FLORA OF THE DISTRICT OF COLUMBIA. 115 



3. SPIRODELA Schleid. 



1. Spirodela polyrhiza (L.) Schleid. 



Common on the surface of stagnant or slow-moving water, nsually associated with 

 Lemna. Blooms in midsummer, but rarely sets seed, and goes through the winter in 

 a condensed resting stage in the mud on bottoms of ponds. Generally distributed 

 in temperate and tropical regions. {Lemna polyrhiza h.) 



20. XYRIDACEAE. Yellow-eyed grass Family. 



1. XYRIS L. Yellow-eyed grass. 



Bracts of the head with a definitely marked green middle part; lateral sepals mi- 

 nutely erose on the keel; stems subcompressed and narrowly 2-winged toward 

 the summit, slender, scarcely flexuous; leaves mostly flat, grasslike. 



1. X. caroliuiana. 



Bracts of the head usually green-tipped but with no marked line between the middle 

 and the margins; lateral sepals ciliolate on the keel, a tuft of silky hairs fit the 

 apex; stems not compressed, obscurely 3-angled toward the summit, usually wiry 

 and flexuous; leaves usually firm and flexuous -. .2. X. flexuosa. 



1. Xyris caroliniana Walt. 



Among sphagnum in white sand of magnolia bogs; Powder Mill Swamp region and 

 near Savage Station. Aug.-Sept. Atlantic Coastal Plain and at the head of Lake 

 Michigan. 



2. Xyris flexuosa Muhl. 



Wet sandy sunny places in bogs and along streams and ditches; Takoma Park, 

 southward and eastward, and at Arlington; infrequent. July-Aug. Eastern U. S. 



21. ERIOCAULACEAE. Pipewort Family. 



1. ERIOCAULON L. Pipewort. 



Scapes weak, not rigid, not over 15 cm. tall unless submerged. Sheaths loose; leaves 



scarcely longer than the sheaths 1. E. septangulare. 



Scapes firm, strongly 10-14-ridged, commonly 50 cm. tall, more or less twisted. 



2. E. decangulare. 



1. Eriocaulon septangulare With. 



Alluvial soil by streams and tidal flats; near the mouth of the Eastern Branch and 

 at the mouth of Fourmile Run. Aug.-Sept. Eastern U. S. 



2. Eriocaulon decangulare L. 



Among sphagnum, mostly in magnolia bogs; Takoma Park, southward and eastward. 

 Aug.-Sept. Atlantic Coastal Plain from N. J. southward. 



The seeds of Eriocaulon are dispersed by the wind. At maturity in late September 

 and October the chaffy floral bract and the fertile flower with its dry sepals and 

 corolla and ripened capsule break away from the receptacle as a whole and float 

 in the breeze, buoyed up by the tufts of copious hairs. 



22. COMMELINACEAE. Spiderwort Family. 



Most of the plants of this family are somewhat succulent, and have mucilaginous juice 

 and frequent cells with needle-shaped crystals. Several species of different genera 

 are cultivated as house plants, the most common being one of the Wandering Jews 

 (Zebrina pendula Schnizl.\ with leaves purple beneath. 



Petals equal, purplish blue; filaments bearded; perfect stamens G; bracts leaflike or 

 inconspicuous; leaves long-linear. Flowers in terminal umbels. 



1. TRADESCANTIA. 



