Isoëtes (sometimes spelled Isoetes), commonly known as the quillworts, is a genus of plants in the class Isoëtopsida and order Isoëtales. 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 Isoëtes, so the genus does not warrant taxonomic recognition.
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.
- Isoetes alpina – New Zealand quillwort
- Isoetes andicola – (syn. Stylites andicola)
- Isoetes appalachiana – Appalachian quillwort
- Isoetes australis – Australian quillwort
- Isoetes beestonii – (extinct)
- Isoetes bolanderi – Bolander's quillwort
- Isoetes brevicula
- Isoetes brochonii – Pyrenean quillwort
- Isoetes butleri – Butler's quillwort
- Isoetes caroli
- Isoetes caroliniana - Carolina quillwort
- Isoetes drummondii – Drummond's quillwort
- Isoetes durieui – Durieu's quillwort
- Isoetes echinospora – spring quillwort
- Isoetes ecuadoriensis
- Isoetes eludens – elusive quillwort
- Isoetes engelmannii Engelmann's quillwort
- Isoetes flaccida – southern quillwort
- Isoetes gemmifera – (syn. Stylites gemmifera)
- Isoetes histrix – land quillwort
- Isoetes howellii – Howell's quillwort
- Isoetes inflata
- Isoetes ivones
- Isoetes lacustris – lake quillwort
- Isoetes louisianensis – Louisiana quillwort
- Isoetes maritima – maritime quillwort
- Isoetes melanopoda – black-footed quillwort
- Isoetes melanospora – black-spored quillwort
- Isoetes mongerensis – Monger quillwort
- Isoetes muelleri – Mueller's quillwort
- Isoetes nuttallii – Nuttall's quillwort
- Isoetes piedmontana – Piedmont quillwort
- Isoetes setacea
- Isoetes sinensis
- Isoetes taiwanensis
- Isoetes tegetiformans – mat-forming quillwort
- Isoetes tenella – spiny-spore quillwort
- Isoetes tenuissima – French quillwort
- Isoetes toximontana - green-spored quillwort from South Africa
- Isoetes tripus
- Isoetes valida – Carolina or strong quillwort
- Isoetes velata
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 endagered species I. tegetiformans and I. virginica.
Fossilised specimens of I. beestonii have been found in rocks dating to the early Triassic. Quillworts are considered by some 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.
- Reichenbach, H. G. L. (1828). Conspectus Regni Vegetabilis. p. 43.
- 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.
- Musselman, Lytton John. 2001. "Georgia quillworts". The Journal of the Georgia Botanical Society 16. 2-19, 40.
- 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.
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