This section is from the book "Wayside And Woodland Blossoms", by Edward Step. Also available from Amazon: Wayside And Woodland Blossoms: A Guide To British Wild-Flowers.
The Duckweeds - Shakespeare's "Green mantle of the standing pool" - are plants that are well-known to everybody, and consequently very few persons know anything of them. This is a paradox; but they are so common and so small that the average man or woman is content to know them in the aggregate, and cannot condescend to a more intimate acquaintance with individuals, or with the different species, yet like many other small things - "unconsidered trifles" - they are very interesting to the botanist; for these are among the smallest and simplest of the flowering plants. Taking up two or three plants from one pond and comparing them with some from another piece of water, we shall probably find a difference in them; but they are all possessed of a more or less flattened green body that floats on the water, and which we shall be inclined to call a leaf. It is not a leaf, however, but a plant that produces no leaves, though it has roots and flowers. To be more accurate we will call it a frond, from whose under-surface there goes down one or more simple unbranched roots, and in clefts of whose margin are simple flowers. The flower consists of an envelope or spathe (see page 15), within which is a bottle-shaped pistil, with one or two stamens beside it. Some authorities contend that the pistil and each of the stamens is really a distinct flower similar to those in Arum. These flowers are so minute that they are rarely seen, and so are thought to flower only occasionally. The plant is chiefly multiplied by the production of new fronds from its edges. The four species figured give the whole of the genus, so far as Britain is concerned; but three others are known in foreign waters. The differences in the natives may be thus briefly enumerated:
I. Least Duckweed (Lemna minor). The most frequent species. Frond not more than a quarter of an inch long, egg-shaped, the top flat and bright green, underside very pale green and slightly convex, with a single root. Spathe two-lipped, one much larger than the other. Stamens two, one maturing before the other; style long. Flowering in July.
1. Lemna minor. 3. Lemna gibba.
2. - Tisulca. 4. - polyrhiza.
- Lemnaceae. -
Anthemis arvensis. - Compositae. II. Ivy-leaved Duckweed (L. trisulca). Frond thin and flat, nearly an inch long, tailed at one end, coarsely toothed at the other. New fronds emerge at right angles to the parent. Roots solitary. Stamens two; style short. June and July.
III. Thick-leaved Duckweed (L. gibba). Frond nearly round, narrowed at one end, large, almost flat, green opaque on top, greatly swollen beneath, whitish, clear, the cell-structure being very noticeable. Root solitary, stamens two. Flowers June to September.
IV. Great Duckweed (L. polyrhiza). At once distinguished from the others by its bunch of roots from each frond. Upper surface slightly convex, dark green with seven nerves. Underside purple, as also the upper margins. Stamens two. Flower has been rarely, if ever, seen in this country.
Late in Autumn the fronds sink to the bottom of the ponds and ditches, and remain there hibernating till Spring, when they arise to the surface, and again vegetate. The name of the genus is the old Greek appellation of the plant Lemna, supposed to be derived from Lepis, a scale.