DUCKWEED FAMILY. 365 
Family ro. LEMNACEAE Dumort. FI. BeleerA7.) 1827." 
DUCKWEED FAMILY. 
Minute perennial floating aquatic plants, without leaves or with only very 
rudimentary ones. The plant body consists of a disc-shaped elongated or irreg- 
ular thallus, which is loosely cellular, densely chlorophyllous and sometimes 
bears one or more rootlets. The vegetative growth is by lateral branching, the 
branches being but slightly connected by slender stalks and soon separating. 
In the autumn these disconnected branches fall to the bottom of the ditch or 
pond, but rise and again increase in size in the spring. ‘The inflorescence con- 
sists of one or more naked monoecious flowers borne on a slight lateral promi- 
nence on the edge or upper surface of the plant. ach flower commonly con- 
sists of but a single stamen or a single flask-shaped pistil. The anther is pro- 
vided with two to four pollen-sacs, containing spherical minutely barbellate 
grains. ‘The pistil is gradually narrowed to the funnel-shaped scar-like stigmatic 
apex, and produces 1-6 erect or inverted ovules. The fruit is a 1-6-seeded utricle. 
The family comprises the smallest of the flowering plants and contains 3 genera, all of which 
are represented in North America, and about 26 species of wide distribution. 
Thallus prominently nerved; with a membranous spathe and a cluster of several rootlets; ovules 2 
1. Spzrvodela. 
Thallus slightly nerved; with a membranous spathe and a single rootlet; ovules 1-6. 2. Lemna. 
Thallus nerveless; without spathe or rootlets; ovule r. 3. Wolffia. 
1. SPIRODELA Schleid. Linnaea, 13: 391. 1839. 
Thallus disc-shaped, 7-12-nerved. The lateral branches subtended by a single bipartite 
basilar rudimentary leaf. The thinly-capped rootlets as well as the nerves are provided 
with a single bundle of vascular tissue. The ovary produces two anatropous ovules. Fruit 
unknown. ([Greek, in allusion to the cluster of rootlets. ] 
Two species, the following, and .S. O/igorhiza, a native 
of southern Asia, Australia and the Fiji Islands. ie 
1. Spirodela polyrhiza (L,.) Schleid. 
Greater Duckweed. (Fig. 884.) 
Lemna polyrhiza V,. Sp. Pl. 970. 1753. 
Sptrodela polyrhiza Schleid. Linnaea, 13: 392. 1839. 
Thallus round-obovate, 2//-5’’ long, thick, flat and 
dark green above, slightly convex and purple beneath, 
palmately 5-11-nerved. Each thallus bears a central 
cluster of from 5-11 elongated rootlets. Rootcap 
pointed. 
In rivers, ponds, pools and shallow lakes, Nova Scotia 
to British Columbia, south to South Carolina, Texas, 
northern Mexicoand Nevada. Widely distributed in the 
Old World and in tropical America. Seldom collected in 
flower. 
2. LEMNA IL, Sp. Pl. 970. 1753. 
Thallus dise-shaped, usually provided with a central nerve and with or without two or 
four lateral nerves. Each thallus produces a single rootlet, which is devoid of vascular tis- 
sue and is commonly provided with a thin blunt or pointed rootcap. The ovary contains 
from one to six orthotropous amphitropous or anatropous ovules. Fruit ovoid, more or less 
ribbed. Endosperm in one or three layers. [Greek, in allusion to the growth of these 
small plants in swamps. ] 
Seven species, in temperate and tropical regions, Besides the following species, Lemna An- 
golensis is a native of Lower Guinea and Lemna paucicostata has been found in Asia, Africa and 
South America. 
Ovule 1, amphitropous or orthotropous; endosperm in three layers. 
Thalli of two kinds, ovule amphitropous. 1. L. trisulca. 
Thalli all alike. 
Thallus without lateral nerves; ovule orthotropous. 2. L. Valdiviana. 
Thallus with lateral nerves; ovule orthotropous. 3. L. perpusilla, 
Thallus with lateral nerves; ovule amphitropous. 4. L. minor. 
Ovules 2-6, anatropous; endosperm in one layer. 5. L. gtbba. 
 * Text contributed by Mr. EDMUND P. SHELDON. 
