For the most part, the flowers of the 2015 VNPS Wildflower of the Year, Clethra alnifolia (Sweet Pepperbush), are unremarkable. Five separate sepals, 5 sepa rate petals, 10 stamens in 2 whorls, and a 3-carpellate superior ovary—an organization that can only be considered prosaic among the dicots. One floral feature, however, stands out: the anthers in the open flowers are upside-down! (See Figure 1A.) Further, these upside-down anthers open by pores (Figures 1B, 1C) rather than longitudinal slits, as in most flowering plants. These pores initially form on what would normally be the lowermost extremity of the anther, the inversion of which brings the pores to a forward position in the open flower. The significance of these acrobatic anthers—found not just in Sweet Pepperbush but through all 65 members of the genus Clethra—is twofold: anther inversion is essential to the plant’s pollen-presentation strategy, and the odd morphology taxonomically links Clethra with the large and diverse heath family (Ericaceae).
Copyright © 2015, Virginia Native Plant Society. This article first appeared in Sempervirens (Spring 2015), 1,3.
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Hayden, W. John. "Upside-down Anthers of Clethra Stand Out." Sempervirens, Spring 2015, 1, 3.