 

 effective, and temperatures as high as 9 to 15°C have some effect. 

 Temperatures a few degrees below freezing are also effective. In 

 later experiments, a shorter vernalization time was used, and the 

 temperature optimum was found to be slightly more sharp. That is, 

 the curves had a peak (although not a very sharp one) at about 

 6°C. As we might expect, the exact optimum temperature depends 

 upon the species. 



As also might be expected, the optimum times for cold treatment 



F 



