FLOKA OF THE DISTRICT OF COLUMBIA. 21 



ets of this evergreen shrub are conspicuous in winter and in early 

 spring before the deciduous trees have produced their foliage (Plate 

 38). In such localities one finds several species of heaths, such as 

 Vaccinium vaciUans, Gaylussacia haccata, Polycodium stamineum, 

 Azalea nudiflora, ChimapMla umheJlata, and C. maculata. The 

 characteristic grass of such localities is Panicum ashei. Similar to 

 the laurel thickets are the pine and oak woods, consisting of Pinus 

 virginiana and Quercus marilandica. In sandy pine woods one often 

 finds the beautiful moccasin flower. Another type of barrens is 

 found in the pure forests of scrub pine occurring on both the Piedmont 

 and the Coastal Plain. The undergrowth is very sparse, consisting 

 of such plants as Pyrola secunda, the two species of ChimapMla, 

 Mitchella re pens, and Cypripedium acaule. 



Our knowledge of the flora of the Washington area is far from com- 

 plete. Certain parts of the area are fairly well known; other parts are, 

 so far as our records show, almost whoUy unexplored. The best 

 known parts are the District itself and a rather narrow zone around 

 it; the valley of the Potomac from Great Falls to Alexandria for a 

 mile or two back from the river; and the valley of Eastern Branch 

 (or Anacostia River) as far as Beltsville. The unexplored parts 

 lie in general toward the periphery of the fifteen-mile circle, espe- 

 cially from Mount Vernon to Vienna, and the sector from the Penn- 

 sylvania Railroad to the Potomac River. The region west of 

 Brandywine is practically unknown botanically. 



SYSTEMATIC TREATMENT OF THE VASCULAR PLANTS. 



KEY TO THE FAMILIES (IN SOME CASES TO THE GENERA) BASED MAINLY 



ON VEGETATIVE CHARACTERS. 



I. Trees and Shrubs. 



Leaves needle-like or scalelike, or narrowly linear. 



Stems jointed, the joints flattened, succulent, armed with spines; fruit a juicy berry. 



CACTACEAE (p. 211). 



Stems not jointed or flattened, woody, unarmed; fruit a dry cone or a berry-like cone. 



PINACEAE (p. 60). 

 Leaves flat, broader than linear, never scalelike. 



1. Leaves compound, composed of few or many leaflets. 



Leaves opposite. 

 Stems climbing, or clambering over other plants. 

 Leaflets 3-7; corolla none, the 4 distinctseiDals colored and petal-like; fruit of numer- 

 ous hairy long-tailed achenes RANUNCULACEAE (p. 156). 



Leaflets 9-11; corolla funuelform, orange, large and showy; sepals united; fruit a 



2-celled capsule 10-15 cm. long Bignonla (p. 254). 



Stems erect, never climbing. 

 Leaflets 3 or 5, never more. 

 Plants shrubs; leaflets 3, finely toothed; flowers perfect, white or whitish; fruit 



bladder-like, 3-lobed STAPHYLEACEAE (p. 200). 



Plants trees; leaflets 3 or 5, coarsely toothed or lobed; flowers pistillate and 

 staminate, greenish; fruit a double winged samara Rulac (p. 200) 



