Dormancy in Betula as a quantitative state.

Klebs (12) reported in 1914 that the growth of beach, oak, ash, and hornbeam could be maintained even in winter if they were placed under continuous electric light. Since Klebs' report on the effect of day length on the growth of woody plants, there have been many studies pertaining to photoperiodism and dormancy in woody plants. They were amply reviewed by Samish (21), Wareing (24,26), and Nitsch (16). In general, a short photoperiod induces a reduction in growth of woody plants through a shortening of internodes and a decrease in the number of new nodes formed. These same plants would grow continuously under long photoperiods. The breaking of dormancy induced by either artificial or natural short photoperiods has been accomplished by three different treatments: A: long photoperiods (7, 10, 18, 19, 22, 23, 25, 27); B: chilling (2, 5, 7, 9, 19), and C: application of gibberellic acid (1, 3,4, 6, 13,14, 15). Previous workers such as Van der Veen (23) and Downs and Borthwick (7) have suggested that, when leaves remain on the plant and are subjected to short days after visible growth has ceased, dormancy becomes increasingly more difficult to break. This project was undertaken to study the effect of photoperiods on dormancy with special consideration on the degree of dormancy.