PONTEDERIACEAE. 53 



Family 18. LEMNACEAE. DUCKWEED FAMILY. 



Very small thallose plants floating free on the water, propagat- 

 ing by the division of the thallus; flowers 1-3, monoecious, from 

 the edge or upper surface, rare; fruit a 1-7-seeded utricle. 



Thallus 1-5-nerved, with a single rootlet. 71. LEMNA, 53. 



Thallus 5-12-nerved, with several rootlets. 71a. SPIRODELA, 53. 



71. LEMNA. 



Thallus 1-5-nerved, producing a single rootlet beneath; 

 flowers produced from a cleft in the margin of the thallus, usually 

 3 together, surrounded by a spathe; two staminate, each of a 

 single stamen, the other pistillate of a simple pistil; ovary 

 1 -celled. 



Thalli oblong, separate or soon separating. L. minor. 



Thalli oblong, each narrowed into a sheath-like base, remaining 



connected. L. Irisulca. 



Lemna minor L. Thalli round to elliptic-ovate, 2-5 mm. long, very ob- 

 scurely 3-nerved; seeds oblong-obovate, amphitropous, with prominent oper- 

 culum. Common in ponds, very rarely fruiting. 



Lemna trisulca L. Thalli thin, oblong to oblong-lanceolate, attenuate 

 into a stalk-like base, usually several remaining connected, each faintly 3- 

 nerved; seeds ovate, amphitropous, with a small round operculum. In ponds 

 near Potlatch, Idaho. 



71a. SPIRODELA. 



Very similar to Lemna but rootlets several, with axile vascular 

 tissue; anther-cells divided by a vertical partition and dehiscing 

 longitudinally. 



Spirodela polyrhiza (L.) Schleid. Thallus round-obovate, 3-8 mm. long, 

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

 7-nerved. Silver Lake, Pullman, Darlington. 



Family 19. PONTEDERIACEAE. PONDWEED FAMILY. 



Aquatic herbs with perfect more or less irregular flowers, sur- 

 rounded by a spathe ; perianth of 6 petal-like segments free from 

 the 3-celled ovary ; stamens 3 or 6, unequal or dissimilar, on the 

 throat of the perianth ; style 1 ; fruit a 1-3-celled capsule or 

 1 -celled utricle. 



72. HETERANTHERA. 



Low herbs living in mud or shallow water, with a 1-few- 

 flowered spathe bursting from the sheathing side or base of a 

 petiole ; perianth-limb somewhat equally 6-parted ; stamens in the 

 throat, usually unequal; capsule 1 or incompletely 3-celled. 



