Myriophyllum

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Myriophyllum

Description

Aquatic or terrestrial, glabrous herbs, perennial or (not in Mal.) sometimes annual. Leaves often very constantly flanked by 1 (-3) ‘stipular’ outgrowths, which are filiform to subulate, mostly short, becoming dark, caducous; Flowers mostly sessile, 1 or more in the axil of a bract or leaf, each with 2 sometimes very inconspicuous bracteoles; Sepals 4 or 0, mostly very small, erect. Petals 4 or 2, whether or not caducous, in the ♀ flowers strongly reduced and covered by the styles or 0. Stamens 8 or 4 epipetalous, or 1, in the ♀ flowers 0; Ovary more or less urceolate, nearly always (mostly alternisepalous) 4-sulcate, 4-2-celled, in ♂ flowers reduced or 0; Fruit mostly ± urceolate, breaking up into 4 or 2 one-seeded mericarps;

Distribution

Africa: present Almost ubiquist: present Asia-Tropical: Australasia: most of the Arctic: absent
Almost ubiquist, except in most of the Arctic and rare in Africa, c. 40 spp. with a distinct centre in Australia; in Malesia 8 or 9 native spp. from which two endemic.
The distribution of most SE. Asiatic and Australian species is insufficiently known, as such inconspicuous aquatics are still 'under-collected'.

Taxonomy

In the vegetative parts and habit the species are very variable: plants with immersed leaves easily produce aerial leaves when the water recedes, vice versa. Sizes of leaves and bracts respectively show great variation, especially towards the inflorescence. In general it is impossible to identify sterile specimens. Varieties or even species distinguished merely by vegetative or quantitative characters deserve no systematic recognition.
SCHINDLER'S subdivision of the genus into 3 subgenera is unsatisfactory; this is in part caused by the fact that he did not examine material of a number of critical species, and in part by the fact that he used unreliable characters, paying amongst others too much attention to the number of stamens. Within Myriophyllum (‘Eumyriophyllum) he distinguished for example 2 sections, viz Pentapteris DC. em. O.K. and Tessaronia SCHINDL., differing in the number of stamens, viz 8 and 4 respectively. However, in M. spicatum of § Pentapteris their number varies from 8 to 2. M. indicumwith 8 and M. tetrandrum with 4 stamens, are certainly very closely allied, but are placed in two different sections, the first in § Pentapteris and the second in § Tessaronia. In M. dicocum of subg. Dicarium the number of stamens varies from 4 to 8.
An inadmissible procedure has been to subordinate Pelonastes HOOK.f. as a subsection to subg. Myriophyllum in an emended sense, although none of HOOKER'S original species is attributed to it; these are arranged by SCHINDLER in the new subg. Brachytheca SCHINDL.
In my opinion it serves no good use to distinguish subgenera or even sections in the genus.

Notes

The sculpture of the fruits becomes only clearly visible in dried material.
Largely because of the supposed incompleteness of the knowledge on the distribution of the species, I have included 5 extra-Malesian species in the key, wich in this way covers all species from Malesia, SE. Asia, Madagascar, and Africa.
For literature on the extra-Malesian species, their distribution and ecology, I refer to my precursory paper in .
Pre-identification of some species with simple opposite leaves and male flowers (notably M. pygmaeum and M. pedunculatum) is rather difficult, but they can easily be assigned to the genus by the presence of dark, subulate pseudostipules and dark-tipped leaves.

Citation

LINNÉ, Sp. Pl., ed. 1. 1753: 992
DC. - in Prod. 1828: 68
MEIJDEN - in Blumea. 1969: 306
BTH. - in Fl. Austr. 1864: 486
SCHINDL. - in Pfl. R. 1905: 77