Isoetes

Isoetes
Isoetes lacustris
Scientific classification
Kingdom:	Plantae
Division:	Lycopodiophyta
Class:	Isoetopsida
Order:	Isoetales
Family:	Isoetaceae Rchb.
Genus:	Isoetes L.

Isoetes, 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.

The name of the genus is sometimes 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 is correct.

Description

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.

Classification

Compared to other genera, the genus Isoetes is a technically difficult and relatively poorly known. 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 for Isoetes instead rely on spore characteristics, which make identification to species difficult to impossible without microscopy. Placed in its own family Isoetaceae, the genus includes about 150 species worldwide.

Selected species

This list is incomplete; you can help by expanding it.

• 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 endangered species I. tegetiformans and I. virginica.

Evolution

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.

	Lepidodendrales Pleuromeia Nathorstiana Isoetes

External links

• Checklist of World Ferns, Family Isoetaceae, genus Isoetes; world species list. (143 species)
• Flora of North America - Isoetes