2 PROCEEDINGS OK THE ACADEMY OF [Jan., 



Original Centers Concerned in North Arnerican Plant Dispersal. — 

 John W. Harshberger, Ph.D., remarked that the northern part of 

 North America was covered during Miocene times with a dense forest 

 of trees, the living representatives of which include the sequoias, mag- 

 nolias, oaks, eucalyptus and species of the genus Cinnamoynum and 

 certain palms, and that these extended as far north as Greenland. 

 As early as the close of the Cretaceous period we find an indication of 

 the separation of the American flora into an eastern and a western 

 division. In the eastern division, the deciduous trees perhaps predomi- 

 nated ; in the western, the coniferous vegetation formed a large per- 

 centage of the floral elements. The great continental glacier de- 

 stroyed this forest in the north, but remnants of it remained in the 

 south . 



At the close of the glacial period, the following centers of distribu- 

 tion of plants might have been recognized : first, the deciduous forest 

 in the east; second, the prairie flora at the center of the continent; 

 thii-d, the great coniferous forest of the Pacific northwest; fourth, the 

 desert or xerophytic flora of the Mexican tableland; fifth, the great 

 American tropic flora which occupied at one time an Antillean conti- 

 nent that later broke up into several physiographic units, viz., the 

 islands of the Greater Antilles, Central America and northern South 

 America. Wliile this Antillean landmass existed, Mexico was sepa- 

 rated from it. 



With the disappearance of the glacial ice sheet, the equilibrium 

 between these separate floras was disturbed. The tundra vegetation 

 and other Arctic species occupied during the glacial period the margin 

 of the great ice sheet. These plants migrated north to the Arctic 

 regions, but many remained behind to form the vegetation of sphagnum 

 bogs and alpine summits of the higher mountains. The deciduous 

 floras spread northward and northwestward, such plants as Populvs 

 tremuloides and Betula papyrifera reaching to Cook Inlet, Alaska. 

 The prairie flora spread northward, reaching Saskatchewan, and south- 

 ward to Texas. The Pacific coast conifers spread northward to Cook 

 Inlet, Alaska, eastward to the Rocky Mountains and then' southward, 

 supplying one of the elements of the Rocky jMountain flora. The 

 Mexican xerophytes — yuccas, agaves, cacti and the like — spread north- 

 ward into a territory which was before the glacial period characterized 

 by a more humid climate (hence the presence of many eastern decidu- 

 ous trees), and which later assumed an arid climate with the consequent 

 destruction of the deciduous element and the spread of the coniferous 

 associations. The tropic center of northern South America, the West 

 Indies, and Central America supplied part of Florida and much of low- 

 land Mexico with tro]:)ic plants. The differences, now found, being 

 emphasized b}^ the separation of the islands from each other and the 

 isolation of the floras by physiographic changes. 



A study of the several regions above mentioned emphasizes the 

 fact, that centers of distribution are determined by the following cri- 

 teria: location of the greatest differentiation of type; of dominance or 

 great abundance of individuals; of the presence of peculiar endemic 

 forms; of the continuity and convergence of lines of dispersal. 



