Citation: L., Sp. Pl. 992 (1753).

Derivation: Greek myrios, numberless, countless; phyllon, leaf; an allusion to the finely divided pectinate submerged leaves of many species.

Synonymy: Pelonastes Hook. f., London J. Bot. 6:474 (1847).

Common name: Water-milfoils.

Description:
Annual or perennial herbs growing in or adjacent to water; roots fibrous, often adventitious from nodes; stems weak, often floating, glabrous (except M. crispatum); leaves whorled, opposite or alternate, usually dimorphic, submerged leaves finely pectinate, emergent (out of water) leaves entire or toothed, or rarely all of one type or the other.

Inflorescence simple (rarely branched), spike-like, with flowers borne singly (rarely in dichasia) in the axils of the emergent leaves, each with 2 bracteoles; flowers unisexual or bisexual, plants monoecious or dioecious; male flowers at the top of the spike, 4-merous, sepals and petals usually present, stamens 2, 4 or 8, styles and ovary vestigial or absent; a few transitional bisexual flowers sometimes present; female flowers (monoecious plants) below males, 4-merous, sepals present or absent, petals vestigial or absent, stamens 0, styles free, 1 per carpel, sessile or clavate, stigma usually fimbriate, ovary of 4 carpels.

Fruit dry, variously ornamented, splitting at maturity into 1-seeded mericarps.

Distribution:  About 60 species, almost cosmopolitan, but the major centre of diversity is Australia with 36 species (31 endemic). Other major centres are New Zealand (6 spp., 3 endemic), North America (13 spp., 7 endemic), and India/Indochina (10 species, 7 endemic). (Schindler (1905) Pflanzenr. 23:77-104; Aston (1973) Aquatic plants of Australia 79-98; Orchard (1986) Brunonia 8:173-291).

Biology: No text

Taxonomic notes: "Water-milfoil" a European common name referring to the finely divided submerged leaves of all species there, has been slavishly transferred to the Australian species by some authors. Its use for species with all leaves entire (about one-third of the Australian spp.) is inappropriate.

Key to Species:

1. All, or at least the lower (submerged), leaves in whorls
2. All leaves in whorls
3. Emergent leaves pectinate	M. verrucosum 11.
3. Emergent leaves ovate, terete or lanceolate, entire or toothed but not pectinate
4. Emergent leaves terete or linear (more than 3 times as long as broad)
5. Flowers and fruits in fascicles; styles mauve to purple	M. papillosum 7.
5. Flowers and fruits solitary; styles white to pink
6. Emergent leaves in whorls of 5-8; emergent spikes fusiform in outline; stem usually bearing crisped hairs	M. crispatum 3.
6. Emergent leaves in whorls of 3-6 or sometimes slightly irregular; emergent spike cylindrical and truncate; stem glabrous
7. Fruit yellow-brown, mericarps cylindrical, 1.4-1.5 mm long; emergent leaves usually strictly in whorls	M. variifolium 10.
7. Fruit reddish-purple, mericarps ovoid, 0.5-1.1 mm long; emergent leaves in whorls but usually with some irregularly arranged as well	M. simulans 9.
4. Emergent leaves ovate to oblong (up to 3 times as long as broad)
8. Mericarps oblong to ovoid, smooth or coarsely warty.
9. Mericarps smooth or minutely papillose	M. salsugineum 8.
9. Mericarps bluntly angled on the dorsal surface, coarsely and irregularly warty	M. verrucosum 11.
8. Mericarps cylindrical, with dense fine spines on the dorsal surface when mature	M. caput-medusae 2.
2. Only submerged leaves in whorls; emergent leaves irregular or in pseudowhorls	M. simulans 9.
1. All leaves alternate or opposite
10. All leaves alternate
11. Lowermost leaves pectinate; stamens 8; perennials	M. simulans 9.
11. Lowermost leaves entire or 3-fid; stamens 2 or 4; ephemeral annuals
12. Flowers and fruits in fascicles; stamens 4; mericarps urceolate	M. glomeratum 4.
12. Flowers and fruits solitary in axils; stamens 2; mericarps ovoid	M. integrifolium 5.
10. All leaves opposite
13. Submerged leaves pectinate; male flower enclosed at first in a hooded bract; mericarps cylindrical	M. muelleri 6.
13. Submerged leaves oblanceolate to obovate; male flower not enclosed in a hooded bract; mericarps tear-shaped	M. amphibium 1.

Author: Not yet available