STATISLICS 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 southernrange. 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 it 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—thatis 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- 
