Whyte — 19 — Research in Vernalization 



ply and subjected to temperatures of 1°C. and 18°C. respectively during 

 germination ; a second series was grown in short day of 10 hours with sim- 

 ilar temperature treatments. In the long-day series, cold germination re- 

 duced the time required for earing and flowering, not so in the short-day 

 series. Low temperature in the long-day treatment also reduced the rate of 

 tillering, not in the short-day plants. The effects of temperature were found 

 to be independent of nitrogen manuring. 



Under long days and after germination at 1°C., flowering occurred 10 

 weeks after planting out; after germination at 18°C. it was delayed until 

 autumn when a few ears appeared. Under short days ear emergence was 

 indefinitely delayed after both temperature treatments. Examination of 

 the growing points revealed, however, that under both temperature treat- 

 ments and daylengths ear initials are laid down. After germination at 

 18°C. flower initials are laid down earlier in short days than in long days 

 and the nuijiber of initials is considerably increased, and with it the potential 

 grain-yielding capacity Such ears, however, die within the leaf sheaths, 

 and the stems fail to elongate. After germination at 1°C. the characteristic 

 temperature after-effect, namely, early flower differentiation, was manifest 

 only under long days while under short days the effect was slight. When 

 plants which had received some weeks of short-day treatment were sub- 

 jected to long days, ears were rapidly formed after germination at 1°C. and 

 more slowly after germination at 18°C. Even with plants germinated 

 at high temperature, flower differentiation may be induced by short-day 

 treatment alone, showing that cold germination is not essential for re- 

 production. Stem elongation and ear emergence, however, fail unless 

 long days are given subsequent to ear differentiation. These results il- 

 lustrate a point stressed from the start in the work from this Institute, 

 namely, the great importance of determining the time of formation of the 

 flower primordia rather than the time of emergence of the inflorescence in 

 studies of this kind. 



The concept of minimal leaf number has also been a characteristic of 

 the work of Gregory and Purvis. Observations made by Purvis (1934) on 

 the spike primordia indicate that a minimal number of leaves must be formed 

 before the differentiation of flower initials can occur. Under short-day con- 

 ditions this is always about twenty-two, irrespective of variety and tem- 

 perature, while under long-day conditions the minimal number is less for 

 plants germinated at low than at high temperatures, namely, 12 for the 

 former (in this particular experiment) and 22 to 25 for the latter. This 

 reduction in leaf number combined with the more rapid elongation of the 

 spike leads to acceleration in the onset of floral differentiation and thus 

 induces a behaviour more closely resembling that of spring varieties in 

 which the minimal number is only seven. The effect of the greater minimal 

 number of leaves required before "ripeness to flower" is attained in short 

 days is partially counterbalanced by the higher rate of differentiation of the 

 ear in these plants, particularly in those germinated at low temperatures. 

 The increased rate of floral differentiation in short days also affords an ex- 

 planation of the photoperiodic after-effect observed by Rasumov in short-day 

 plants like millet. However, in long-day plants like oats or barley, the floral 

 differentiation is almost independent of daylength, only the later stages being 



