Herbs creeping or floating, with the leaves petiolate or sessile, circinate in vernation. Fruit (sporocarps) situated at the base of the leaves or leafstalks, containing the capsular sporanges of one kind with 2 kinds of spores, or of 2 kinds with the different spores separated.
Genera 6. species 20? inhabiting ditches and inundated places in nearly all countries, but chiefly in temperate latitudes.
1. MARSIL'EA, L. Sporocarps at the base of the leaf-stalks, of one kind, 2-celled, cells transversely many-celled; spores inserted on each horizontal placenta. - Stems creeping, rooting; lvs. petiolate.
1 M. quadrifolia L? Glabrous; prostrate stems slender, wiry, 8 to 16' long; lvs. palmately 4-foliate, on filitorm petioles 1 to 3' high, lfts. broadly obovate or fan-shaped, obtuse; fr. (sporocarps) round-oval, borne on short, axillary stalks, and as large as a pepper-corn. - Sent from La. by Dr. Hale. Perhaps the locality is beyond our limits.
2 M. vestita, a very delicate species, with stems and petioles as fine as threads, with the quaternate leaflets and the very small sessile sporocarps clothed with minute, silky, brown hairs, is sent from Iowa, near the Mississippi R. by Dr. Couzens. It probably grows in Ill. Height of lvs. 1 to 2'.
2. ISOE'TES, L. Quill-wort. (Gr. equal, year; alike all the year round?) Sporocarps oval, membranous, 1-celled, immersed in the dilated base of the frond; spores subglobous, slightly angular, attached to numerous filiform receptacles, those in the outer fruits larger, angular, triple or in 4s, apparently of a different nature.
I. lacustris L. Lvs. caespitous, subulate, semiterete, dilated and imbricated at base. - A curious aquatic, in water at or near the margin of ponds and rivers, N. Eng. and Mid. States, often wholly submersed. Lvs. radical, numerous, tufted, simple, 2 to 10' long, somewhat spreading, containing numerous cells divided by longitudinal and transverse partitions. Fr. whitish, rather large, in the excavated base of the leaves which dilated portion is ordinarily as long as wide; in var. riparia, broader than long; in var. Exgelmanm, longer than broad.
3. AZOL'LA, Lam. (Gr. to dry, to kill; quickly killed by drought.) Fruit sessile on the under side of the branches, of 2 kinds; the sterile smaller, opening all around, containing a thick body bearing 3 angular lobes (antheridia) above; the fertile a thin pericarp bursting irregularly, containing many globular, stalked sporangia each with a few spores. - Minute, floating, resembling a Jungennannia, with filiform stems and lobed fronds.
A. Caroliniana Willd. Lvs. ovate-oblong, obtuse, imbricated, fleshy, floating, reddish beneath, scarcely more than 1/3" in length; sterile fruits in pairs or solitary, at the base of the fertile, many times smaller than it. - Lakes and marshes N. Y. to Ill. and S. States.
4. SALVIN'IA natans L, inserted in previous editions on the authority of Pursh, has not been observed since.