Wood Anatomy of Onagraceae, with Notes on Alternative Modes of Photosynthate Movement in Dicotyledon Woods

Abstract

Vessel elements in Onagraceae correlate perfectly with groups of species; elements are long and wide in mesomorphic species, shortest and narrowest in the most xeromorphic species. Libriform fibers in the family are thin-walled, but many have gelatinous inner walls. The possibility that these represent not tension wood but a water-storage mechanism is examined. Libriform fibers are mostly septate or nucleate or both in the family; this indicates longevity and simulation of parenchyma in starch-storage function. These fibers may compensate for the paucity of axial parenchyma. Interxylary phloem ("included phloem") does not occur in Fuchsia, Hauya and Ludwigia, the genera in the family most generalized in other respects, and is absent in most annuals studied. Selective pressure for formation of interxylary phloem in the 3 genera may be minimal because of slow rates of photosynthate translocation within wood and selective pressure for formation of interxylary phloem in annuals may be minimal for spatial reasons. Interxylary phloem may be related to massive flowering that draws rapidly on stored starch, chiefly in the shrubby perennials. The relationships of Onagraceae seem closest to Lythraceae, Sonneratiaceae, Punicaceae, Crypteroniaceae and Combretaceae; also close are other myrtalean families: Melastomaceae, Myrtaceae, Penaeaceae, Oliniaceae. These affinities are clearly evident from wood features alone: vestured pits on vessels; ray cells upright to square; intraxylary phloem present adjacent to pith; libriform fibers septate or nucleate; prismatic crystals in fibers and rays. Onagraceae tend to show herbaceous characteristics in wood of herbaceous genera; woody genera such as Hauya show no indicators of herbaceous structure. The ancestral habit of Onagraceae was probably shrubby, and without interxylary phloem; interxylary phloem may have evolved more than once in the family.