THE LOW TEMPERATURE PROMOTION OF FLOWERING 57 



of the seed, take part in the chemistry of the vernalization process 

 (see Chapter 6 for related phenomena in photoperiodism). 



8. Interactions of Day-Lengths and Cold 



We saw in Chapter 2 that temperature and day-length may interact 

 in all sorts of ways. Short-days will essentially replace the effects of 

 cold in rye. Furthermore the response in rye is not absolute, since 

 neither cold nor short-days are required for ultimate flowering. In 

 Campanula medium, the response is absolute, and short-days and low 

 temperatures are completely interchangeable. Either one or the 

 other is required for flowering. A certain variety of radish is a long- 

 day plant unless it is treated with cold, after which it becomes 

 day-neutral. In this case, long-days and low temperature might be 

 considered as interchangeable. A long-day variety of spinach has 

 a critical day of about 14 hr unless it has been vernalized, in which 

 case the critical day decreases to about 8 hr. Such temperature 

 interactions could play an important role in future research on 

 vernalization. 



Results with Applied Pure Chemicals and 

 Plant Extracts 



As mentioned above, there is some evidence for a positive flower- 

 promoting substance which is formed in response to cold treatment. 

 In Chapter 1 we also introduced the concept of a general flowering 

 hormone (to be discussed further in Chapters 9 and 10). Obviously 

 an important goal of plant physiologists is to discover the chemical 

 nature of such growth regulating compounds and to understand the 

 biochemistry of their formation and action. The auxins (stem growth 

 hormones) were the first such compounds to be isolated and at least 

 partially characterized, but in recent years other compounds have 

 begun to yield to this approach. Thus we now know about cell 

 division factors such as kinetin and the coconut milk factors with 

 all their relatives, growth inhibitors such as the unsaturated lactones 

 and naringenin from dormant peach buds, and an extracted potato 

 tuber-inducing principle. Whether or not the flowering hormone 

 itself has been extracted remains in doubt at the time of this writing 

 (see Chapter 9), but three classes of compounds extracted from plants 

 are known to replace the requirement for vernalization. Of these 



