Influence of Leaf Type and Plant Age on Leaf Structure and Sclerophylly in Hakea (Proteaceae)

Abstract

Hakea Schrader (Proteaceae) species possess one of two contrasting leaf morphologies—broad or terete. Terete leaves are either simple (needle-like) or two- or three-pronged, and are further characterised by their greater thickness (> 1 mm), smaller projected area and mass, higher mass per unit area (a measure of sclerophylly) and lower density than broad leaves. Broad leaves are much more variable in their morphology, ranging from narrow-linear to fan-shaped or ovoid-elliptic, and may be flat, undulate, shell-shaped or spiralled. The greater thickness and sclerophylly of terete leaves can be partially attributed to the presence of a prominent, thick-walled parenchyma core and increased palisade thickness. The core contains a compact conduit of fibre-capped vascular bundles. The sclerophyllous nature of broad leaves is due to their high density, attributable to their thin palisade and large fibre caps surrounding the main vascular bundles. Both leaf types have a thick cuticle (> 20 µm) in mature plants, and sunken stomates, with terete leaves possessing a greater stomatal density than broad leaves. Both leaf types are isolateral and hence amphistomatous. Within a species, adult and seedling leaves having a similar leaf type differ morphologically rather than anatomically, with an overall increase in leaf thickness and higher levels of sclerophylly in adult leaves. Some species produce broad seedling leaves that are eventually replaced by terete adult leaves.