The Morphogenesis of Apple Buds

The formation of the terminal resting bud of apple spurs was studied on untreated and defoliated spurs by dissection of buds collected at intervals during the season. A distinctive pattern of bud development was found in all treatments, the principal features being firstly the steady rate over long periods of the season at which successive primordia began or completed bud-scale development, and secondly a change in rate of initiation of bud-scales several weeks after the resting bud had begun to form. Bud-scale initiation was invariably delayed in primordia in the early period of bud development, when primordia adjacent to the apical meristem were affected directly by the foliage. The rate at which the number of mature bud-scales in the bud increased during the season was not related to the extent of foliar development, but appeared to depend upon the degree to which the bud, during its early development, was affected by the foliage. Whilst defoliation of spurs showed that the formation of bud-scales was dependent upon an effect of the foliage, no simple relationship was found. The number and size of leaves on defoliated spurs was much less than on untreated spurs, but the pattern of bud development was very similar. Since the morphogenetic effect of foliage is known to vary with its age, the constant rate of formation of bud-scales during the season was considered to be evidence of a second factor in the bud which counteracted variations in effect of the foliage. A change in rate of increase in number of bud-scales could only occur therefore if the effective level of one of these factors was altered independently of the other. This, it is suggested, is brought about by the accumulation of a further factor in the bud-scales, at a rate determined during the early formation of each primordium. From the similarity between the effects of these hypothetical factors and those of growth-regulating substances extracted from or applied to buds of fruit trees it is thought that the second factor may be a gibberellin-like substance, and the bud-scale factor a growth inhibitor.