• Isoetes
• Stylites

Quillworts are plants of the genera Isoetes and Stylites in the class Isoetopsida and order Isoetales. The order Isoetales is sometimes placed in the class Isoetopsida, sometimes in the Selaginellopsida or Lycopsida. They are considered "fern allies". These plants occur mainly in ponds and slow-moving streams, generally in the swampy and cooler parts of northern North America and Eurasia.

Quillworts are mostly aquatic or semi-aquatic. Quillwort leaves are hollow and quill-like, arising from a central corm. Each leaf is narrow, 1-2 mm (less than 1/8in) wide. They broaden to a swollen base where they attach in clusters to a bulb-like, underground rhizome characteristic of all quillwort species except the Mat-forming Quillwort , Isoetes tegetiformans. 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.

Many species, such as the Louisiana Quillwort and the Mat-forming Quillwort, are endangered.

Quillwort species are very difficult to distinguish by general appearance. The best way to identify them is by examing the megaspores under a microscope.

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.

The name quillwort is also used for plants of the genus Acorus.

Species list (incomplete)

• Louisiana Quillwort, Isoetes louisianensis
• Black-spored Quillwort , Isoetes melanospora
• Mat-forming Quillwort , Isoetes tegetiformans
• Appalachian Quillwort , Isoetes appalachiana
• Lake Quillwort, Isoetes lacustris
• Spiny Spore Quillwort , Isoetes tenella
• Southern quillwort , Isoetes flaccida