Isoetes

Quillwort megasporangia

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 water dispersion. Quillworts have different ways to spread their spores based on the environment. Quillwort leaves are hollow and quill-like, with a minute ligule at the base of the upper surface.[5]^:7 arising from a central corm. Each leaf is narrow, 2–20 centimetres (0.8–8 in) long (exceptionally up to 100 cm or 40 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.[6] Isoetes andicola is unusual in being the only known terrestrial vascular plant that take up all its CO₂ through the roots. Only 4% of total biomass, the tips of the leaves, is chlorophyllous.[7] The roots broaden to a swollen base up to 5 mm (0.2 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 their megaspores under a microscope. Moreover, habitat, texture, spore size, and velum provide features that will distinguish Isoëtes taxa.[8]

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.[9]

As of November 2019, Plants of the World Online accepted the following extant species:[10]

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.

Fossilised specimens of I. beestonii have been found in rocks dating to the latest Permian.[12][13] 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.

Wikimedia Commons has media related to Isoetes.

Wikispecies has information related to Isoetes

• Checklist of World Ferns, Family Isoetaceae, genus Isoetes; world species list. (143 species)
• Distribution and classification list for world isoetes