Isoetales, sometimes also written Isoëtales, is an
order of plants in the class
Lycopodiopsida.
There are about 140-150 living species, all of which are classified in the genus ''
Isoetes'' (quillworts), with a
cosmopolitan distribution
In biogeography, a cosmopolitan distribution is the range of a taxon that extends across most or all of the surface of the Earth, in appropriate habitats; most cosmopolitan species are known to be highly adaptable to a range of climatic and en ...
, but often scarce to rare. Living species are mostly aquatic or semi-aquatic, and are found in clear ponds and slowly moving streams. Each
leaf
A leaf (: leaves) is a principal appendage of the plant stem, stem of a vascular plant, usually borne laterally above ground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leav ...
is slender and broadens downward to a swollen base up to 5 mm wide where the leaves attach in clusters to a bulb-like, underground
corm characteristic of most quillworts. 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. 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'' are the only living
pteridophyte
A pteridophyte is a vascular plant (with xylem and phloem) that reproduces by means of spores. Because pteridophytes produce neither flowers nor seeds, they are sometimes referred to as " cryptogams", meaning that their means of reproduction is ...
s capable of
secondary growth.
Fossils
Some authors include the tree-like "aboresecent lycophytes", which formed forests during the
Carboniferous
The Carboniferous ( ) is a Geologic time scale, geologic period and System (stratigraphy), system of the Paleozoic era (geology), era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the ...
period, and often assigned to their own order,
Lepidodendrales, within Isoetales.
Fossilised specimens of ''
Isoetes beestonii'' have been found in rocks dating to the latest
Permian
The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years, from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the s ...
-earliest
Triassic
The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...
.
During the Early Triassic, Isoetales, such as the long-stemmed ''
Pleuromeia'' were dominant over large areas of the globe. The oldest fossil closely resembling modern quillworts is ''Isoetites rolandii'' from the Late Jurassic of North America.
References
External links
*
World list of all species in order Isoetales from World Ferns by M. Hassler in the Catalogue of Life 203 species.
*
{{Authority control
Lycophytes
Lycophyte orders