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.

The foliar characters in Callitris (Callitroideae, Cupressaceae s. str.) and their evolutionary and ecological significance

“…The transfusion tissue extending from the xylem lies between this cell layer and the mesophyll cells similarly to the situation in Casuarina and Allocasuarina where the layer has been termed a “foot layer” (Dörken & Parsons ; Dörken et al. ).…”

“…) and is well developed in Allocasuarina species that have similarly reduced adult leaves (Dörken et al. ). Suggested functions of such tissue include protecting the mesophyll from excess solar radiation, reducing water loss via the lamina and preventing leaf collapse during low water potential in drought (Dörken & Lepetit ).…”

“…Each fused leaf is termed a phyllichnium (Johnson and Wilson ) These are separated by well marked furrows in which files of stomata are exclusively encrypted (Dörken et al. ) , as in C. gracilis . In Callitris the leaves are in whorls of two to four while in the Casuarinaceae there are from 4 – 20 leaves in a whorl (Wilson & Johnson ).…”

“…; Dörken & Parsons , , ; Dörken et al. , ). In addition, soil salinity, the length of the vegetative period and the temperature during the vegetative period have a strong impact on leaf size.…”

“…It can occur after years as is the case within several Cupressus species (Cupressaceae, Coniferales) or it can occur in the earliest developmental stages as is the case with all members of the angiosperm family Casuarinaceae (Fagales), where the primary leaves are already strongly reduced scales and all subsequent leaves form the phyllichnia typical of the family (Hwang & Conran ; Dörken & Parsons ; Dörken et al. ).…”