Leaf anatomy of tropical fern rheophytes, with its evolutionary and ecological implications

Abstract

Rheophytes are restricted to stream beds that are regularly flooded by swift-running water after rains and are morphologically characterized by having narrow, (ob)lanceolate leaves – leaflets (stenophylls) and other features that are adapted to the unique habitat, decreasing resistance to the swift-running water. The present study characterized anatomically the leaves of fern rheophytes. Generally, the mesophyll cells of rheophytes are less expanded, and therefore their intercellular spaces are smaller than those of related dryland species. Furthermore, frequency of occurrence of stomata per unit leaf area is greater, the cuticular layer is thicker, and the epicuticular wax deposits on the leaf epidermis are denser in at least some rheophytes than in related dryland species. It can be assumed that the stenophylls of the rheophytes are produced by developmental events, including weaker cell expansion than in dryland species, and that a phylogenetic decrease in cell expansion in leaves was involved in the origin of stenophylls from broader leaves of ancestral dryland species. The leaf anatomical features are discussed in relation to the ecological specificity of the rheophytes. Key words: evolution, flood resistance, intercellular space, leaf anatomy, rheophytic ferns, stenophyll.