Leaves and epiphyllous shoots in Chisocheton (Meliaceae): a continuum of woody leaf and stem axes

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Abstract
The pinnately compound leaves of Chisocheton tenuis, a small tree from Papua New Guinea, exhibit indeterminate growth and periodically produce new pinnae from a leaf tip bud. Inflorescences and vegetative shoots arise from epiphyllous buds on the adaxial surface of the rachis between the pinna pairs. Axillary buds occur on the stem but are always vegetative. The structure and ontogeny of leaves, axillary buds, and epiphyllous buds are documented with sections and scanning electron microscopy. Although epiphyllous inflorescences are described from herbarium specimens of C. tenuis, only vegetative shoots were collected as epiphyllous outgrowths. These epiphyllous shoots formed woody stem and rachis axes similar to the stem and rachis on the original shoot. Epiphyllous buds first appear on a leaf primordium without evidence of an ontogenetic displacement from an earlier axillary site. Later, epiphyllous buds and pinnae arise in an acropetal order close to the meristem at the tip of the leaf. Epiphyllous inflorescences in Chisocheton pohlianus are described from herbarium material. Leaf and bud structure and ontogeny in a related species, Chisocheton montanus, which lacks epiphyllous buds and has axillary inflorescences, are similar to C. tenuis except that no meristems occur on the rachis. Possible morphological interpretations for these examples of unusual organography are presented. Epiphyllous buds and leaf tip meristems are examples of heterotopy.

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