Isoetes

Isoetes
Isoetes lacustris[1]
Scientific classification
Kingdom: Plantae
Division: Lycopodiophyta
Class: Isoetopsida
Order: Isoetales
Family: Isoetaceae
Rchb.[2]
Genus: Isoetes
L.

Isoetes, commonly known as the quillworts, is a genus of plants in the class Isoetopsida and order Isoetales. They are lycopods. There are about 140–150 species, with a cosmopolitan distribution but often scarce to rare. Some botanists split the genus, separating two South American species into the genus Stylites, although molecular data place these species among other species of Isoetes, so the genus does not warrant taxonomic recognition.

The name of the genus is sometimes spelled Isoëtes. The diaeresis (two dots over the e) indicate that the o and the e are to be pronounced in two distinct syllables. Including this in print is optional; either spelling is correct.[3]

Description

Quillworts are mostly aquatic or semi-aquatic in clear ponds and slow-moving streams, though several (e.g. I. butleri, I. histrix and I. nuttallii) grow on wet ground that dries out in the summer. Quillwort leaves are hollow and quill-like, arising from a central corm. Each leaf is narrow, 2–20 centimetres (0.8–7.9 in) long (exceptionally up to 100 cm or 39 in) and 0.5–3.0 mm (0.02–0.12 in) wide; they can be either evergreen, winter deciduous, or dry-season deciduous. They broaden to a swollen base up to 5 mm (0.20 in) wide where they attach in clusters to a bulb-like, underground rhizome characteristic of most quillwort species, though a few (e.g. I. tegetiformans) form spreading mats. This swollen base also contains male and female sporangia, protected by a thin, transparent covering (velum), which is used diagnostically to help identify quillwort species. They are heterosporous. Quillwort species are very difficult to distinguish by general appearance. The best way to identify them is by examining the megaspores under a microscope.

Classification

Compared to other genera, Isoetes is poorly known. Even after studies with cytology, scanning electron microscopy, and chromatography, species are difficult to identify and their phylogeny is disputed. Vegetative characters commonly used to distinguish other genera, such as leaf length, rigidity, color, or shape are variable and depend on habitat. Most classification systems for Isoetes rely on spore characteristics, which make species identification nearly impossible without microscopy.[4]

Placed in its own family, Isoetaceae, the genus includes about 150 species worldwide.[5][6]

Selected species
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Many species, such as the Louisiana quillwort and the mat-forming quillwort, are endangered species. Several species of Isoetes are commonly called Merlin's grass, especially I. lacustris, but also the endangered species I. tegetiformans and I. virginica.

Evolution

Fossilised specimens of I. beestonii have been found in rocks dating to the early Triassic.[7] Quillworts are considered by some[7] to be the last remnant of the fossil tree Lepidodendron with which they share some unusual features including the development of both wood and bark, a modified shoot system acting as roots, bipolar growth, and an upright stance.

Lepidodendrales

Pleuromeia

Nathorstiana

Isoetes





References

  1. ↑ illustration from Otto Wilhelm Thomé Flora von Deutschland, Österreich und der Schweiz 1885, Gera, Germany
  2. ↑ Reichenbach, H. G. L. (1828). Conspectus Regni Vegetabilis. p. 43.
  3. ↑ International Code of Nomenclature for algae, fungi, and plants (Melbourne Code) see section 60.6
  4. ↑ Cody, William; Britton, Donald (1989). Ferns and Fern Allies of Canada. Agriculture Canada.
  5. ↑ 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. p. 64.
  6. ↑ Musselman, Lytton John. 2001. "Georgia quillworts". The Journal of the Georgia Botanical Society 16. 2-19, 40.
  7. 1 2 3 Retallack, G. J. (1997). "Earliest Triassic Origin of Isoetes and Quillwort Evolutionary Radiation". Journal of Paleontology 71 (3): 500–521. doi:10.2307/1306630. JSTOR 1306630.

External links

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