| Abstract: | The formation of the terminal resting bud of apple spurs wasstudied on untreated and defoliated spurs by dissection of budscollected at intervals during the season. A distinctive patternof bud development was found in all treatments, the principalfeatures being firstly the steady rate over long periods ofthe season at which successive primordia began or completedbud-scale development, and secondly a change in rate of initiationof bud-scales several weeks after the resting bud had begunto form. Bud-scale initiation was invariably delayed in primordia inthe early period of bud development, when primordia adjacentto the apical meristem were affected directly by the foliage.The rate at which the number of mature bud-scales in the budincreased during the season was not related to the extent offoliar development, but appeared to depend upon the degree towhich the bud, during its early development, was affected bythe foliage. Whilst defoliation of spurs showed that the formation of bud-scaleswas dependent upon an effect of the foliage, no simple relationshipwas found. The number and size of leaves on defoliated spurswas much less than on untreated spurs, but the pattern of buddevelopment was very similar. Since the morphogenetic effectof foliage is known to vary with its age, the constant rateof formation of bud-scales during the season was consideredto be evidence of a second factor in the bud which counteractedvariations in effect of the foliage. A change in rate of increasein number of bud-scales could only occur therefore if the effectivelevel of one of these factors was altered independently of theother. This, it is suggested, is brought about by the accumulationof a further factor in the bud-scales, at a rate determinedduring the early formation of each primordium. From the similaritybetween the effects of these hypothetical factors and thoseof growth-regulating substances extracted from or applied tobuds of fruit trees it is thought that the second factor maybe a gibberellin-like substance, and the bud-scale factor agrowth inhibitor. |
