On Arborescence in Gahnia clarkei and Gahnia sieberiana

Specimens of Gahnia sieberiana from Brisbane, Queensland, and of Gahnia clarkei from near Orbost, Victoria, were collected and examined both morphologically and anatomically. The species grow in wet areas and are of interest because they represent the largest arborescent species known in the Cyperaceae. Stem diameters up to 120 mm and stems up to 10 m long have been observed. Such long stems tend to be supported by nearby vegetation. Although fresh stems are tough and woody, they are brittle. Branching of the stems is sympodial, and numerous branches are produced by plants growing in exposed habitats. There is less branching in plants from shaded habitats. Basal shoots may also occur. Adventitious roots develop basally on most plants, but with G. sieberiana, some adventitious roots form near the shoot apex and grow in and around leaf bases. Anatomical features of interest are an endodermoid layer composed of sclereids with elongate, undulated, outer tangential walls that are lignified and suberized, short vessel elements with horizontal to oblique simple perforation plates, and relatively short sclereids surrounding vessel elements in the vascular bundles. Some vascular bundles are bipolar. The presence of short vessel elements here is in marked contrast to the longer tracheary elements in other arborescent monocotyledons.