LBMNACEAE (DUCKWEED FAMILY) 269 



LEMNACEAE (Duckweed Family) 



* 

 Minute stemless plants, floating free on the water, destitute of distinct stem 



and foliage, being merely a frond, producing one or few monoecious flowers 



from the edge or upper surface, and commonly hanging routs from underneath ; 



ovules rising from the base of the cell. Fruit a l-T -seeded utricle. Seed large. 



Embryo straight. — The simplest, and some of them the smallest of flowering 



plants, propagating by the proliferous growth of a new individual from a cleft 



in the edge or base of the parent frond, also by autumnal fronds in the form of 



minute bulblets, which sink to the bottom of the water, but rise and vegetate in 



spring ; the flowers (in summer) and fruit scarce, in some species hardly ever 



seen. — These plants may be regarded as very simplified Araceae. 



1. Spirodela. Frond S-lS-nerved, with several rootlets. 



2. Lemna. Frond 1— 5-nerved, with a single rootlet. 



3. Wolffla. Frond thick, ovoid or ellipsoidal, very minute (0.5-1.3 mm. locg), without rootlets. 



4. Wolfflella. Fronds strap-shaped, thin, without rootlets. 



1. SPIRODELA Schleid. 



Anther-cells bilocellate by a vertical partition and longitudinally dehiscent 

 Ovules 2. Rootlets several, with axile vascular tissue. Otherwise as Lemna. 

 (From (TTretpa, a cord, and 5ij\os, evident.) 



1. S. polyrhiza (L.) Schleid. Fronds round-obovate (3-8 mm. long), thick, 

 purple and rather convex beneath, dark green above, palmately (mostly 

 7-) nei-ved. — Common in ponds and pools, except near our n. limits. (Temp, 

 and trop. regions.) 



3. LEMNA L. Duckweed. Duck's-meat 



Flowers produced from a cleft in the margin of the frond, usually three 

 together surrounded by a spathe ; two of them staminate, consisting of a stamen 

 only ; the other pistillate, of a simple pistil ; the whole therefore imitating a 

 single diandrous flower. Ster. Fl. Filament slender ; anther 2-celled, didy- 

 mous ; the cells dehiscent transversely. Fert. Fl. Ovary 1-celled ; style and 

 truncate or funnel-shaped stigma simple. Ovules and seeds 1-7. — Fronds 

 1-5-nerved, producing a single rootlet beneath (which is destitute of vascular 

 tissue), proliferous from a cleft in the margin toward the base. (An old Greek 

 name of uncertain meaning.) 



* Fronds oblong, long-stalked at base, remaining connected. 



1. L. trisiilca L. Fronds oblong to oblong-lanceolate (6-10 mm. long), at- 

 tenuate at base into a slender stalk, denticulate at the tip, very obscurely 3-nerved, 

 often without rootlets, usually several series of offshoots remaining connected ; 

 spathe sac-like ; seeds ovate, amphitropous, ,with small round operculum. — ■ 

 Ponds and springy places, N. S. to N. J., Tex., and w. to the Pacific. (Temp, 

 and trop. regions.) 



* * Fronds oblong to elliptical or round-ovate, sessile, soon separating. 



2. L. valdivikna Philippl. Fronds elliptic-oblong, small (2.5-4 mm. long), 

 rather thick, usually somewhat falcate, obscurely l-nerved; spathe broad-reni- 

 form ; utricle long-ovate, pointed by the long style ; seed orthotropous, oblong, 

 with a prominent acute operculum. (L. minor, var. cyclostasa Ell. ; L. cyclo- 

 stasa of auth.) — Pools, Mass. to Fla. and westw. across the continent. 

 (S. A.) 



3. L. perpusilla Torr. Fronds obovate or roundish-oborttte, oblique (2-3 

 mm. long), obscurely 3-nerved; utricle ovate ; style rather long ; seed orthotro 

 ports, ovate or oval, obtuse, with scarcely apiculate operculum. — Mass, to Fla., 

 and w. to Dak. ajid Kan. Var. tkinervis Aust. has larger distinctly 

 3-nerved fronds, and an equally cordate seed. — N. J. to Kan. and I. T. 



