552 LEMNACE^E. (DUCKWEED FAMILY.) 



some of them the smallest of flowering plants, propagating by the prolif- 

 erous growth of a new individual from a cleft in the edge or base of the 

 parent frond, remaining connected for some time or separating, 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. Frond more or less 

 cavernous ; the upper surface furnished with stomata. These plants 

 may be regarded as very simplified Araceae. 



1. Spirodela. Frond 7 - 11-nerved, with several rootlets. 



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



3 Wolffia. Frond thick, very minute (J - " broad), without rootlets. 



1. SPIRODELA, Schleiden. 



Anther-cells bilocellate by a vertical partition and longitudinally dehiscent. 

 Ovules 2. Frond 7 -11-nerved or more; rootlets several, with axile vascular 

 tissue. Otherwise as Lemna. (From <T7re?pa, a cord, and S^Aos, evident.) 



1. S. polyrrhiza, Schleid. Fronds round-obovate (2 -4" long), thick, 

 purple and rather convex beneath, dark green above, palmately (mostly 7-) 

 nerved. (Lemna polyrrhiza, L.) Very common in ponds and pools, through- 

 out N. Am., but very rarely found in flower or fruit. (Eu.) 



2. LEMNA, L. DUCKWEED. DUCK'S-MEAT. 



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

 gether 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 ; pollen-grains large, spherical, muri- 

 cate. 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 root- 

 let beneath (which is destitute of vascular tissue), proliferous from a cleft in 

 the margin toward the base, and at length stipitate ; the tissue abounding with 

 bundles of rhaphides. (An old Greek name of uncertain meaning.) 



* Ovule solitary, orthotropous or nearly so; frond 1 -3-nerved, thin. 

 - Fronds oblong, stalked at base, remaining connected. 



1. L. trisiilca, L. Fronds oblong to oblong-lanceolate (6-9" 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 oper- 

 culum. Ponds and springy places, N. Scotia to N. J., west to the Pacific. (Eu.) 



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



2. L. Valdiviana, Philippi. Fronds elliptic-oblong, small (about V 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. Torreyi, Austin.) Pools, N. J. and 

 southward, westward across the continent. (S. Am.) 



3. L. perpusilla, Torr. Fronds obovate orroundish-obovate, oblique (1 - 

 \$" long), obscurely 3-nerved ; utricle ovate ; style rather long ; seed orthotro- 



