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Anatomy and Morphology |
Institute of Systematic Botany, University of Zurich, Zollikerstrasse 107, 8008 Zurich, Switzerland
ABSTRACT
The buzz-pollinated genus Senna (Leguminosae) is outstanding for including species with monosymmetric flowers and species with diverse asymmetric, enantiomorphic (enantiostylous) flowers. To recognize patterns of homology, we dissected the floral symmetry character complex and explored corolla morphology in 60 Senna species and studied floral development of four enantiomorphic species. The asymmetry morph of a flower is correlated with the direction of spiral calyx aestivation. We recognized five patterns of floral asymmetry, resulting from different combinations of six structural elements: deflection of the carpel, deflection of the median abaxial stamen, deflection or modification in size of one lateral abaxial stamen, and modification in shape and size of one or both lower petals. Prominent corolla asymmetry begins in the earl-stage bud (unequal development of lower petals). Androecium asymmetry begins either in the midstage bud (unequal development of thecae in median abaxial stamen; twisting of androecium) or at anthesis (stamen deflection). Gynoecium asymmetry begins in early bud (primordium off the median plane, ventral slit laterally oriented) or midstage to late bud (carpel deflection). In enantiostylous flowers, pronouncedly concave and robust petals of both monosymmetric and asymmetric corollas likely function to ricochet and direct pollen flow during buzz pollination. Occurrence of particular combinations of structural elements of floral symmetry in the subclades is shown.
Key Words: buzz pollination • enantiomorphy • enantiostyly • floral asymmetry • floral development • functional morphology • homology • petal venation
Received for publication 23 May 2007. Accepted for publication 8 November 2007.
FOOTNOTES
1 The authors thank the curators of the cited herbaria; I. Alvárez, A. Conceição, G. Flores, G. López, L. Paganucci de Queiroz, M. Quintana, and R. Vanni for valuable help in the field; the Australian National Botanic Gardens (ANBG), the Botanic Garden of the Canton Ticino (Switzerland), and the Botanic Gardens of the Universities of Munich, Basel, and Zurich for providing plant material for this study; U. Jauch for assistance with the SEM; people of the Endress laboratory for assistance with microtome sectioning; the ANBG and M. Belgrano for providing photographs of flowers of three Senna species; A. Bruneau and two anonymous reviewers for comments on the manuscript; Q.C.B. Cronk for information on Gesneriaceae; and the Georges-und-Antoine-Claraz-Schenkung (University of Zurich) and the Kommission für Reisestipendien der Schweizerischen Akademie der Naturwissenschaften for financial support of the collecting trips.
2 Author for correspondence (e-mail: brigitte.marazzi{at}systbot.uzh.ch)
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