Isoetes mexicana Underw.
Descripción
Rootstocks bilobed; leaves ca. 20, ca. 25 cm long, pale toward bases, half-terete in mid-length transection; velum covering less than 25% of each sporangium; sporangial walls unspotted; megaspores white, average 275–375 µm in diam., smooth or nearly so with girdle smooth; microspores light gray in mass, average 25–35 µm long, echinate-tuberculate; 2n=22 (Dgo, Jal).A
Ejemplar revisado
Chih (Pringle 1713, G, MO, UC). Dgo (Breedlove 18848, NY; McVaugh 21725, MICH, NY). Gto (McVaugh 24072, MEXU, MICH, NY). Hgo (Pringle 8796, BM, C, E, G, M, MO, MEXU, NY, PH, UC, US, VT; Rzedowski 36110, ENCB). Jal McVaugh 17600, CAS). Mich (González 260, ENCB, MEXU, NY). Qro (Rzedowski & McVaugh 483, MEXU, MICH, NY).A
Distribución
México (país) EndémicoA
Elevación
1750 – 2500 mA
Ecología y Hábitat
Ponds, streams, grassy meadows in seasonally wet, September–November.A
Tipo de vegetación
(oak-pine woodlands) Bosque de otro tipoA
Categoría IUCN
No incluidaB
Categoría NOM-059-SEMARNAT-2010
No incluidaC
Discusión taxonómica
Isoëtes mexicana and I. montezumae are similar in general appearance. However, most collections of I. mexicana can by distinguished by their smooth, shiny, porcelain-like megaspores averaging less than 375 µm in diameter, each contained by an unspotted sporangial wall. Occasionally, megaspores of I. mexicana are obscurely tuberculate. In contrast, I. montezumae has roughened, usually lusterless, obscurely to boldly tuberculate megaspores that average more than 375 µm in diameter, each contained by a spotted sporangial wall. Sometimes, megaspores of I. montezumae lack apparent tubercles, but spore diameter will usually distinguish these smooth-spored individuals from I. mexicana. In addition, according to their original descriptions, microspores of I. mexicana average less than 35 µm in length, whereas those of I. montezumae will average more than 35 µm in length. When making these distinctions, it is important to use mature spores.
Study of herbarium specimens indicates that Isoëtes mexicana and I. montezumae could be treated as a single variable species or two intergrading species. There are collections of these two taxa that are easily assignable to either I. mexicana or I. montezumae, but there are also many collections of specimens with intergrading characters that are difficult to place. However, within the overlapping ranges of I. mexicana and I. montezumae there are specimens bearing irregular spores that vary in size and shape. Production of irregular spores is a characteristic of interspecific hybrids. Collections of what appear to be interspecific hybrids within the overlapping ranges of these taxa are evidence that I. mexicana and I. montezumae are genetically divergent to an extent that they can be treated as two species.A
Study of herbarium specimens indicates that Isoëtes mexicana and I. montezumae could be treated as a single variable species or two intergrading species. There are collections of these two taxa that are easily assignable to either I. mexicana or I. montezumae, but there are also many collections of specimens with intergrading characters that are difficult to place. However, within the overlapping ranges of I. mexicana and I. montezumae there are specimens bearing irregular spores that vary in size and shape. Production of irregular spores is a characteristic of interspecific hybrids. Collections of what appear to be interspecific hybrids within the overlapping ranges of these taxa are evidence that I. mexicana and I. montezumae are genetically divergent to an extent that they can be treated as two species.A