The transition from the juvenile to the adult condition (‘phase change’) in woody plants normally occurs at an age and a size characteristic for the species and flowering does not occur until this change has taken place. By experiments with seedlings of birch (Betula verrucosa) grown either continuously under long days in a glasshouse, or periodically under alternating periods of long and short days in association with chilling treatments, it was shown that flowering occurs much sooner in the seedlings grown continuously (i.e. without periods of dormancy) than in seedlings of this species grown under normal outdoor conditions, or in the experimental seedlings grown ‘periodically’. Similar results were obtained with Japanese larch (Larix leptolepis) and black currant (Ribes nigrum) when flower-inducing treatments appropriate for the species, viz. horizontal training or short days, respectively, were applied after continuously grown seedlings had attained a certain size. Further experiments were carried out with black currant to determine whether the attainment of a certain minimum size is necessary for phase change. The shoot tips of seedlings were removed when they had attained a size at which flowering does not occur and were rooted as cuttings; the latter plants were similarly treated and this process was repeated again twice. The final series of plants which had been derived in this manner initiated flowers in response to short days and thus had attained the adult condition, although at no stage had any of the successive rooted cuttings attained the minimum size at which phase change normally occurs in this species. It is concluded that size, per se, is not the primary factor determining the stage at which phase change occurs.
Experiments were carried out with Japanese larch and European larch (Larix decidua) in which scions from juvenile (non-coning) seedling trees were grafted on to mature trees which produced abundant cones but little evidence was obtained that phase change was hastened by this treatment. On the other hand, a considerable proportion of scions from young trees, which were approaching the adult condition, initiated cones following grafting on to adult trees.
Phase change takes place in the shoot apical meristems, which appear to behave as autonomous units in the sense that their transition to the adult condition is apparently not determined by the amount of already differentiated tissue. It is suggested that phase change occurs after the meristems have undergone a certain number of cell divisions, so that the change is correlated with, but not determined by, the attainment of a certain size.