Origin and Development of Secondary Capitula in Calendula officinalis L. in Response to Gibberellic Acid

Foliar application of gibberellic acid greatly enhanced theformation of secondary capitula. The proportion of primary capitulashowing this feature increased from 6% in the controls to 28,58, and 54% at 100, 250, and 500 parts 10^–6 GA₃, respectively.Secondary capitula were initiated either along the peripheryor in the centre of the receptacle or from both; their totalnumbers for 15 plants per treatment were 53, 215, 660, and 404in response to 0, 100, 250, and 500 parts 10^–6 GA₃, respectively.Secondary capitula were smaller and had fewer disc florets andfruits, particularly of the ‘wingless and small’type. Tertiary capitula, not observed in nature, formed on 2.72and 3.21% of the secondary capitula on plants sprayed with 250and 500 parts 10^–6 GA₃.The formation of a secondary inflorescencemeristem in treated plants in place of a floret primordium canbe detected histologically early in its development, and itspattern of development resembles that of the primary capitulum.