Vegetative morphology and anatomy ofMaundia(Maundiaceae: Alismatales) and patterns of peripheral bundle orientation in angiosperm leaves with three-dimensional venation

Abstract

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.

A new anatomical interpretation for abaxialization in unifacial leaf blade of stone plants (Aizoaceae, Caryophyllales)

“…Thus, in some species, photosynthesis and water storage occur in the same tissue (''all cell succulents''), whereas others may exhibit a subepidermal chlorenchyma encircling the inner water storage parenchyma (''storage succulents'') (Ihlenfeldt 1985). Variations can also be found in the organization of the vascular system, while the vascular system of usual flat bifacial leaves present xylem poles usually oriented toward the leaf adaxial side and phloem poles toward its abaxial side (Eames and MacDaniels 1947;Esau 1960;Kaplan 2001); the vascular system in terete leaves frequently exhibits a 3D organization, with small vascular bundles presenting a radial distribution along the leaf periphery (Ogburn & Edwards 2013;Hernandes-Lopes et al 2016;Melo-de-Pinna et al 2016;Platonova et al 2016). In these terete leaves, a larger central vascular bundle may be present in some cases (as seen in Aizoaceae), or the main vascular bundle can have a peripheral position, occurring between the peripheral vascular bundles (PVBs) and occupying an abaxial position as in some monocots with terete leaves (Melo- de-Pinna et al 2016;Platonova et al 2016).…”

Towards stable classifications

“…Bone, Cribb & Buerki, ; Chase et al ., ). Finally, in this issue, we present papers on fungi and lichens (Auxier et al ., ; Widhelm et al ., ), Alismatales (Platonova et al ., ; Volkova et al ., ), Amaryllidaceae (Souza et al ., ), Ranunculaceae (Lehtonen, Christenhusz & Falck, ), Malvales (Aubriot, Singh & Knapp, 2016b; Aubriot et al ., 2016b) and Malpighiales (Ghislain et al ., ). These and other similar papers are leading towards ever more stable classifications.…”