Ovary, Fruit, and Seed Morphology of the Lythraceae

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Abstract

Premise of research. The ovary, fruits, and seeds of the Lythraceae present an array of morphological characters that have not been fully explored for their utility in clarifying taxonomic, phylogenetic, and historical biogeographic questions. This study provides a morphological record of ovary, fruit, and seed characters for the family that can be used to address these questions. The seeds, in particular, display diverse adaptations to dispersal and are emphasized in the study. A more complete knowledge of the fruits and seeds of the Lythraceae further promises to aid identification of lythracean fossil fruit and seed remains from late Cretaceous and Cenozoic floras around the world.Methodology. The morphology of alcohol-preserved and softened dried ovaries, fruits, and seeds is described based on hand sections studied using LM. SEM further details the external features of mature seeds of all genera of the family. Selected character states are shown on a previously produced molecular-based phylogeny to examine distribution of adaptations that influenced the radiation and diversification of the family.Pivotal results. Diversity in the mostly superior ovary of the family includes forms with elongated or globose axile placentas, ovarian stipes, and differing degrees of septal development. Tropical tree genera show tendencies toward an inferior ovary; semi-inferior to fully inferior ovaries occur in Punica and Trapa. The most common fruit type, occurring in 10 of the 28 genera, is a dry, thin- to thick-walled capsule of two to five locules that splits irregularly to free the seed. Other capsules open loculicidally, septicidally, by a circumscissile opening, or are indehiscent. Capuronia, Punica, and Sonneratia produce dissimilar indehiscent berrylike fruits; a drupe is unique to Trapa. Seed size in the Lythraceae ranges widely, with 18 of 28 genera producing seeds less than 1 mm long. The largest seeds are winged seeds in Lagerstroemia (to 1.6 cm) and Lafoensia (to 3.5 cm). Seed shape is obpyramidal to bilateral, and seed coats, composed mainly of polygonal or rectangular cells, are smooth surfaced with a reticulate or scalariform pattern. Aerenchymatous float tissue, wings, and unique inverted mucilaginous trichomes in the seed coat that evaginate upon wetting to form long “hairs” are specialized features of adaptive significance for dispersal, germination, and establishment.Conclusions. Ovaries, fruits, and especially the seeds are morphologically well diversified in the Lythraceae. Character states are phylogenetically autapomorphic on terminal branches or homoplastic and do not support the major lineages or clades. The relatively extensive fossil record of the family, including seeds of Decodon (73.5 Ma) and pollen of Lythrum/Peplis (82–81 Ma), suggests early adaptations of seeds for aquatic dispersal by development of extensive float tissue (Decodon) and for enhanced dispersal and germination by inverted mucilaginous seed coat trichomes (inferred from extant Lythrum/Peplis). The adaptations are responsible in large part for the present global distribution of the family.

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