FLORA OF THE DISTRICT OF COLUMBIA. 145 



6. Rumex patientla L. , 



Dumps and waste places; apparently not very common. May-June. North- 

 eastern states westward ; naturalized from Eur. 



6. Rumex crispus L. Narrow dock. 



Roadsides and waste places; common. Summer. Naturalized from Eur., nearly 

 throughout N. Amer. 



Hybrids between this species and R. obtusifoUus are reported by Ward. 



2. POIYGONITM L. Knotweed. 



Stem and branches angled. 



Leaves plicate; fruits erect, on stout pedicels 6. P. tenue. 



Leaves flat, the margins merely revolute; fruits drooping on slender reflexed pedicels. 



6. P. douglasii. 

 Stem and branches terete or nearlj^ so, usually striate. 



Plants chiefly prostrate, the branches mostly basal; flowers comparatively small, 



the sepals less than 2 mm. long. 



Leaves usually acute; achene ovoid, acute 1. P. aviculare. 



Leaves obtuse or subacute; achene broadly ovoid, constricted below the apex, 



often conspicuously so 2. P. buxiforme. 



Plants erect or ascending, more or less diffusely branched above the base; flowers 



larger, the sepals 2-3 mm. long. 



Leaves elliptic to obovate, obtuse to acutish, yellowish green 3. P. erectum. 



Leaves linear-elliptic to lance-elliptic, acuminate, green (not yellowish). 



4. P. atlanticum. 



1. Polygonum aviculare L. Knotgrass. 

 Cultivated and waste places; a common weed. Summer and autumn. N. Amer. 



and Eur. 



2. Polygonum buxiforme Small. 



Waste ground; apparently rare locally, a single record. July. Widely distributed 

 in N. Amer. 



3. Polygonum erectum L. 



Moist or dry situations; common. July-Sept. Widely distributed in N. Amer. 



4. Polygonum atlanticum (Robinson) Bicknell. 



Vicinity of Government fish ponds, a single record (July, 1902). Adventive; native 

 of salt marshes along the N. Eng. coast. 



6. Polygonum tenue Michx. 



Dryish, moreorlesssandy slopes; not very common. July-Sept. Eastern N. Amer. 



6. Polygonum douglasii Greene. 



Old circus grounds; a single record (Aug. 7, 1912). Western U. S.; adventive east- 

 ward. 



3. TO VARA Adans. 



1. Tovara vlrginica (L.) Raf. Jumpseed. 



Low moist woods and alluvial thickets; common. Late summer. Eastern N. Amer. 



4. PERSIC ARIA Mill. 



Spikes solitary or in pairs. 

 Plants usually aquatic ; leaves floating, long-petioled , obtuse or acutish ; spikes ovoid 



or oblong, not more than 2.5 cm. long, the peduncles glabrous. . . 1. P. amphibia. 

 Plants usually of swamps or stream banks, erect or ascending; leaves short-petioled, 



acuminate; spikes 3-10 cm. long, the peduncles hispid.. 2. P. muhlenbergli. 



6928&— 19 10 



