384 NAIADACEiE. 



Order CXLIV. PISTIACEJE.— Duckweeds. 



Flowers 2 — 3, appearing from the margin of a flat frond, en- 

 closed in a spathe but without a spadix, monoecious ; the sterile 

 consisting of 1 — 2 stamens ; the fertile of a 1 -celled ovary, a 

 short style and a simple stigma. Fruit membranous or cap- 

 sular, not opening. — Floating or land plants, with very cellular, 

 lenticular or lobed fronds, destitute of proper stems and leaves. 



LEMNA. Z/m%.— Duckweed. 

 (From the Greek X£/;(/ia, hark or scale ; in allusion to the form of the fronds.) 

 Spathe membranous, urceolate, with 2 sterile flowers. Sta- 

 mens 2, rarely wanting. Filaments longer than the style, curved. 

 Style usually elongated. Stigma flat. Fruit an utricle. 



1. L. trisulca Linn. : fronds thin, elliptic-lanceolate, cordate at one ex- 

 tremity, at the other serrate ; root solitary. 



Ditches and ponds. N. Y. to Virg. ; rarely in flower. July. 0. — Fronds 

 half an inch or more in length, thin, margin pellucid ; young fronds produced 

 from lateral clefts, of the same shape as the parent plant, and again proliferous 

 before they are detached. Flowers very minute. Root a single fibre. 



Star Duckweed. 



2. L. minor Linn. : fronds nearly ovate, compressed ; root solitary. 

 Stagnant waters. N. Y. to Car. June, July. (T). — Fronds a line and a half 



long, slightly convex beneath, somewhat fleshy, increasing rapidly by gemmce 

 (young fronds) so as often completely to cover the surface of stagnant water. 



Lesser Duckweed. 



3. L. gibba Linn. : fronds obovate, almost flat above, hemispheric and 

 pale beneath ; root subsolitary. 



Stagnant waters, near Uverpool, Onondaga county, N. Y. Pursh. Brad- 

 dock's Bay, Lake Ontario. Torr. June, July. (T). — Distinguished from the 

 former by its being pale and hemispheric beneath, and appearing reticulated. 



Gibbous Duckweed. 



4. L. perpusilla Torr. : fronds obovate, thin ; root solitary ; seed erect. 

 Ponds on Staten Island, IS. Y. Aug. (I). ? — Fronds a line and a half long, 



bright-green on both sides. Flowers bursting from a cleft in the side of the 

 frond. Spathe cyathiform. Seed oblong, erect. Smallest Duckweed. 



5. L. polyrhiza Linn. : fronds roundish-obovate, compressed ; roots 

 numerous, fascicled. 



Stagnant waters. N. Y. to Car. June, July. 0. — Fronds 3 — 4 lines long, 

 succulent, of a firm texture, distinctly nerved above and often dark purple be- 

 neath. Foot a bundle of 8 or 10 simple fibres in the middle of the frond. The 

 largest of all the species. It is said never to have been seen in flower either in 

 North America or in Great Britain. Larger Duckweed. 



Order CXLY. N AI AD ACE JE.— Pond weeds. 



Flowers mostly diclinous. Perianth of 2 or 4 pieces, rarely 

 wanting. Stamens definite. Ovaries 1 or more, superior. Stigma 



