Myriophyllum L.
Annual or perennial herbs, usually glabrous, monoecious, dioecious, or fls ⚥; stems rooting at nodes, prostrate to erect. Lvs usually whorled, sometimes opposite or alternate, usually dimorphic, the submerged lvs pectinate and finely divided, the aerial lvs ± entire, rarely all lvs of either form. Hydathodes often present near lf axils. Infl. a simple or rarely branched, terminal spike; fls usually solitary, in the axils of aerial lvs, occasionally in the axils of upper submerged lvs, each with 2 bracteoles; ♂ fls usually above ♀ within same infl. in monoecious spp. ♂ fls (2)-4-merous; sepals present; petals not clawed, ± hooded; stamens (1)-4 or 8; filaments short; anthers linear; pistil rudiments present or 0. ♀ fls (2)-4-merous; sepals and petals very reduced or 0; ovary (2)-4-celled; styles as many as cells of ovary; stigmas capitate, fimbriate; staminodes rudimentary. Fr. of 2-4, 1-seeded mericarps; mericarps usually smooth and rounded on dorsal face, lacking a differentiated pericarp, sessile or shortly pedunculate.
Key
c. 40 spp., widespread, especially in Australia and N. America, aquatic. Native spp. 5, naturalised 1.
The treatment below follows Orchard, A. E., Brunonia 2: 247-287 (1980).
