Embryo and seedling morphology in T rithuria lanterna ( H ydatellaceae, N ymphaeales): new data for infrafamilial systematics and a novel type of syncotyly

The monogeneric early‐divergent angiosperm family H ydatellaceae ( T rithuria ) was formerly placed in the monocots and shows several features that are at least superficially monocot‐like. Seedlings of H ydatellaceae are unusually diverse and have been interpreted as either dicotylar or monocotylar. We provide the first detailed developmental description of seedlings of T rithuria lanterna (including the first data on mature embryos of tropical H ydatellaceae) as a basis for the general discussion of seedling diversity in H ydatellaceae. Seedlings at various stages after germination were studied using serial sections and scanning electron microscopy. The embryo is dicotylar. It lacks pronounced asymmetry and lacks a plumule before seed germination. In the majority of seedlings, the cotyledons are free and appear attached to the seedling axis at different levels. In other seedlings, the cotyledons are united via a non‐haustorial leaf‐like organ; this alternative condition represents a novel type of syncotyly for seed plants and a second type of syncotyly recorded for H ydatellaceae. Seedling morphology is determined by strong one‐sided growth of the hypocotyl, which is an unusual way of overcoming the basic seed plant spatial constraint at germination. The direction of one‐sided growth is independent of cotyledon orientation and could be environmentally determined. Seedlings provide synapomorphies for the two major clades of T rithuria , which can be regarded as subgenera. Although no direct homology is inferred, the exceptional degree of morphological variation in H ydatellaceae seedlings, including the variable occurrence of several superficially monocot‐like features, leads us to hypothesize that the stem group of monocots could have exhibited an analogous degree of variation in cotyledon morphology. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society , 2014, 174 , 551–573.