88 LEMNACEAE. 



of the frond. Staminate flower consisting of a single stamen and the pistillate 

 flower of a single ovary containing 1 to 7 ovules. Perianth none. Flowers 

 Mini fruil scarce, in 1 species unknown. Vegetative reproduction active and 

 taking place by lateral branching, the branches being attached by slender stalks 

 (stipes). These branches soon Beparate or remain connected for some time; 

 they may at certain seasons sink to the bottom of the pond or ditch and under- 

 go a resting period. The account of genera and species here given has been 

 adapted almost entirely from Chas. II. Thompson's Revision published in the 

 9th Report of the Missouri Botanical Garden. 



Frond with 1 to several nerves and a single root; ovules 1 to 7 1. Lemna. 



Frond without nerves; root none; ovule 1 2. Wolffia. 



1. 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 sur- 

 rounded by a spathe. Ovary with 1 to 7 ovules. Fruit ribbed. (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. 



Unsymmetrical, 



Obliquely obovate; obscurely 3 to 7-nerved, papillate along the median line. 



1. 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, com- 

 monly 2, orbicular to obovate, slightly to very unsymmetrical, usually 3 to 

 5-nerved, 1 to 2 lines wide, 1 to 2^ lines long, thick, convex and slightly 

 keeled above, flat to strongly gibbous beneath; base usually acute and com- 

 monly with narrow wing margins; pistil clavate; ovules 1 to 7; fruit sym- 

 metrical, purple-tinted, winged with rounded lobes at the upper margin on 

 either side of the stigma. 



Abundant in ponds. 



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. 



Covering the surface of stagnant ponds. Variable. 



3. L. trisulca L. Ivy-leaved Duckweed. Fronds forming dense masses, 

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

 falcate, 2% to 5 lines long and 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 prominent- 

 ly L2 to lo -ribbed. 



