Very small plants floating about freely on the surface of ponds and ditches, consisting merely of a little frond, commonly with a single root or a tuft of roots from the lower surface, and producing minute monoecious flowers from a cleft in the edge of the frond, or from the upper surface.
Fruit a 1-7-seeded utricle. The flowers are rarely to he seen.
1. Lem'na. Flowers from a cleft in the edge of the frond, usually 2 of a single stamen each, and 1 of a simple pistil, the three surrounded by a spathe. Roots several or only one.
2. Wolff'ia. Flowers from the centre of the upper surface of the frond, only 2; one of a single stamen, the other of a simple pistil. Boots none. The smallest of flowering plants.
1. L. polyrrhi'za, L. (Spirodela polyrrhiza, Schleiden.) Fronds round-obovate, green above, purplish beneath, mostly 7-nerved. Rootlets several. - Common in ponds and pools.
2. L. trisul'ca, L. Fronds oblong to oblong-lanceolate, half an inch or more long, narrowed at the base into a slender stalk, denticulate at the tip, obscurely 3-nerved. Rootlet single, often wanting. - Ponds and springy places.
3. L. minor, L. Fronds round to elliptic-obovate, very small. Rootlet single. - Stagnant waters.
1. W. Columbia'na, Karsten. Globular or nearly so, light green all over, not dotted. - Stagnant waters, usually rather below the surface.
2. W. Brasilien'sis, Weddell. Oblong, deep green above, pale below, dotted all over with brown. - With the last, but floating.