56 • Temperature and Flowering 



understanding of such low-temperature effects are in relatively 

 small-scale horticultural and floricultural practices. 



Vernalization in winter rye 



Although accounts of the effects of chilling seeds and seedlings 

 abound in the literature, there have been relatively few extensive 

 studies of vernalization. The work of F. G. Gregory, O. N. Purvis, 

 and their collaborators in England since about 1931, on the effects 

 of vernalization and photoperiodism on flower initiation, develop- 

 ment, and vegetative growth of spring and winter strains of the 

 Petkus variety of rye, Secale cereale, is by far the most thorough. 



The spring strain is a typical quantitative LDP. Under 

 sufficiently long days, flower initiation begins after approximately 

 seven leaves have differentiated, whereas under short days (10 

 hours light) it occurs only after at least 22 leaves have been pro- 

 duced. The winter strain, when germinated at relatively high tem- 

 peratures (for example, 18° C) , is not an LDP, but flowers equally 

 slowly— again after about 22 leaves— under both long and short 

 days. However, if the germinating winter strain is vernalized by 

 holding it at 1° C for several weeks before planting, it subsequently 

 responds to long days in the same way as does the spring strain 

 (Purvis, 1934). The effect of vernalization is thus to render the 

 seedling sensitive to long days; early flower initiation does not take 

 place as a result of vernalization alone, or vernalization followed 

 by short days. 



The effect of vernalization is proportional, within limits, to the 

 duration of the cold treatment. Four days' exposure is sufficient to 

 increase the subsequent relative growth rate of the stem apex, but 

 has no effect on either the number of days from planting to full 

 anthesis or the number of leaves preceding flower initiation. Both 

 values are reduced to a minimum (under subsequent long days) by 

 increasing the length of the cold treatment up to 14 weeks (Purvis 

 and Gregory, 1937). 



To determine what portion of the germinating seed perceives 

 the cold treatment, Gregory and Purvis (1938a) and Purvis (1940) 

 studied the effects of low temperature on excised intact embryos 

 and parts of embryos. Not only the intact embryo itself, separated 

 from the rest of the seed, but even its isolated apex alone are 

 susceptible to vernalization, giving rise to plants responding op- 



