Devernaijzation • 59 



ing seeds, however, are not vernalizable; this distinction between 

 biennials and winter annuals is not always clear-cut, but in 

 Hyoscyamus at least it is clear that seedlings are not sensitive to 

 vernalization before 10 days of age, and not maximally sensitive 

 until they are 30 days old (Sarkar, 1958). Work on the vernalization 

 of Hyoscyamus has been reviewed by the original workers, Melchers 

 and Lang (1948) and Lang (1952). Evidence for the existence of a 

 translocatable product of vernalization has also been put forward 

 and will be discussed in Chapter Five. 



An exception to the observations that vernalization is per- 

 ceived by the stem apex is found in Streptocarpus wendlandii 

 (Oehlkers, 1956), in which the leaf appears to be the receptive 

 region and neither embryo nor stem apex can be vernalized at all. 



Several varieties of ornamental Chrysanthemum (Chrysanthe- 

 mum morifolium) require vernalization. Here again the apex is the 

 site of vernalization, and all the laterals subsequently derived from 

 it over a long period of time show the vernalized condition 

 (Schwabe, 1954). While most of the vernalizable plants studied 

 require the treatment in order to respond as LDP, or are daylength- 

 indifferent, vernalized Chrysanthemum is a quantitative SDP for 

 both flower initiation and development. Three or four weeks at 

 4 to 5° C has an optimum vernalizing effect. Low temperature is 

 effective even if given discontinuously, and a particular total 

 number of hours given during each dark period is more effective 

 than the same number of hours given only during light periods, 

 at least under short-day conditions. Chrysanthemum is a perennial, 

 and yet requires renewed vernalization each year (Schwabe, 1950), 

 a situation probably characteristic of many such plants. This brings 

 up the general topic of "devernalization," which has been observed 

 in a number of plants. 



DEVERNALIZATION 



Vernalized seeds of Petkus winter rye can be devernalized 

 simply by drying them and holding them in the dry condition for 

 several weeks. However, only the effects of vernalization on the 

 subsequent flowering response (to long days) are so reversed; the 

 effects on vegetative growth are more complex. This is well illus- 

 trated by some data from Gregory and Purvis (1938a). Their unver- 



