CLEVINGER, JENNIFER A. AND ELIZABETH M. HARRIS.* Department of Biology, James Madison University, Harrisonburg, VA 22807, Department of Evolution, Ecology and Organismal Biology, Ohio State University Herbarium, Columbus, OH 43212. - Floral primordia complexes in the Engelmanniinae (Asteraceae: Heliantheae).
Asteraceae inflorescence meristems are physically constrained as to
the position of the individual floral meristems so that the floral
primordia are tightly packed. New floral meristems are generally
initiated in regular phyllotactic spirals at an angle of 137.5 degrees
that converge at the apex of the inflorescence as shown by several
recent studies that have plotted and quantified the positioning of
floral primordia on simpler Asteraceae models. The models are not
absolute for all taxa in the Asteraceae, however. Certain basal taxa
in the tribe Heliantheae display a marked deviation from the expected
pattern of packing. Most of the taxa of the subtribe Engelmanniinae
(sensu Clevinger and Panero), including Engelmannia,
Berlandiera, Chrysogonum, and Lindheimera produce
peripheral multi-meristem complexes that consist of one ray flower
primoridium, two or more disk flower primordia as well as the
associated involucral and receptacular bract primordia. These
primordia complexes act as coordinated units as far as timing of
initiation and development is concerned and also display distinct
heterochronic development when compared to their closest neighbor
floral and bract primordia. They remain evident at anthesis and
beyond as cypsela complexes that presumably aid in dispersal. Some
(but not all) species of the terminal taxon of the Engelmanniinae,
Silphium, possess similar primordia complexes but do not go on
to develop cypsela complexes. The trait is completely lacking in some
of the more basal members of the subtribe: Dugesia,
Wyethia, and Borrichia.
Key words: cypsela complex, Engelmanniinae, inflorescence bauplan, phyllotactic spirals, primordia complexes