English description top ↑
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.