106 LEMNACEAE (DUCKWEED FAMILY) 



separate as distinct thalli (the stalk, if it remains attached, constituting the 

 stipe). Each produces one or more roots from the under side and on the edge 

 or upper surface is borne the inflorescence which consists of 1 or more naked 

 monoecious flowers. Each flower is either a single stamen, with 2-4-pollen 

 sacs, producing barbellate pollen grains, or a single flask-shaped pistil, pro- 

 ducing 1-6 ovules. Fruit a utricle and seeds large. 



Thallus 1-3-nerved, or nerveless; rootlet solitary . . . . . .1. Lemna. 



Thallus 7-12-nerved; rootlets several, fascicled ...... 2. Spirodela. 



1. LEMNA * L. DUCKWEED 



Flowers produced from a cleft in the margin of the thallus, 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 imitat- 

 ing a single diandrous flower. Anther cells transversely divided by a partition, 

 each cavity opening transversely. Seeds 1-6. 



Thalli long-stipitate . . . . . . . . . . 1. L. trisulca. 



Thalli short-stipitate or sessile. 

 Spathe open. 



Thalli 1-nerved or nerveless. 



Thalli thin, without papules; root-cap strongly curved, tapering . 2. L. cyclostasa. 

 Thalli thick with a row of papules along the nerve; root-cap little 



curved, cylindric . . . . . . . . . 3. L. minima. 



Thalli 3-nerved; root-cap cylindric 4. L. perpusilla. 



Spathe sac-like. 



Thalli green or purplish beneath; fruit not winged . . . . 5. L. minor. 

 Thalli pale beneath, usually strongly gibbous; fruit winged . . 6. L. gibba. 



1. Lemna trisulca L. Sp. PL 970. 1753. Thallus lanceolate, submerged 

 and devoid of stomata in the primary aquatic form, ovate to oblong-lanceolate, 

 5-15 mm. long, floating and provided with stomata in the later flowering stage: 

 the later and more common form is narrowed at the base to a slender stipe, 

 thin, denticulate, with or without rootlets, and slightly 3-nerved: several in- 

 dividuals often remain connected, so as to form a chain-like series: root-cap 

 pointed: fruit symmetrical: seed deeply 12-15-ribbed. Across the continent; 

 also in the Old World. 



2. Lemna cyclostasa (Ell.) Chev. Fl. Par. 2: 256. 1827. Thallus oblong- 

 elliptic, 2-4.5 mm. long, thin, subfalcate and shortly stalked at the base, pro- 

 vided with numerous stomata, except on the borders, nerveless: root-cap 

 blunt: spathe reniform: fruit ovoid-oblong, unsymmetrical : seeds prominently 

 12-29-ribbed. Pools and slow-flowing streams; northern Wyoming, and 

 across the continent. 



3. Lemna minima Philippi, Linnaea 33: 239. 1864. Thallus oblong to 

 elliptic, 2-4 mm. long, obscurely 1-nerved, or nerveless, with a row of papules 

 along the nerve, the lower surface flat, or slightly convex, the apex rounded: 

 root-cap usually short, a little curved, rarely perfectly straight, cylindric, 

 blunt: spathe open: pistil short-clavate ; stigma concave: ovule solitary: 

 seed oblong, pointed, about 16-ribbed, with many transverse striations. 

 Wyoming to Florida and to California. 



4. Lemna perpusilla Torr. Fl. N. Y. 2: 245. 1843. Thallus small, 2-3.5 mm. 

 long, obovate, often purplish tinged beneath, unsymmetrical and abruptly 

 narrowed to a very short stalk, provided throughout with numerous stomata, 

 obscurely 3-nerved: root-cap straight, pointed: fruit ovoid: seed 12-20- 

 ribbed. Frequent; northern Wyoming to New York. 



5. Lemna minor L. Sp. PI. 970. 1753. Thallus obovate or subcircular, 

 1.5-5 mm. long, thickish, rarely reddish or purplish tinged, short-stalked 

 when young, provided throughout with stomata, nearly symmetrical, ob- 

 scurely 3-nerved, very rarely 4-5-nerved: root-cap obtuse or sub truncate: 



* The Key and specific descriptions have been taken without change from Britton's 

 Manual. 



