Dormancy in Betula as a quantitative state

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Abstract
The growth of leafy seedlings of Betula pubescens and B. lutea was studied under photoperiods ranging from 10 to 18 hours. The rate of growth was found to increase in proportion to the length of the photoperiod. Dormancy was induced after 2-4 weeks of the shortest photoperiod. The greater the number of short days given, the longer it subsequently took to break this dormancy. Dormancy could be broken either by cold treatment or by the longer photoperiods, but best of all by a cold treatment followed by a long photoperiod. Either an interruption of the long night at its mid-point or the application of gibberellic acid reduced the short-day effect. When one branch of a bifurcated seedling was exposed to short days and the other to long days, the long-day branch was retarded in its growth. It was suggested that the photoperiodic regime controls the level of some substance which in turn controls the rate of the growth reaction.