Monographs Details: Isoetes
Authority: Mickel, John T. & Beitel, Joseph M. 1988. Pteridophyte Flora of Oaxaca, Mexico. Mem. New York Bot. Gard. 46: 1-580.
Description:Genus Description - Aquatic, amphibious, or wet-terrestrial; leaves from an erect 2- or 3-lobed corm (or rarely with a creeping rhizome, but not in ours); roots in rows in the grooves between the lobes; leaves linear, grass-like, erect to recurved or flaccid, 1-veined, with longitudinal air chambers interrupted by transverse septa; sporangia embedded in leaf bases, one per leaf, covered partially to entirely with a thin velum and with a minute, triangular ligule on the adaxial surface just distal to the sporangium; heterosporous; microsporangia (generally on the inner leaves) with thousands of monolete bilateral microspores; megasporangia with ca. 100 larger, trilete, tetrahedral-globose megaspores.
Discussion:Type: Isoetes lacustris Linnaeus.
For additional synonymy, see Pichi Sermolli (Webbia 26: 175. 1971).
Isoetes is a worldwide genus of 60-100 species, the species limits being very difficult to determine. Megaspore surface characters are generally the primary basis for classification, although characters of rhizome lobing and habit, leaf size, stomata and peripheral leaf strands are also used. Interpretation of the characters is open to much question and few characters can be used with great confidence. Five species have been reported from Mexico, but the spotty distribution, often with highly disjunct populations, the paucity of records and the subtle and often unreliable char-
acters combine to make it difficult to ascertain the actual numbers of species involved.
R. James Hickey, working on Isoetes in the neotropics, has provided the identification of the single collection from Oaxaca (in litt.) as well as supplemental material (see Hickey’s treatment of Isoetes in Stolze, 1981). The ranges of the other species to be looked for are based on specimens at NY annotated by Dr. Hickey.
Reference: Pfeiffer, N. E. 1922. Monograph of the Isoetaceae. Ann. Missouri Bot. Gard. 9: 79-232.