422 THE DUCKWEED FAMILY. [ Lemna. 
like those of Z. minor, but rather larger and much thicker, flat above, 
spongy, and almost hemispherical underneath, with a single root to each. 
Stamens 2. Ovary with 2 or more ovules. 
With the station and range of ZL. minor it is everywhere less common. 
Local in England and Ireland, and rare in Scotland. 7%, summer, very 
rarely. 
4, %. polyrrhiza, Linn. (fig. 947). Greater Duckweed.—Fronds 
larger than in any other species, attaining 3 or 4 lines diameter, broadly 
ovate or orbicular, rather thick, with a cluster of roots under each one. 
Flowers of Z. gibba. 
As widely dispersed as the other species, and rather more frequent than ~ 
LL. gibba, except in Scotland. The flowers have never been observed in 
Britain. 
5, &. arrhiza, Linn, (fig. 948). Rootless Duckweed.—Fronds rather 
thick and swollen on the underside, but much smaller than in any other 
British species, only half a line long and rarely emitting any root. Anther 
solitary, globose, sessile. Ovary with a single ovule, the flower and fruit 
issuing from the upper surface, not from the edge of the frond as in the other 
species, on which account this one has been separated as a genus under the 
name of Wolffia. 
On ponds and still waters, common in the warmer regions of Asia and 
Africa, and perhaps in South America, more rare in Europe. In Britain 
flowerless, and confined to the south-eastern counties. 2. summer. 
LXXVITI. NAIADEZ, THE NAIAD FAMILY, 
Aquatic floating or submerged plants; the Jeaves either 
sheathing at the base or accompanied by sheathing stipules, 
alternate or sometimes opposite. Flowers axulary, incon- 
spicuous, solitary or spiked, usually proceeding from a sheathing 
bract. Perianth none, or consisting of 4 small, scale-like seg- 
ments. Stamens 1, 2, or 4. Ovaries either of 2 or 4 distinct 
carpels, each with a single ovule and a separate stigma, or 
single, with 1 ovule and 2 to 4 stigmas. Fruit consisting of 1, 
2, or 4 seed-like nuts, each with one seed, without albumen. 
An Order not numerous in species, but abundantly diffused over all parts 
of the world, in the sea as well as in fresh waters. 
Stems creeping in sand or mud under salt-water. Leaves very 
long and linear. Flowers within the base ofa long, linear 
bract, like the stem-leaves : . ° : - . . Ll. ZOosrera. 
Stems floating. Flowers axillary. 
Ovaries simple, with 1 styleand 2 or3 stigmas. Leaves oe 
site or whorled, and often toothed : . . 2. NatAs, 
Ovaries of 4 carpels, each with a separate stigma. 
Flowers and carpels axillary and sessile. Leaves opposite, 
very slender f : . 3. ZANNICHELLIA, 
Flowers usually 2, on an axillary peduncle. Carpels 
stalked, pear-sb aped, leaves alternate, very slender . 4, RUpprA. 
Flowers in pedunculate spikes or heads. Carpels sessile. 
Leaves alternate or rarely opposite . ° ° ° . 5. PoTAMOGETON, 
