Morpho-anatomical studies on the leaf reduction in Casuarina: the ecology of xeromorphy

The ontogenetic changes in the foliage of two Casuarina species were investigated. While the cotyledons are flattened linear structures, all other leaf-types are strongly reduced. Except for the two primary leaves, all subsequent leaves are strongly fused to each other and also to the shoot axis, except for the leaf tips; the shoot axis is completely surrounded by photosynthetic leaf tissue and the branchlet is not made up of cladodes but of extended leaf sheaths which are a novel strategy for achieving reduced photosynthetic area. In seedlings there are four leaves per node, forming four shallow vertical furrows where light-exposed and non-encrypted stomata are developed. These features are also developed in the adult foliage within the strictly scleromorphic genus Gymnostoma, clearly the most mesic of the present day genera of Casuarinaceae and very likely to include the ancestral types. Thus, we assume that the Casuarina-seedling leaves reflect the ancestral scleromorphic condition. In the adult foliage, the number of leaves per node is strongly increased, which leads to the formation of several nearly closed vertical furrows on the shoot, where stomata are shaded and strongly encrypted. Thus, the adult foliage shows several xeromorphic features that are absent in the juvenile foliage. Our morpho-anatomical data mapped on ecological and palaeobotanical data show that within Casuarinaceae the foliage shifted from scleromorphic to xeromorphic. Thus, the adult xeromorphic foliage in Casuarina is the derived, advanced state.