Phylogenetic trends in the Alismatales with reference to pollen grains

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Abstract
The pollen morphology of Butomaceae, Limnocharitaceae and Alismataceae, the three families comprising the Alismatales, was studied using light and scanning electron microscopy. Pollen grains of the monotypic Butomaceae are monosulcate and boat‐shaped, those of Limnocharitaceae have 4–10 ill‐defined, fimbriate, globally distributed pores, and those of most Alismataceae are pantoporate and spheroidal, or rounded polyhedral. However, Alisma oligococcum has unusual 2‐porate, lens‐shaped pollen grains, indicating an isolated position in the family so far as apertures are concerned. Pollen morphological characters of the order were analysed using cladistic methods and a hypothesis for pollen evolution in the Alismatales is presented. Monosulcate pollen is clearly primitive in the order, as amongst monocotyledons in general, whilst derived pollen grains possess higher numbers of pores. Fimbriate aperture margins, areas of circumporal ornamentation and sunken apertures are also considered derived. The general trend towards higher aperture numbers parallels aperture evolution amongst certain dicotyledons.