DUCKWEED FAMILY 245 



Eefs. SPIRODELA POLYRHIZA Schleiden, Linnaea 13:392 (1839). Lemna polyrhiza L. 

 Sp. PI. 970 (1753), type European. 



2. LEMNA L. DUCKWEED. 



Fronds disk-shaped, usually with a central nerve and with or without several 

 lateral nerves, each with a single root which is commonly provided with a root 

 cap. Reproductive pouches 2, appearing as clefts in either margin of the basal 

 portion of the frond, each containing a cluster of 3 flowers surrounded by a 

 spathe. Ovary with 1 to 7 ovules. Fruit ribbed. Species about 8, all continents. 

 (Ancient Greek name.) 



Frond with a long stipe, mostly submerged and forming large masses; papillae none 



3. L. trisulca. 

 Frond with a short stipe, floating on the surface. 



Symmetrical or nearly so, papillate along the median line. 

 Oblong-ovate; fruit more or less lenticular. 



Upper surface uniformly green; margin of the fruit without appendages; seed 



always 1 2. L. minor. 



Upper surface mottled with irregular brown streaks; margin of the fruit with 



rounded wing lobes; seeds 1 to several 1. L. gibba. 



Oblong to elliptical, small, green on both surfaces, with a row of papillae along the 



mid-nerve; fruit elongated 5. L. minima. 



Unsymmetrieal. 



Obliquely obovate, obscurely 3 to 7-nerved, papillate along the median line....l. L. gibba. 

 Long oblong, thin, obscurely 1-nerved; papillae none 4. L. cyclostasa. 



1. L. gibba L. GIBBOUS DUCKWEED. Fronds 1 to 4 in a group, commonly 

 2, orbicular to obovate, slightly to very unsymmetrical, usually 3 to 5-nerved, 

 1 to 2 lines wide, 1 to 2i/ lines long, thick, convex and slightly keeled above, flat 

 to strongly gibbous beneath ; base usually acute and commonly with narrow wing 

 margins ; pistil clavate ; ovules 1 to 7 ; fruit symmetrical, purple-tinted, winged 

 with rounded lobes at the upper margin on either side of the stigma. 



Stagnant ponds. All continents except South America. 



Locs. Pasadena, Compton, Santa Monica Canon, San Pedro, San Juan Capistrano (Erythea 

 4:195); Mound City, San Bernardino Valley (Erythea, 7:90); San Francisco (Bot. Cal. 

 2:190). 



Eef. LEMNA GIBBA L. Sp. PI. 970 (1753), type European. 



2. L. minor L. SMALLER DUCKWEED. Fronds solitary or few in a cluster, 

 round to elliptic-obovate, green or purplish beneath, uniformly bright-green 

 above, convex on both sides, upper surface sometimes slightly keeled and with a 

 row of papillae along the mid-nerve, the apical one usually quite prominent ; 

 pistil clavate ; ovule 1 ; fruit not winged, projecting about % beyond the margin 

 of the frond. 



Stagnant pools, abundant. All continents except South America. 

 Loc. Lobos Creek, San Francisco (Bot. Cal. 2:190). 

 Ref. LEMNA MINOR L. Sp. PI. 970 (1753), type European. 



3. L. trisulca L. IVY-LEAF DUCKWEED. Fronds forming dense masses, ob- 

 long to oblong-lanceolate, slightly unsymmetrical and frequently a little falcate, 

 21/0 to 5 lines long and l 1 /^ lines wide, the long stipe attached to the basal margin ; 

 floating fronds with shorter stipes and cavernous throughout the central portion ; 

 submerged fronds with long twisted stipes ; seed prominently 12 to 15-ribbed. 



Cold springs and running water, Sierra Nevada. All continents except South 

 America. 



Locs. Plumas Co. (Bot. Cal. 2:189); Bouldin Isl. (Zoe, 4:217); San Francisco (Bot. Cal. 

 2:189); San Gabriel Creek, San Gabriel Mts. (Bot. Cal. 2:189). 



Ref. LEMNA TRISULCA L. Sp. PI. 970 (1753), type European. 



4. L. cyclostasa Chev. Fronds solitary or more commonly 2 to 8 cohering 

 in a more or less curved chain, thin, oblong to obovate-oblong, usually somewhat 

 falcate, % to % lines wide by 1 to I 1 /* lines long, without papillae ; base of the 



