16 CONTKIBUTIONS FROM THE NATIONAL HERBARIUM. 



that belong to introduced species may require proof that they are 

 established. 



All the species admitted to the formal list are based upon specimens 

 in the District Flora Herbarium, which has been segregated from 

 the main collection of the National Herbarium. Species reported ^ 

 but which are not supported by specimens have been mentioned in 

 notes. All the species listed in Ward's Flora and its Supplements 

 have been accounted for, even though they can not now be verijfied 

 by specimens. Some of these are accounted for by synonymy; 

 others are shown by the specimens to be errors of identification; 

 a few of which there are no specimens in the Herbarium have been 

 mentioned in notes. 



The more commonly cultivated species have been noted under 

 the family or genus to which they belong. No attempt has been 

 made to include the vast array of ornamental plants of greenhouses 

 and parks, except a few of the well-known species, such as trees that 

 are planted along the streets. 



The nomenclature is in acc»/rd with the American Code of Botanical 

 Nomenclature, except that so-oalled duplicate binomials are not used. 



Synonyms have been given where necessary to coordinate the 

 names with Gray's Manual, Britton and Brown's Illustrated Flora, 

 and Ward's Flora. Errors of identification in Ward's Flora have 

 been indicated thus, EriantJius atopecuroides of Ward's Flora. 



Mr. L. V. Hallock contributed the photographs for plates 5, 8, 15, 

 18, 19, 20B, 25, 28, 29, 30, 31, 34, 37, 39, 41 ; Mr. E. L. CrandaU those 

 for plates 11, 17, 20A, 26, 35, 36; the photographic laboratory of the 

 Department of Agriculture that for plate 13; Mr. Hitchcock furnished 

 those for the remainder. 



The geology of the region as related to the flora has been briefly 

 discussed by Edgar T. Wherry.^ The chief ph^^siographic feature 

 is the "fall fine," which separates the Piedmont Plateau on the north- 

 west from the Coastal Plain on the southeast, being named from the 

 fact that the rivers and smaller streams, flowing southeastward to the 

 sea, often have waterfalls or rapids near this line. On a topographic 

 map of the region the course of the fall line can be readily traced by 

 the change in the contours along the stream valleys. Northwest of 

 the line the streams run in narrow steep-sided gorges, the contoui 

 lines being close together for some distance back of the stream; t3"pical 

 illustrations of this are shown in plates 5, 6, and 7. Southeast of it 

 they have broad open valleys marked by wide spacing of the contour 

 lines. This is shown by the following streams: Back Lick Run and 

 Holmes Run near Lincolnia, Fourmile Run at Barcroft, the Potomac 

 River below Georgetown, Sligo Branch near Riggs School, Northwest 



' See footnotes on page 13. 



2 In a paper read before the Botanical Society of Washington and abstracted in the 

 Journal of the "Washington Academy of Sciences (7: 435. 1917). 



