Comparative leaf morphology and anatomy of six Selaginella species (Selaginellaceae, subgen. Rupestrae) with notes on xerophytic adaptations

Abstract

The morphology and anatomy of vegetative leaves and sporophylls of six isophyllous species of Mexican Selaginella (subgen. Rupestrae): S. arsenei, S. extensa, S. peruviana, S. rupincola, S. sellowii and S. wrightii are described. The six species show small size of vegetative leaves (1.82–3.22 mm long × 0.32–0.62 mm wide), and lanceolate shape. The sporophylls are ovate (1.77–2.30 mm long × 0.63–0.84 mm wide). All species have hypostomatic leaves, with a central longitudinal stomatal groove on the abaxial surface. The central area is longitudinally flanked by long epidermal cells, and these by two rows of papillate cells. Epicuticular waxes as thin films, crusts or folds are present on the abaxial epidermal cells. The mesophyll has two longitudinal rows of sclerenchyma beneath the abaxial epidermis. Thickened epidermal cell walls are also present. These features are consistent with the climatic characteristics of the xeric environments these species live in, and can be considered xeric adaptations.