94 NAIADACEAE 
disk less specialized than a leaf (a thallus). The rootlet is a 
floating thread extending from the under side of the green disk 
downward about an inch. From one side of the disk grows the 
minute flower, or, technically, flowers, for the minute group con- 
sists of two stamens and a pistil which, although in close relation 
are not considered as in the same receptacle. These are surrounded 
by a green envelope which is regarded as a spathe, hence these 
plants have been classified as closely related to the Arums. 
In the genus Wol/fia, minute floating bodies, there is no root- 
like appendage. 
1. LEMNA, L. 
Characters included above, with the root-like appendages. 
1. L. trisulca, L. (Fig. 3, pl. 6.) Ivy-LEAveD DucKweep. Fronds 
in form of a snow shoe, often more than 3 in. long. Ponds, ditches, ete. 
2. L. perpusilla, Torr. Minute DvucKWEED. Seale or frond about 
1/10 in. long, pear-shaped, with 3 nerves on the surface, purplish be- 
neath. Ponds, lakes, ete. 
3. L. minor, L. (Fig. 4, pl. 6.) Lesser DucKweEED. Nearly round, 
more or less 3-nerved, about 4 in. diameter, several sticking together. 
Common in stagnant pools. 
4. L. cyclostasa, (EIl.) Chev. VaupiviA DucKWEED. Oval with one 
extremity pointed, 1/12 in. diameter. No nerves on surface. 
5. L. polyrhiza, L. (Fig. 2, pl. 6.) GREATER DucKWEED. Disk 
rounded or oblong, 7 to 12 nerved. MRootlets in a cluster of 5 to 11. 
(This plant is assigned te a genus Spirodella, 8. polyrhiza, by Schleiden.) 
2. WOLFFIA, Horkel. 
In our region, minute flowering plants, the smallest of flowering plants, 
small grains which can only be distinguished from the lower, non-flower- 
ing plants, by the aid of a strong magnifying glass. There is no root- 
like thread, the plant is not flattened like Lemna, but more or less glob- 
ular. 
1. W. columbiana, Karst. (Fig. 11, pl. 6.) Plant nearly spherical, 
about 1/50 in. diameter. Not dotted. Floating near the surface of 
stagnant waters. 
2. W. punctata, Griseb. (W. brasiliensis, Eng!m.) Plant oblong, 
about 1/50 in. long, with brown dots above and below. Floating in simi- 
lar situations to No. 1. 
Famity I].—NAIADACEAE. Ponpweep FAMILY 
Plants of fresh or salt water, entirely immersed except that the 
flowers may appear above water, and the upper leaves may float. 
Upper leaves usually in pairs, the lower generally alternate, and in 
2 rows. Flowers usually small, inconspicuous, in lengthened 
