1. Ceratophyllum Linnaeus, Sp. Pl. 2: 992. 1753; Gen. Pl. ed. 5, 428, 1754.
Hornwort, cornifle [Greek ceratos, horn, and phyllon, leaf]
Plants , submersed perennials. Stems suspended or anchored by branches buried in substrate; branches 0-3 per node. Leaves 3-11 per whorl, cauline; petiole inconspicuous. Leaf blade simple or forked into linear-filiform, denticulate segments. Inflorescences extra-axillary, alternating with leaves. Flowers: bracts basally connate, foliaceous; pedicel less than 1 mm or essentially absent. Staminate flowers: anther 2-locular, connective projecting as apical appendage. Pistillate flowers: ovary tapering to persistent, spinelike style; ovule pendulous. Achene ellipsoid, moderately compressed, surface smooth or tuberculate, basal or marginal spines (or both) present or absent, terminal spine present, marginal wing present or absent. x = 12, 19, 20.
Useful in identification of species of Ceratophyllum are leaf-forking characteristics. Leaves with no forking are "0-order"; they consist only of a primary segment. Those forking once are "1st-order"; their ultimate segments are secondary. Those in which at least one secondary segment forks are "2d-order"; their ultimate segments are tertiary. Those in which at least one tertiary segment forks are "3d-order"; their ultimate segments are quaternary. Those in which at least one quaternary segment forks are "4th order."
Plumule features used in the key below are observable through dissection of softened (soaked) seeds removed from achenes.
SELECTED REFERENCES
Fassett, N. C. 1953. North American Ceratophyllum. Comun. Inst. Trop. Invest. Ci. Univ. El Salvador 2: 25-45. Herendeen, P. S., D. H. Les, and D. L. Dilcher. 1990. Fossil Ceratophyllum (Ceratophyllaceae) from the Tertiary of North America. Amer. J. Bot. 77: 7-16. Jones, E. N. 1931. The morphology and biology of Ceratophyllum demersum. Stud. Nat. Hist. Iowa Univ. 13: 11-55. Les, D. H. 1985. The taxonomic significance of plumule morphology in Ceratophyllum (Ceratophyllaceae). Syst. Bot. 10: 338-346. Les, D. H. 1986. The phytogeography of Ceratophyllum demersum and C. echinatum in glaciated North America. Canad. J. Bot. 64: 498-509. Les, D. H. 1986b. The evolution of achene morphology in Ceratophyllum (Ceratophyllaceae), I. Fruit spine variation and relationships of C. demersum, C. submersum, and C. apiculatum. Syst. Bot. 11: 549-558. Les, D. H. 1988b. The evolution of achene morphology in Ceratophyllum (Ceratophyllaceae), II. Fruit variation and systematics of the "spiny-margined" group. Syst. Bot. 13: 73-86. Les, D. H. 1988c. The evolution of achene morphology in Ceratophyllum (Ceratophyllaceae), III. Relationships of the "facially-spined" group. Syst. Bot. 13: 509-518. Les, D. H. 1989. The evolution of achene morphology in Ceratophyllum (Ceratophyllaceae), IV. Summary of proposed relationships and evolutionary trends. Syst. Bot. 14: 254-262. Lowden, R. M. 1978. Studies in the submerged genus Ceratophyllum L. in the neotropics. Aquat. Bot. 4: 127-142. Muenscher, W. C. 1940. Fruits and seedlings of Ceratophyllum. Amer. J. Bot. 27: 231-233.
