Foliar characters were studied in 25 European, North African, North American, and South American terrestrial, amphibious, and aquatic taxa of the genus Isoetes L. (Pteridophyta, Isoetaceae). The foliar characters investigated have a variable diagnostic value. The presence and types of cuticular ornamentations, types of margins, epidermal patterns and cell ratios of the epidermis, types of IPP (intercellular pectic protuberances) in the cells of the leaf diaphragms, epidermal patterns and cell ratios of velum, epidermal patterns, and nature of walls in the adaxial epidermis of sporangia may be good diagnostic specific characters that should be included in taxonomic descriptions. Transverse sections of the microphylls, the types and distribution of collenchyma (fascicular, lacunar, or partially laminar collenchyma, with the latter two types described here for the first time), the cell types of leaf diaphragms, the presence of IPP in these cells, the partial or complete endodermis in the air chambers, and the presence of a persistent mucilage-secreting or an early lignified ligule are features that characterize species groups in Isoetes. The mucilage-secreting ligula and the types of endodermis can be considered as generic traits in Isoetes. The genus was largely considered as lacking good diagnostic characters or having rather environmentally influenced traits, two concepts that are discussed here after presentation of the new evidence.
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Vol. 94 • No. 1