86 PONTEDERIACEAE. 



Family 19. LEMNACEAE. Duckweed Family. 

 Very small thallosc 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-ncrvcd, with a single rootlet. 102. Lemna, 86. 



Thallus 5-12-ncrvcd, with several rootlets. 103. Spirodela, 86. 



102. 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 single pistil; ovary 1-celled. 



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



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



connected. L. tristdca. 



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

 scurely 3-nerved; seed oblong-obovate, amphitropous, each with a prominent 

 operculum. 



Ver>' common on still ponds, often mixed with Spirodela. 



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

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

 nerved; seed ovate, amphitropous, with a small round operculum. 



Still ponds and ditches, rare. 



103. SPIRODELA. 



Very similar to Lemna but rootlets several and with axile 

 vascular tissues; 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. 



Common on the surface of still ponds. 



Family 20. 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. 



104. 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. 



