Postglacial spreadings of Plants. 211 
Mac MILLAN') remarks on this point, with reference to the character of the 
Minnesota flora, that it has been shown that while the valley of the Minnesota 
is geographically central, it is by no means botanically central, but, on the 
contrary, strongly southern and eastern. BESSEY?) has shown that the trees 
and shrubs of Nebraska have come up the Missouri bottoms and spread from 
the southeastern corner of the state west and northwest. Mason states that 
the trees of Kansas show the same origin. Apams?) has demonstrated this 
clearly in a study of the faunal distribution of animals and in a cursory way 
with plants. BRAY alludes to the same eastern origin in speaking of the east 
Texas timber belt. “The further extension of this forest is checked near the 
Brazos River by the drier climate of the southwest. Here its vanguard is 
broken into straggling detachments, of which only the hardier push onward 
along the prairie stream ways or up the deeper canyons of the hills. It is a 
striking phenomenon, this breaking up and gradual dwindling away of so vast 
and vigorous a forest. Not only in Texas, but far to the north, through the 
Indian Territory, Kansas, Nebraska, and the Dakotas, the same thing may 
be seen. Like a vast wave that has rolled in upon a level beach, the Atlantic 
forest breaks upon the dry plains, halting, creeping forward, thinning out, and 
finally disappearing, except where, along a river course, it pushes far inland *). 
In line with this argument, the conclusions of TRANSEAU are apropos. Transeau 
recognizes four centers of distribution in eastern North America in which the 
complex of climatic factors is most favorable to the type of forest vegetation 
there at present localized. When we depart from such centers, we find the 
conditions more and more unfavorable to the four particular types of forest 
which are: (1) the northeastern coniferous centering in the St. Lawrence basin 
and corresponding with our St. Lawrence Great Lake Region (see colored 
map): (2) the deciduous forest centering in the lower Ohio basin and southern 
Appalachians (our Piedmont-Appalachian-Ozark Plateau Region); (3) the south- 
western coniferous forest, centering’ in the south Atlantic and Gulf coastal plain 
(our Atlantic-Gulf Coastal Region); and the insular tropic forest in the southern 
part of Florida on the West Indies (our Bahaman Region). TRANSEAU) be- 
lieves, that the present limits of these centers (which implies that in these places 
the trees attain their best development) is due to the influence of present 
climatic factors, viz., temperature, relative humidity, wind velocity and rainfall 
which most powerfully influence plant growth. These factors can be expressed 
1) er ConwAv: The Metaspermae of the Minnesota Valley 1892: 
2) Bessev, C. E.: The Forests and forest Trees of Nebraska. Annual Report State Board 
ae ie 79—102. 
3) ADAMS, CHAS. C.: Southeastern United States as a Center of geographical Distribution of 
Flora and Fauna. Biological Bulletin III: 115— 129. 2. 
4) Bray, WILLIAM C.: Forest Resources of Texas. Bulletin 47, Bureau of Forestry U. S. 
Department Agriculture 1904: 15. 
5) Transeau, E. N.: Forest Centers of eastern America. American Naturalist XXXIX: 
875—889. Dec. 1905. With 6 maps. 
14* 
