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Morphology of Hydatellaceae, an anomalous aquatic family recently recognized as an early-divergent angiosperm lineage¹

Jodrell Laboratory, Royal Botanic Gardens, Kew, Richmond, Surrey TW9 3AB, UK; Department of Higher Plants, Biological Faculty, Moscow State University, 119992, Moscow, Russia; CEBB, EB/EES, Benham Building, DP312, University of Adelaide, Adelaide, SA 5005, Australia; L. H. Bailey Hortorium and Department of Plant Biology, Cornell University, Ithaca, New York 14853 USA; CALM, c/o Manjimup Research Centre, Brain Street, 6258 Manjimup, WA, Australia; New York Botanical Garden, Bronx, New York 10458 USA

ABSTRACT

The family Hydatellaceae was recently reassigned to the early-divergent angiosperm order Nymphaeales rather than the monocot order Poales. This dramatic taxonomic adjustment allows comparison with other early-divergent angiosperms, both extant and extinct. Hydatellaceae possess some monocot-like features that could represent adaptations to an aquatic habit. Ecophysiological parallels can also be drawn from fossil taxa that are known from small achene-like diaspores, as in Hydatellaceae. Reproductive units of Hydatellaceae consist of perianthlike bracts enclosing several pistils and/or stamens. In species with bisexual reproductive units, a single unit resembles an "inside-out" flower, in which stamens are surrounded by carpels that are initiated centrifugally. Furthermore, involucre development in Trithuria submersa, with delayed growth of second whorl bracts, resembles similar delayed development of the second perianth whorl in Cabomba. Several hypotheses on the homologies of reproductive units in Hydatellaceae are explored. Currently, the most plausible interpretation is that each reproductive unit represents an aggregation of reduced unisexual apetalous flowers, which are thus very different from flowers of Nymphaeales. Each pistil in Hydatellaceae is morphologically and developmentally consistent with a solitary ascidiate carpel. However, ascidiate carpel development, consistent with placement in Nymphaeales, is closely similar to pseudomonomerous pistil development as in Poaes.

Key Words: angiosperm • flower • Hydatella • Hydatellaceae • inflorescence • ontogeny • pseudanthium • Trithuria

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