104 PUBLIC HEALTH BULLETIN NO. 286 



4. Joints with unequally rounded sides, thick, 3- 4. L. perpusilla, 



nerved or -veined. 

 4. Joints with parallel sides, thin, 1-nerved or 6. L. valdiviana. 



-veined. 



1. L. trisulca L. Star Duckweed — Found in only the western part 

 of our territory, not noted by the writers. 



2. L. minor L. Duckweed — Extremely difficult to distinguish from 

 the following species except in fruit, and fruits very rarely in nature. 

 The range of this species includes all of our territory, and it is common. 

 A dense covering of this plant often inhibits mosquito breeding, but 

 partial growths often harbor Anopheles quadrimaculatus larvae. 



3. L. minima Philippi — The remarks under the preceding apply 

 aUke to this species, although its distribution is probably more limited. 



4. L. perpusilla Torr. — Ponds, range includes all of our territory. 



5. L. valdiviana Philippi — The remarks under the preceding species 

 apply also to this. 



4. Spirodela Schleid. 



S. polyrhiza (L.) Schleid. — This easily recognized, large Duckweed 

 occurs over practically all of our territory and is very frequent. It 

 has often been foimd in association with Anopheles quadrimaculatus 

 except when in dense, blanket-like growth. 



MAYACACEAE Bog-moss family 



1. Mayaca Aubl. 



1. Stalks of flowers much longer than the leaves 1. M. aubletii. 



1 . Stalks shorter than the leaves 2, M. fiuviatilis. 



1. M. aubleti Michx. Bog-moss — Ranges over most of our Coastal 

 Plain, generally in acid ponds. 



2. M. fiuviatilis Aubl. — Range more restricted, known only from 

 Florida to Georgia and Mississippi. Much less frequent than the 

 preceding species and is considered by some to be only a form of it. 



XYRIDACEAE Yellow-eyed Grass famUy 



1. Xyris [Gronov.] L. 



1. Leaves round, filiform; sterile filaments glabrous 1. X. baldmniana. 



1 . Leaves flat ; sterile filaments bearded 2. 



2. Lateral sepals included under bracts 3. 



2. Lateral sepals partially exserted 11. 



3. Keel or wing of the lateral sepal ciliate 4. 



3. Keel or wing of the lateral sepal incised, cross-toothed 6. 

 or lacerate. 



4. Lateral sepals winged, the wing partially 5. 



ciliate, not bearded. 



4. Lateral sepals keeled, the keel mainly cilio- 2. X. torta. 

 late its entire length, bearded at the apex. 



