698 BIOLOGICAL EFFECTS OF RADIATION 



by the aftereffects of germination in the cold. However, on the basis 

 of experimental data obtained, this investigator concludes that length of 

 day is more effective and more general in its action as a formative factor, 

 only relatively few plants being indifferent to the light period. 



In a study of interrelationship of temperature and length of day 

 as affecting winter and spring types of cereals Enomoto (20) found that 

 in all varieties of wheat and barley tested heading was accelerated by 

 high temperature, wheat being somewhat more sensitive than barley. 

 As a whole the wheat varieties were much less sensitive to day length 

 than the barley varieties. The most typical winter varieties are least 

 sensitive to both high temperature and long day, while the most typical 

 spring varieties are highly sensitive either to high temperature or to day 

 length or they are moderately sensitive to both. Among varieties show- 

 ing a similar grade of response to one of these factors the grade of the 

 spring-growing habit is correlated with the grade of response to the other 

 factor. Further, the grade of the spring-growing habit may be roughly 

 proportional to the sum of the grades of responses to high temperature 

 and to long day. 



The comparative responses of early-, medium- and late-maturing 

 varieties of soy beans to temperature and day length were studied under 

 partially controlled conditions by Garner and Allard (25). In greenhouse 

 plantings made at short intervals through the year under favorable 

 temperature conditions all three varieties reached the flowering stage in 

 about 25 days when grown during the period of 6 months in which the 

 day length was relatively short and, therefore, behaved as early-maturing 

 sorts. With the increasing day length of spring, however, first the late 

 and subsequently the medium forms suddenly changed from the spring- 

 fiowering to the fall- or late-summer-flowering type, that is, flowering 

 was deferred till the return of short days. In the early sort the duration 

 of the vegetative stage did not undergo any marked change. The 

 increasing length of day apparently exercised a definite selective action 

 on the different varieties. With a fixed day length of 10 hr., outdoor 

 plantings made at intervals through the growing season showed close 

 correlation of the length of the preflowering stage with the mean tempera- 

 ture, but relatively low temperature delayed flowering to about the same 

 extent in the latest and earliest varieties. Apparently change in tempera- 

 ture exercised no selective action on the different varieties. These 

 data and the results of extensive field plantings indicate that in this 

 particular species variations in duration of the vegetative stage from 

 year to year in both early and late varieties, when planted on any particu- 

 lar date, are due chiefly to differences in temperature, while length of day 

 is the principal external factor responsible for the fact that in higher 

 latitudes one variety is always relatively early and another late in 

 flowering and fruiting. 



