STATISTICS OF METASPERMAE. 759 



the summer are pretty generally southern in their character, 

 and there is no drainage towards the valley from the far west- 

 ern regions of the continent. Geologically, too, the valley has 

 belonged, since the very early ages, to the Atlantic North 

 American continent. Before the union of the eastern and 

 western halves of the continent, Minnesota and much sur 

 rounding territory was formed as a portion of the eastern area. 

 The present topography of the continent is such that a district 

 situated as is that of the Minnesota valley must perforce receive 

 its population of plants from the east and from the south, 

 rather than from the west or north. It, appears, therefore, 

 that the geographically central position of the valley does not 

 by any means counterbalance its geological, topographical, 

 hydrographical southeasternness. This southeasternness is 

 reflected in a preponderantly southeastern metaspermic flora. 



From another point of view it will be seen that the equa- 

 torial pressure of plant population tends to crowd into the 

 valley species of southern range. This biological phenomenon 

 may be deemed of importance scarcely second to the physical 

 phenomena named above, as a determinant of the southeastern- 

 ness of the Minnesota valley plant inhabitants. Not only does 

 the equatorial pressure tend to inject southern forms into the 

 valley area, but ii tends also to fill the valley with species 

 strong on account of their southernness. As has been seen it 

 is particularly the newest and most vigorous group of plants — 

 the Metachlamydeae — that is characterised by a general north- 

 bound movement. Thus, doubly, the biological conditions 

 of plant immigration favor an extensive movement from the 

 south rather than from the north. More plants and stronger 

 plants may be expected from that direction than from any 

 other. The various modifications of this general movement 

 have already been discussed. 



The two groups of causes for the southern and eastern char- 

 acter of the Minnesota valley plant-population will, upon an- 

 alysis, be found adequate to explain the preponderance of 

 species. 



In conclusion the following tabulation is presented as a sum- 

 mary of the characters peculiar to each of the three taxonomic 

 groups represented in the valley of the Minnesota. The num- 

 erals indicate the order of the importance of each group in 

 the character in question. For example, the Monocotyledones 

 show a larger percentage of aquatic plants than the Archich- 

 lamydeae and these a larger percentage than the Metachlamy- 



