Home / Anomalous_Items / Aquatic_Macrophytes / Emergent_Leaves / Isoetes
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Name derivation:
Classification:
Isoetes L.; there are 50 species descriptions (and 6 varieties) currently listed by the USDA Natural Resources Conservation Service occurring in the United States, and ~150 species recognized worldwide (Taylor et al. 1993). Class Lycopodiopsida; Order Isoetales The plant form, with narrow elongate hollow leaves emanating from a basal corm, is readily distinguishable from all other plant families. Identification to species requires microscopic description of the macrospores.
Evolution:
Isoetes is in a class (Isoetales) within the lycopsids, the earliest group of plants to have vascularization (xylem, phloem). They have microphylls – early ‘true leaves’ that have a single vascular strand. Early fossils from Australia have been identified as the earliest known Isoetes species, from the early Triassic Period, ~250 million years ago. Isoetes is the most primitive of all lycopsids, as it has undifferentiated sporophylls, and the ancestor of all quillworts. All quillworts except Isoetes itself are now extinct , so it has survived as a genus for an exceptionally long period of time. I. beestonii is the ancestral species (Retallack 1997).
Morphology:
Quill-shaped leaves of the dominant and photosynthetic sporophyte are microphylls, and some are sporophylls bearing sporangia at their base. The genus is heterosporous and monoceious, producing both female megaspores (~ 350-450 µm diameter) on outer sporophylls, and male microspores (~ 21-27 µm diameter) on more inner sporophylls, on the same plant. Diminutive gametophytes are non-photosynthetic and are supplied with abundant nutrients from the spores. At least some species utilize soil carbon dioxide as a partial (Richardson et al.) or entire source exclusive of atmospheric carbon, unusual in vascular plants. Submerged leaves of some species utilize ony carbon dioxide from surrounding water, even using CAM to do so at night when not competing with other aquatic flora (Sandquist and Keeley 1990);. Many photographs, SEM images, habitat and distribution notes of several species of Isoetes by Lytton John Musselman are available online.
Similar genera:
Habitat:
Variable from submerged to emergent foliage, and from freshwater to damp terrestrial habitats.
References:
Richardson, K., H. Griffiths, m.lo. Reed, J.A. Raven, and N.M. Griffiths 1984. Inorganic carbon assimilation in the Isoetids, Isoetes lacustris L. and Lobelia dortmanna L. Retallack, G.J. 1997. Earliest triassic origin of Isoetes and quillwort evolutionary radiation. J. Paleont. 71:500-521. Sandquist, D.R., and J.E. Keeley 1990. Carbon uptake characteristics in two high elevation populations of the aquatic CAM plant Isoetes bolanderi (Isoetaceae). Amer. J. Bot. 77:682-688. Taylor, W. Carl; Neil T. Luebke; Donald M. Britton; R. James Hickey; & Daniel F. Brunton 1993. Isoëtaceae. Flora of North America 2. Oxford University Press