Abstract
A hypothesis in the literature suggests that apical size is one of the critical determinants of the number of organs a flower will form. Genetically selected and control lines of Linanthus androsaecus ssp. androsaecus (Polemoniaceae) were used to test this hypothesis. Plants in the selection up (SU) line of this species produce flowers with greater than the normal number of corolla lobes, and plants in the selection down (SD) line produce less than the normal number of corolla lobes. Apices from lateral branches collected from plants just prior to flowering differed in size among these two lines and controls. Plants in the SU line showed an increase in apical diameter in median section while plants in the SD line showed a decrease in apical diameter in comparison to controls. SU plants may be flowering later, and SD plants earlier, than controls. SU and control apices from the main stem, collected from plants in a late stage of vegetative growth, also differed in apical diameter. Increase in the number of apical cells is associated with increased size in SU apices in comparison to controls. Apical size differences associated with petal number appear to become established during vegetative growth by the time the first two pairs of foliage leaves are formed. The selected and control lines of L. androsaecus offer support for the hypothesis that apical size is an important factor in the determination of petal number.
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