Order LXXXVII. LEMNACEÆ.
Minute gregarious floating water-plants, without distinct stems or true leaves, consisting of green scale-like fronds free from one another or 2–3 cohering by their margins, either rootless or more generally giving off 1 or several capillary rootlets from the undersurface. Flowers very seldom produced, most minute, placed in clefts on the edges of the frond, or sunk on its surface, naked or enclosed in a spathe, usually a single female with 1 or 2 males by its side. Perianth wanting in both sexes. Male flower: Stamens 1 or 2; filaments short; anthers 1–2-celled. Female: Ovary sessile, 1-celled, narrowed into a short and stout style; stigma simple; ovules 1–7. Fruit a somewhat fleshy utricle, with 1 or several seeds; albumen fleshy or wanting; embryo straight, axile.
An order of 2 genera and 20 species, found in still waters in all countries, both temperate and tropical. It contains the smallest of all known flowering-plants, all of them being of exceedingly simple structure, and very seldom found in flower.
1. LEMNA, Linn.
Fronds proliferous from the sides near the base, with one or several delicate root-fibres descending from the under-surface. Flowers in marginal clefts of the fronds. Filaments slender; anthers didymous, 2-celled; pollen globose, muricate. Ovary with 1–7 ovules. Fruit a 1–7-seeded utricle. Seeds erect or horizontal; embryo conic or ovoid.
A small genus of 8 or 9 species, most of them of almost worldwide distribution. The two following are all that are positively known to occur in New Zealand, but others will probably be found.
| Root single. Fronds broadly ovate, thin, almost flat on both surfaces. Ovule solitary | 1. L. minor. |
| Root single. Fronds broadly ovate, thick, conspicuously tumid beneath. Ovules 2 | 2. L. gibba. |
1. L. minor, Linn. Sp. Plant. 970.—Root solitary, the sheath at its base without appendages. Frond symmetrical, ⅛–¼ in. long, ovate or oblong, nearly flat on both surfaces, green above, paler beneath; young ones growing from one side but soon becoming disconnected. Spathe unequally 2-lipped. Stamens 2, appearing successively (each a distinct male flower). Style rather long. Seed horizontal, hemi-anatropal, albuminous.—Hook. f. Fl. Nov. Zel. i. 239; Handb. N.Z. Fl. 278; Benth. Fl. Austral. vii. 163; Hegelm. Lemn. 141, t. 9, 10.
North and South Islands: Still waters, abundant throughout. Sea-level to 2000 ft. Duckweed. Almost cosmopolitan in its distribution.
2. L. gibba, Linn. Sp. Plant. 970.—Root solitary, the sheath at its base elongate, cylindric, rootcap acute. Frond symmetrical, ¼–⅓ in. long, thick and spongy, flat above, convex beneath; young ones growing from one side, but soon separating. Spathe sac-like. Stamens 2. Ovules 2 or more. Utricle bursting transversely. Seeds erect, anatropous; albumen scanty or wanting.—Hook. f. Fl. Nov. Zel. i. 239; Handb. N.Z. Fl. 278; Benth. Fl. Austral. vii. 163; Hegelm. Lemn. 145, t. 11–13.
North Island: Poverty Bay, Colenso.
I have seen no New Zealand specimens of this species, which is almost as widely distributed as L. minor. It is recognised at a glance by the conspicuously tumid under-surface of the frond.