GEOGRAPHIC RELATIONS OF FLORA. 459 



An analysis of these columns shows thai more than two-thirds of 

 the genera occurring in the Dismal Swamp region winch are endemic 

 in eastern North America are small ones, numbering 1 to G species, 

 and of these a half dozen are monotypic. Of the 20 genera common 

 to eastern North America and eastern Asia 2 are monotypic, 9 have 

 2 species each, and 4 consist of .'} species each. The others, with one 

 exception (Azalea), number 4 to 10 species each. On the other hand, 

 the genera which are widely distributed in America, or throughout 

 the Northern Hemisphere, are mostly of considerable size. The major- 

 ity, which have still more extensive ranges, include several of the 

 largest of the genera of vascular plants. It is generally admitted that 

 in small genera of comparatively rest ricted distribution we have to do 

 in many cases with very old and failing types. On the other hand, the 

 large, widely dispersed genera are dominant and in many cases com- 

 paratively modern types. 



The total number of species of pteridophytes and embryophytes 

 collected or observed is, roughly, 720, of which about 100 have been 

 introduced by the direct or- indirect agency of man from other regions, 

 while the remainder are indigenous. Of the indigenous species over 

 500 are endemic in extratropical North America, the great majority 

 in the country east of the Rocky Mountains and a large percentage in 

 the Austroriparian area. The nonendemic but indigenous species 

 occur likewise in the following regions: 



1. Tropical Zone. 



(a) Of both hemispheres 9 



(b) Of the New World alone _ . - 50 



— 59 



2. North Temperate Zone. 



(a) Europe alone .'. 2 



( b) Europe and Asia _ 23 



(c) Asia alone _ 6 



— 31 



Total 90 



1. 'Tropical Zone. 



(a) BOTH HEMISPHERES. 



Cyperus haspan. Centella asiatiea, 



Cyperus esculentus. Dichondra evolvulacea. 



Spirodela polyrhiza. Physalis angulata. 



Hydrocotyle umbellata. Monniera monniera. 

 Ilydrocotyle ranunculoides. 



