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.
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.
We have already described several species of Compositæ, and now return to that order to describe a type of flower very similar in general appearance to the Daisy (page 1). The Corn Chamomile is an annual plant; the lower portion of its stem is prostrate, sending up erect branches with alternate, prettily cut leaves, twice pinnate. The flower-heads are borne singly on long stalks, and the floral envelope (_involucre_) consists of a number of over-lapping scales (_bracts_), whose margins are dry and chaffy. The base (_receptacle_) upon which the florets are packed is convex and covered with little chaffy scales, which stand up between the florets. The disc-florets contain both anthers and pistil; the ray-florets are pistillate only. The whole plant is downy. It occurs in fields and waste places, flowering from May to August. Though somewhat widely distributed, it is a local plant. The name is an old Greek name for the Chamomile, from _anthemon_, a flower, probably owing to the profusion of its blossoms.