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Collateral bundles with external position of the phloem characterize the stem vasculature of most seed plants. An earlier study highlighted the occurrence of inverted peripheral bundles in the leafless inflorescence peduncle of the rare Australian aquatic Maundia triglochinoides. This unusual feature and other morphological and molecular data supported the recognition of the monogeneric Maundiaceae, but the anatomy of the leaves, rhizomes and roots of Maundia remained unknown and is studied here. Maundia has an iterative sympodial growth with all shoots bearing five tubular cataphylls splitting longitudinally and simulating open sheaths at maturity and two (or three) linear foliage leaves without a conspicuous basal sheath. This morphology distinguishes Maundiaceae from all other Alismatales. The rhizome has an atactostele with collateral bundles of normal orientation; peripheral bundles are absent. Cataphylls have a series of normally oriented bundles. Foliage leaves are thick, bifacial, semi-elliptical in cross-section, with a thin subepidermal layer of chlorenchyma on both sides, accompanied by peripheral bundles with xylem facing outwards (thus abaxial peripheral bundles are inverted) and central large bundles of normal orientation. Strong anatomical similarity between leaves and peduncles is related to their shared function as assimilatory organs. As in angiosperm succulents, the three-dimensional leaf venation in thick aquatic and helophyte leaves of Alismatales serves to reduce transport distances between veins and photosynthetic cells. In both cases, the patterns of orientation of peripheral bundles (with inverted adaxial or abaxial bundles) are unstable in large clades. These slender bundles cannot be used for the identification of unifacial leaves. Some anatomical characters express homoplastic similarities between Maundiaceae and Aponogetonaceae. |
