Vernalization and Photoperiodism — 20 — A Symposium 



hastened by long day. All spring cereals are also of this type. On the other 

 hand, winter rye behaves as a long-day plant only when germinated at low 

 temperatures. When germinated at 18°C. the floral differentiation shows 

 the typical short-day behaviour, being promoted by short day, though stem 

 elongation and further stages of floral development still retain the long-day 

 reaction. Thus the effect of chilling in winter cereals sown in the spring is 

 to reduce the minimal number of leaves required for flower initiation and to 

 enable the plant to attain the condition of "ripeness to flower"' at an earlier 

 date while long days still prevail. It is in this way that Purvis interprets 

 the phenomena connected with vernalization. 



After a review of vernalization as a method of hastening flowering 

 (Purvis, 1936), Purvis and Gregory proceeded to a comparative study 

 of vernalization of winter rye by low temperature and short days, the first 

 of a series of papers published (1937) under the general title of "Studies 

 in Vernalization of Cereals," aiming at a closer analysis of the proximal 

 cause of the efTect of the low temperature applied during germination. A 

 preliminary report appeared in Nature (Gregory and Purvis, 1936a.). 



The treatments with low temperature or preliminary short days both re- 

 sulted in acceleration of flowering. With the low temperature treatment, 

 4 days' exposure produces a definite response ; the magnitude of the response 

 increases with the duration of treatment up to a limit of 14 weeks, after 

 which the flowering behaviour of winter rye is indistinguishable from that 

 of spring rye. With increasing exposure to low temperature the leaf num- 

 ber in winter rye is reduced from approximately twenty-five to seven. 



With the exposure of seedlings of winter rye to preliminary short days 

 (10 hours) the magnitude of the effect on flowering increases with length 

 of treatment up to 6 weeks, but longer exposures retard flowering. Re- 

 duction of leaf number to a minimum of sixteen occurs after 6 weeks' treat- 

 ment with short days, but longer treatment again leads to increase. 



In both spring and winter rye, Purvis and Gregory state that there is 

 a minimal leaf number, seven, which cannot be further reduced, and a 

 maximum, about twenty-five, which cannot be further increased. The 

 primordia between the eighth and twenty-fifth are indeterminate and can 

 produce either leaf or spikelet ; each primordium is double, consisting of a 

 flowering branch (spikelet) with a subtending bract, either part of which 

 may be inhibited according to the length of day or temperature of germina- 

 tion. 



To explain the mechanism of these reactions of a germinating seed or a 

 seedling to low temperature or short day, Purvis and Gregory (1937) 

 postulated the existence of a hypothetical "flower-forming" substance, the 

 effect of which on the labile primordia varies according to the factors to 

 which the plant is exposed. Although stressing that the whole situation is 

 not yet clear, they state that the following facts are established : 



(a) The hypothetical substance is not preformed in the endosperm and 

 transferred to the embryo, as the embryo isolated from the seed behaves 

 similar!}' (see experiments on e.xcised embryos described below). 



{b) The change can be reversed by high temperature (again see below). 

 It appears possible that during vernaHzation some precursor of a "flower- 

 forming" substance accumulates in the embryo. This substance may be 



