This section is from the book "British Wild Flowers - In Their Natural Haunts Vol5-6", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
The habitat of this plant is ponds, still waters, and stagnant waters. The plant is stemless. The root is single. The frond, is thin, transparent, and elliptic, in shape inversely ovate to lance-shaped, tailed at one end, serrate at the other, proliferous at right angles, narrowed below. The young hastate fronds are placed crosswise to the old ones. The plant is submerged, and produces autumn bulblets which persist through the winter. The style is short. The testa is rough. It flowers in June and July, and is a herbaceous annual.
The habitat of this plant is stagnant water, still water, and ponds. The habit is floating. The fronds are flat, inversely ovate, or rounded, spongy and swollen below, opaque, pale-green. There is a solitary, blunt root. The young fronds are stalk-less. The cells below are large, the epidermal cells having flexuous walls. There are 2 stamens. The fruit is a utricle, and bursts transversely. The plant is floating, flowering from June to September, and is a herbaceous annual.
The habitat of this plant is ponds and ditches, in water. The fronds are \ in. long, round to inversely ovate, flattened, plano-convex, green above, purple below, 7-nerved, with numerous tracheae. The epidermal cells have flexuous walls. The roots are numerous, clustered, acute. The spathe is 2-lipped. There are 2 stamens. The plant does not flower in this country. The plant is floating and annual.
Wolffia arrhiza, Wimm. (= W. michelii, Schleid). - The habitat of this species is slow streams, ponds, near London. There are no roots. The fronds are the size of a grain of sand, very small, green, more or less round, flat above, loosely cellular below. The young frond soon separates from the old one, and is solitary at the base. The epidermal cells have straight walls. There are offsets within the base of the old frond. The plant is floating, flowering in June and July, and is a herbaceous annual.