Comparative Anatomy of the Photosynthetic Organs of 39 Xeromorphic Species from Subhumid New South Wales, Australia

Approximately 215 vascular plant species have been recorded at The Rock Nature Reserve, a small island of remnant vegetation in the subhumid South Western Slopes of New South Wales. Although rainfall (585 mm yr−1) is relatively evenly distributed throughout the year, a marked water deficit occurs in summer. About 75% of the species either die (annuals) or die back (majority of the herbaceous perennials) during summer. The anatomy and surface micromorphology of the photosynthetic organs of most of the species that do not die or die back in summer were investigated. A strong correlation was found between species habit and photosynthetic organ anatomy. Of the 39 species examined, 16 were overstory species and 23 understory species. Of the overstory species, 11 possessed an isobilateral mesophyll distribution, four had an essentially centric distribution, and only one species had a dorsiventral distribution. Of the understory species, 17 were dorsiventral or inverse dorsiventral and six were isobilateral or similar. Associated with the differences in mesophyll distribution were other anatomical characteristics. All but two of the isobilateral species were amphistomatous, while all dorsiventral or inverse‐dorsiventral species were hypostomatous or hyperstomatous. In addition, the photosynthetic organs of the isobilateral species were significantly thicker (442 μm average) and had a significantly larger average number of palisade layers (5.6 layers) than the dorsiventral species (293 μm and 2.0 layers, respectively). A correlation between species distribution and anatomy was not established. Several species restricted to the well‐drained, skeletal soils of the higher regions of the reserve had a relatively mesomorphic anatomy, while many species restricted to the lower slopes had multiple characters associated with reducing water loss.