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Isoëtes, also written Isoetes and commonly known as the quillworts, is the genus of plants in the class and order Isoetales. The order Isoetales is sometimes placed in the class Isoetopsida, sometimes in the Selaginellopsida or Lycopsida. They are considered "fern allies". 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.

Quillworts are mostly aquatic or semi-aquatic in clear ponds and slow-moving streams, though several (e.g. I. histrix, 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 cm long (exceptionally up to 100 cm) and 0.5-3 mm wide; they can be either evergreen, winter deciduous, or dry-season deciduous. They broaden to a swollen base up to 5 mm 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 sproangia, 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 bolanderi Bolander's Quillwort
• Isoetes brochonii Pyreneean Quillwort
• Isoetes durieui Durieu's Quillwort
• Isoetes echinospora Spring 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 lacustris Lake Quillwort
• Isoetes louisianensis Louisiana Quillwort
• Isoetes melanospora Black-spored Quillwort
• Isoetes nuttallii Nuttall's Quillwort
• Isoetes tegetiformans Mat-forming Quillwort
• Isoetes tenella Spiny Spore Quillwort
• Isoetes tenuissima French Quillwort
• Isoetes valida Strong 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.

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.