Isoetes (sometimes spelled Isoëtes), 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.
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.
Compared to other genera, the genus Isoetes is a technically difficult and relatively poorly known one. Even after various studies with cytology, scanning electron microscopy, and chromatography, the individual species are still difficult to identify and their phylogeny is still disputed. Vegetative characters that are commonly used to distinguish other genera, such as leaf length, rigidity, color, or shape are variable and depend greatly on habitat. Most classification systems instead rely on spore characteristics, which make field identification to species difficult to impossible without microscopy. Placed in its own family Isoetaceae, the genus includes about 150 species worldwide.
- Isoetes acadiensis
- 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 × eatonii – Eaton'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 hieroglyphica
- Isoetes histrix – land quillwort
- Isoetes howellii – Howell's quillwort
- Isoetes inflata
- Isoetes ivones
- Isoetes jaeger
- Isoetes lacustris – lake quillwort
- Isoetes louisianensis – Louisiana quillwort
- Isoetes macrospora
- Isoetes malinverniana
- 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 occidentalis
- Isoetes piedmontana – Piedmont quillwort
- Isoetes riparia
- 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 tuckermanii
- Isoetes valida – Carolina or strong quillwort
- Isoetes velata
- Isoetes viridimontana – Green Mountain quillwort
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.
- illustration from Otto Wilhelm Thomé Flora von Deutschland, Österreich und der Schweiz 1885, Gera, Germany
- Reichenbach, H. G. L. (1828). Conspectus Regni Vegetabilis. p. 43.
- Cody, William; Britton, Donald (1989). Ferns and Fern Allies of Canada. Agriculture Canada.
- 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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