Isoetes, commonly known as the quillworts, is a genus of plants in the family Isoetaceae. They are lycopods and the only genus in Isoetaceae. There are currently 192 recognized 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 Stylites does not warrant taxonomic recognition.
The name of the genus may also be 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 (Isoetes or Isoëtes) is correct.
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. The Quillworts are spore producing plants and highly rely on the water habits。 Quillwort have different way to speared their spores based on the environment. Quillwort leaves are hollow and quill-like, with a minute ligule at the base of the upper surface.:7 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. Stomata are absent, yet the leaves have a thick cuticle which prevents CO2 uptake, a task that is performed by their hollow roots instead, which absorbs CO2 from the sediment.  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. Moreover, habitat,texture, spore size, and velum provide features that will distinguish Isoëtes tax.
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.
- Selected species
- I. acadiensis
- I. alpina – New Zealand quillwort
- I. andicola – (syn. Stylites andicola)
- I. appalachiana – Appalachian quillwort
- I. australis – Australian quillwort
- †I. beestonii – (Late Permian)
- I. bolanderi – Bolander's quillwort
- I. boomii – Boom's quillwort - SE US (GA, AL, FL)
- I. brevicula
- I. brochonii – Pyrenean quillwort
- I. butleri – Butler's quillwort
- I. caroli- Brazil 
- I. caroliniana - Carolina quillwort
- I. drummondii – Drummond's quillwort
- I. durieui – Durieu's quillwort
- I. echinospora – spring quillwort
- I. ecuadoriensis
- I. eludens – elusive quillwort
- I. engelmannii Engelmann's quillwort
- I. flaccida – southern quillwort
- I. gemmifera – (syn. Stylites gemmifera)
- I. georgiana – Georgia quillwort - SE US (GA)
- I. hieroglyphica
- I. histrix – land quillwort; Channel Islands
- I. howellii – Howell's quillwort
- I. hyemalis – Wintergreen quillwort - SE US
- I. inflata
- I. ivones
- I. jaeger
- I. junciformis – Rush quillwort - SE US (GA)
- I. lacustris – lake quillwort
- I. louisianensis – Louisiana quillwort
- I. macrospora
- I. malinverniana
- I. maritima – maritime quillwort
- I. melanopoda – black-footed quillwort
- I. melanospora – black-spored quillwort
- I. microvela
- I. mississippiensis
- I. mongerensis – Monger quillwort
- I. muelleri – Mueller's quillwort
- I. nuttallii – Nuttall's quillwort
- I. occidentalis
- I. piedmontana – Piedmont quillwort
- I. riparia
- I. setacea
- I. sinensis China
- I. taiwanensis Taiwan
- I. tamaulipana Tamaulipas, México
- I. tegetiformans – mat-forming quillwor
- I. tenella – spiny-spore quillwort
- I. tennesseensis – Tennessee quillwort - SE US (TN)
- I. tenuissima – French quillwort
- I. toximontana - green-spored quillwort from South Africa
- I. tripus
- I. tuckermanii
- I. valida – Carolina or strong quillwort
- I. velata
- I. viridimontana – Green Mountain quillwort
- I. weberi
- Isoetes × eatonii – Eaton's 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 endangered species I. tegetiformans and I. virginica.
Fossilised specimens of I. beestonii have been found in rocks dating to the latest Permian. Quillworts are considered to be the closest extant relatives 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
- Troia, Angelo; Pereira, Jovani B.; Kim, Changkyun; Taylor, W. Carl (2016). "The genus Isoetes (Isoetaceae): a provisional checklist of the accepted and unresolved taxa". Phytotaxa. 277 (2): 101. doi:10.11646/phytotaxa.277.2.1. ISSN 1179-3163.
- Larsén, Eva; Rydin, Catarina (2016). "Disentangling the Phylogeny ofIsoetes(Isoetales), Using Nuclear and Plastid Data". International Journal of Plant Sciences. 177 (2): 157–174. doi:10.1086/684179. ISSN 1058-5893.
- International Code of Nomenclature for algae, fungi, and plants (Melbourne Code) see section 60.6: "The diaeresis, indicating that a vowel is to be pronounced separately from the preceding vowel (as in Cephaëlis, Isoëtes), is a phonetic device that is not considered to alter the spelling; as such, its use is optional."
- Stace, C. A. (2010). New Flora of the British Isles (Third ed.). Cambridge, U.K.: Cambridge University Press. ISBN 9780521707725.
- Ecology of High Altitude Waters
- Isoëtes Linnaeus, Sp. Pl. 2: 1100. 1753; Gen. Pl. ed. 5, 486, 1754.
- Cody, William; Britton, Donald (1989). Ferns and Fern Allies of Canada. Agriculture Canada.
- 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.
- Jovani B. S. Pereira and Paulo.H Labiak. A New Species of Isoetes with Tuberculate Spores from Southeastern Brazil (Isoetaceae) ISSN 1548-2324
- Retallack, Gregory J. (2013). "Permian and Triassic greenhouse crises". Gondwana Research. 24: 90–103. doi:10.1016/j.gr.2012.03.003.