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Herbaceous, perennial, submerged aquatics or marsh plants, usually with annual grass-like leaves arising in a tuft from a lobed, flattened, corm-like stock. Leaves distichous, crowded, with overlapping bases, terete or flattened above, with a broad spoon-like base.


Asia: present Australasia:, Tasmania (Tasmania present) Malaysia: present New Zealand: present Pacific Islands: absent all parts of the world: present
About 75 spp., in all parts of the world except the Pacific Islands (present in Tasmania and New Zealand), but mainly temperate, scarce in Asia, in Malaysia 3 spp., one in the hills and two in the high mountains.


The most recent comparative survey of the family Isoetaceae, regarded as sole family of an order Isoetales, is by Rauh & Falk (). Three genera are included: Isoetes (worldwide), Stylites (Peru), and Nathorstiana (fossil of lower Cretaceous, Germany). Stylites, with its elongate stem and unbranched roots, shovss some resemblances to Nathorstiana, which has been considered to be a possible link between Isoetes and the Triassic fossilPleuromeia. Isoetes has a very short stem of complex structure, and is generally regarded as reduced and specialized. The fossil genus Isoetites, differing from Isoetes in the relative size of megaspores and microspores, in the shape of leaf-tips and in having an unlobed stem, has been most recently discussed by R. W. Brown () who described two species from N. America, ranging from lower Cretaceous to early Tertiary; he considers that a Portuguese fossil from the lower Cretaceous probably belongs to the same genus, though the specimens are imperfect.
There has been no recent monograph of the whole genus Isoetes, and estimates of the number of species vary. C. F. Reed has published a very full list of names, with bibliography, in .
See revision of the genus in New Guinea by , with key, description of new species I. hopei and I. stevensii, and under I. neoguineensis BAKER a new variety rheophila; also SEM photographs of spores of all species and much new information on vegetative morphology and distribution.
Isoetes sp. has been found in Central West Sumatra, in addition to Mindanao and New Guinea the third island where the genus is hitherto discovered in Malesia. Cf. and . It is assumed to be an un- described species, differing from the Philippine one in the megaspores. The exact locality is on the westside of Mt Kerintji, midway the lakes Sati and Landak Panjang, at c. 2080 m altitude, in small, shallow, muddy depressions in swampy forest, 1°42'S and 101°11'E.


Manton recordschromosomenumbers as follows for Isoetes: I. hystrix Durieu, n — 10; f. laeustris L., n = 54-56 (). Rauh & Falk (l.c.) record 2n = c. 50 for Stylites gemmifera W. Rauh.


Leaves of I. philippinensis are said to be eaten.


Linne, Gen. Pl., ed. 5. 1754: 486