The foliar change from seedlings to adults in Allocasuarina (Casuarinaceae): the evolutionary and ecological aspects of leaf reduction, xeromorphy and scleromorphy

Abstract

The morpho‐anatomical structure of the adult photosynthetic organs of the Casuarinaceae is unique in plants. We examine the shoot structure of five species from Allocasuarina ‐ the most derived genus in the family. While the cotyledons of seedlings are similar to those of many other plants the subsequent leaves form a whorl of chlorenchymatic tissue fused to the cortex of the articles of segmented branchlets with only the leaf tips free at the apex of each article. Each leaf in the whorl is called a phyllichnium and consists of a vascular trace below the photosynthetic chlorenchyma with sclerenchyma developed within the mesophyll. The branchlets of Allocasuarina are considered xeromorphic as the stomata of most species are restricted to the sides of the furrows formed by the closely appressed edges of the phyllichnia. The scleromorphic foliage of the ancestral Casuarinaceae may have developed from adaptation to nutrient poor soils but was also preadapted to confer drought resistance. The narrow foliage with abundant sclerenchyma, restricted stomatal area and minimal distance from the atmosphere to xylem tissues ensured that the genus could become competitively successful on impoverished soils and be a distinctive component of arid and even mesic woodlands and forests in Australia.