Isoetes
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Authority
Mickel, John T. & Smith, Alan R. 2004. The pteridophytes of Mexico. Mem. New York Bot. Gard. 88: 1-1054.
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Family
Isoetaceae
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Scientific Name
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Description
Genus Description - Aquatic, amphibious, or seasonally wet-terrestrial, grass-like, tufted; rootstocks buried, subglobose, usually 1-2 cm in diam., 2- to 3-lobed, with sclerified scales present or absent; roots arising along grooves between rootstock lobes, simple or dichotomously branched, in transection containing an eccentric vascular strand and surrounding lacuna; leaves linear, mostly 5-50 cm long, stiff to pliant, erect to reclining or recurved, mostly 1-1.5 mm wide at mid-length, bright green to olive green, attenuate to apices, in transection terete to triangular and containing a central vascular strand surrounded by 4 longitudinal and transversely septate lacunae; ligules triangular, membranous, inserted above sporangia; velum partly or entirely covering adaxial wall of each sporangium; sporangia ovoid, solitary, embedded in leaf bases; spores of two kinds (plants heterosporous); megaspores white to dark gray, tetrahedral-globose, mostly 250-750 µmin diam., tens to hundreds per megasporangium; microspores light gray or brown in mass, ovoid, monolete, mostly 25-50 µm long, thousands per microsporangium; megagametophytes multicellular, white, endosporic, exposed when megaspores open along trilete ridges, with archegonia located by quartets of brownish neck cells; microgametophytes 9-celled, endosporic, each with single antheridium releasing 4 multiflagellate spermatozoids; x=II.
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Discussion
Type: Isoe¨tes lacustris L.
Isoe¨tes is a worldwide genus of more than 200 species. Species are difficult to identify because their morphology yields few characters that easily distinguish species. Spore size, ornamentation, and color, velum cover, and sporangial wall coloration provide character states that will usually distinguish species. A microscope fitted with an ocular micrometer has been used to measure spore size. At least 10 spores were measured to determine their average diameter. Megaspores were measured dry on a glass slide. Microspores were hydrated and measured on a slide in a drop of water under a cover slip. A 10x to 20x hand lens or dissecting microscope is needed to resolve megaspore texture. Normally megaspores are globose and boldly marked with an equatorial ridge encircling the spore and three ridges converging at the spore’s proximal pole. Between these ridges spores may be tuberculate, pustulate, rugate, cristate, verrucate, echinate, or smooth. The girdle, a zone along the distal side of the equatorial ridge, may be textured differently than the rest of the spore. Plants collected at the end of their growing season with mature spores still within sporangia are easiest to identify. Plants collected early in their growing season with small, fragile, immature megaspores are difficult to identify. Mature spores from the previous growing season may be found in decaying leaf bases or in soil around the rootstocks. The velum is a thin flap of tissue that entirely or partly covers the adaxial wall of each sporangium. Isoe¨tes orcuttii has a velum that completely covers the adaxial wall of the sporangium. The velum of the other Mexican species covers less than 50% of the adaxial wall of each sporangium. Sporangial wall coloration appears as spots or streaks of brown colored cells on the semi-transparent sporangial wall. The sporangial walls of Isoe¨tes howellii and I. montezumae usually have brown spots or streaks. The sporangial wall cells of other Mexican species are clear, without brown spots or streaks. Chromosome counts and the production of malformed spores in interspecific hybrids indicate that Isoe¨tes species have evolved in two ways, either by ecological isolation and genetic divergence without increase in chromosome number, or by interspecific hybridization and chromosome doubling. Recognition of basic diploid species with uniform spores and chromosome counts of 2n=22, sterile interspecific hybrids with irregular spores, and allopolyploids with uniform spores and chromosome counts that are multiples of 22, helps in distinguishing Isoe¨tes species. Many Mexican collections of Isoe¨tes show some irregularity in spore size and shape, indicating they could be interspecific hybrids. Interspecific hybrids are recognized by the production of irregular spores that vary in size, shape, and texture. They are usually found where two species occur together. In addition, basic diploid (2n=22) and tetraploid (2n=44) chromosome counts have been reported for Mexican species. Basic diploid species generally have a smaller average spore size than tetraploid species.