Isoetales, sometimes also written Isoëtales, is an order of plants in the class

Lycopodiopsida Lycopodiopsida is a class of vascular plants known as lycopods, lycophytes or other terms including the component lyco-. Members of the class are also called clubmosses, firmosses, spikemosses and quillworts. They have dichotomously branching s ...

. There are about 140-150 living species, all of which are classified in the genus ''

Isoetes ''Isoetes'', commonly known as the quillworts, is the only extant genus of plants in the family Isoetaceae, which is in the class of lycopods. There are currently 192 recognized species, with a cosmopolitan distribution but with the individual ...

'' (quillworts), with a cosmopolitan distribution, 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 any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, ste ...

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 A corm, bulbo-tuber, or bulbotuber is a short, vertical, swollen underground plant stem that serves as a storage organ that some plants use to survive winter or other adverse conditions such as summer drought and heat (perennation). The word ' ...

characteristic of most quillworts. This swollen base also contains male and female

sporangia A sporangium (; from Late Latin, ) is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. Virtually all plants, fungi, and many other lineages form sporangia at some point in their life cyc ...

, protected by a thin, transparent covering (

velum Velum may refer to: Human anatomy * Superior medullary velum, anterior medullary velum or valve of Vieussens, white matter, in the brain, which stretches between the superior cerebellar peduncles ** Frenulum of superior medullary velum, a sligh ...

), 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. ''

'' are the only living

pteridophytes A pteridophyte is a vascular plant (with xylem and phloem) that disperses spores. Because pteridophytes produce neither flowers nor seeds, they are sometimes referred to as "cryptogams", meaning that their means of reproduction is hidden. Ferns, ...

capable of

secondary growth In botany, secondary growth is the growth that results from cell division in the cambia or lateral meristems and that causes the stems and roots to thicken, while primary growth is growth that occurs as a result of cell division at the tips of ...

Fossils

Fossilised specimens of ''

Isoetes beestonii ''Isoetes beestonii'' is the oldest known species of the living quillwort genus from the latest Permian of New South Wales and Queensland. Originally considered earliest Triassic, it is now known to be latest Permian in age, immediately before th ...

'' 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 251.9 Mya. It is the last ...

. All quillworts and their extinct relatives are

heterosporous Heterospory is the production of spores of two different sizes and sexes by the sporophytes of land plants. The smaller of these, the microspore, is male and the larger megaspore is female. Heterospory evolved during the Devonian period from is ...

. Some fossil species are very well known, with many stages of development and the life cycle preserved. Two of the best known are the Carboniferous '' Chaloneria'' and

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

'' Nathorstiana''. During the Early Triassic, the long-stemmed Isoetales genus '' Pleuromeia'' was dominant over large areas of the globe.

References

External links

*
World list of all species in order Isoetales from World Ferns by M. Hassler in the Catalogue of Life
203 species. * {{Taxonbar, from=Q7170041 Lycophytes Plant orders