Robin B. Huck:

Systematics and evolution of Dicerandra (Labiatae)

1987. 344 pages, 70 figures, 22 tables, 850 g
Language: English

(Phanerogamarum Monographiae, Volume 19)

ISBN 978-3-443-78001-2, paperback, price: 82.00 €

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Dicerandra is a genus endemic to the southeastern coastal plain of the United States. Historically, generic delimitation has been based on the presence of appendages on the anthers, described as «spurs» by Bentham, and in this study that circumscription has been maintained. Apparent pollinators are the Apidae, although in extreme ranges there is a shift to the Halictidae, Bombyliidae and Sphingidae. Data from compatibility, self-compatibility and apomictic studies confirm the obligate nature of outcrossing in Dicerandra and the adaptiveness of the spur as a trigger mechanism in pollen dispersal. Epidermal cell shape, arrangement and ornamentation of anther spurs as revealed by SEM correlate with gross morphologies. In the two major complexes of the genus, exserted stamens in flag blossom corollas are oriented ventrally for sternotribic pollination whereas inserted stamens in cucullate-lobed corollas are positioned dorsally for nototribic pollinations. However, high F1 pollen viability percentages obtained in crossing experiments, as well as the discovery of natural hybrids, imply a close relationship between these two complexes.Dicerandra grows in Quercus laevis woodlands and Pinus clausa-scrub oak woodlands in Georgia and Florida on dry sandy soils classified as quartzipsamments. Two species have radiated into more mesic environments dominated by Quercus laurifolia-Q. virginiana. Hydrophilic slime cells on the exocarp of Dicerandra nutlets are theorized to aid in dispersability of fruit by water. Allopatric perennials are isolated geographically and their distant genetic relationship to annuals affirmed by data from crossing experiments. For sympatric species, pre- and postzygotic isolating mechanisms are incomplete; ultimately, linkage of genes associated with pollination syndromes is presumed to preserve species integrity. Evolution of the genus as expressed by the Wagner Groundplan Divergence Method indicates two major phyletic lines. Section Dicerandra includes the standard-lobed corolla species with exserted stamens and the new section Lecontea including D. odoratissima and D. radfordiana is established for cucullate-lobed corolla species with inserted stamens. var. robustior is delimited based on robust habit, compound cymes, wide leaves, purble corollas and reddish-brown anthers. Two previously unknown type specimens are designated holotypes for D. linearifolia and D. densiflora.