Vernalization and Ptiotoperiodism 



■140 — 



A Symposium 



varieties seems to be to the joint effect of light duration and temperature 

 rather than to one of the two factors. 



From the data of Table 3 it appears that the first bloom in Saskatoon, 

 Saskatchewan, for instance, could be interpreted as having been brought 

 about by a combination of 1293 day-degrees and 16.6 hours of average day- 

 light while that in El Centro, California, as having been brought about 

 by a combination of 2112 day-degrees and 10.4 hours. The multiples of 

 the average length of day and the day-degree summation in the two in- 

 stances are very much alike in their numerical expressions. Similarly, as 

 may be seen from the table, the multiples of light duration and summa- 

 tion of day-degrees between the dates of seeding to ripening in Fairbanks, 

 Alaska and El Centro, California, are of very similar magnitudes. How- 

 ever, while a combination of 3175 day-degrees and 17.7 hours of average 

 daylight was required to bring about ripening in Alaska, the ripening in 

 California required a combination of 4876 day-degrees and 11.5 hours of 

 average daylight. One could thus suggest that in the case of these four 

 varieties of flax the first bloom, as well as the ripe stage, is brought about 

 by a combination of either relatively higher summation of day-degrees and 

 shorter days or by relatively lower summation of day-degrees and longer 

 days. In other words, the change from vegetative to reproductive stage 

 seems to occur in some flax varieties under a number of certain combina- 

 tions of temperature and length of day conditions. 



Table 6 



Phenology of Alaska Peas in a Number of North American Areas* 



* Phenological data upon which this chart and table are based were obtained from reports of the 

 University of Alaska, Pennsylvania State College, University of Maryland, and The Joseph Harris 

 Seed Co. of Rochester, N. Y. — Day-degrees were calculated above 40°F. 



The data of Tables and Charts 4 and 5 taken together as a unit for com- 

 parison purposes suggest that the interval between the dates of emergence 

 and first blossom of Alaska peas provide the least variable mathematical 

 expression when measured in terms of a multiple of the average length of 

 day and summation of total day-degrees. The same seems to be true with 

 the interval between the dates of emergence and harvest. It could be there- 

 fore suggested that the response of Alaska peas to the length of day is in- 



