Isoetes is a genus of plants that almost make up the entire group of plants called quillworts. They are one of the most primitive groups of vascular plants that currently exist on Earth. They form a unique class of lycophytes with roughly 150 species still living today, although many species are now endangered.
The quillworts are spore producing plants that rely heavily on water. Many species live entirely submerged while others live in seasonally flooded habitats. They do not produce flowers, fruits, seeds or wood and have only a single vein that runs through their leaves.
Isoetes species are typically small plants with long, narrow leaves that grow from the base of the plant. Their leaves are usually within 5-20 cm long, although a few species have leaves that grow to over 60 cm in length and the largest species has leaves up to 1 m long. One plant tends to have around 10-30 leaves.
A quillwort leaf contains a single vein and four air chambers that run the length of the leaf. It is the presence of a single veined leaf, known as a ‘microphyll’, that distinguishes the quillworts and other lycophytes from all other vascular plants. Submerged species of Isoetes have evergreen leaves whereas wetland species are often deciduous, losing their leaves over the non-growing seasons.
Isoetes plants also have unusual stems. They are short and swollen and grow underground. The unique stem is known as a ‘corm’; it contains two or three lobes that give the corm an overall appearance of a small onion. The outer layers of the corm are shed as the plant grows.
The roots of Isoetes plants grow in clusters, all directly from the base of the corm. Roots also have a single vein and are surrounded by a single air chamber.
Distribution of isoetes
Quillworts are distributed all around the world and are found on all continents excluding Antarctica. They are however often naturally rare or uncommon in many areas.
All Isoetes plants rely heavily on an abundance of water in order to survive. Many species live permanently submerged in lakes and slow moving rivers. Other species inhabit estuaries and wetlands where they often encounter periods flooding.
Taxonomy and diversity
There is approximately 150 species in the genus Isoetes around the world. This makes up the vast majority of quillwort species; the remaining few belong to the genus Stylites.
Quillworts are vascular plants within the division Lycophyta. They form a class of plants called Isoetopsida which includes a single family called Isoetaceae. They are closely related to spikemosses and clubmosses.
The Isoetes have an ancient evolution dating back to over 400 million years ago. They are known to have definitely evolved by at least the Jurassic Period (201-145 million years ago) but may have evolved as early as the Devonian Period between 420 and 360 million years ago.
Reproduction of isoetes
Quillworts have a reproduction cycle similar to ferns where they rely on spores for reproduction rather than seeds. They also do not produce flowers or fruit to attract animals to help with pollination or dispersal. Instead spores tend to be dispersed by water and sometimes wind.
The spores are produced in structures called sporangia. Isoetes have distinct male and female sporangia which are grown at the base of their leaves and each produce multiple spores.
As with all plants, Isoetes reproduce through a cycle of alternating generations between a sporophyte and a gametophyte generation. In quillworts, the sporophyte is the dominant generation and the gametophyte requires the support of the sporophyte in order to survive.
- Isoetes plants are often used as indicators of water quality because they don’t like to grow in poor water.
- The genus Stylites includes a handful of species that live within alpine environments along the Andes of South American.
- Some species of Isoetes are able to absorb CO2 from the mud through their roots rather than from the air via their leaves as most plants do.
Last edited: 31 May 2015