594 METASPERMAE OF THE MINNESOTA VALLEY 
in latitude, would favor the southward and northward move- 
ments more distinctly than would the lower mountain range. 
The writer has shown elsewhere that, of genera which reach 
their maximum number of species in Canada, about twice as 
as many species are distributed south to lat. 30°, and there- 
abouts, in the Rocky and Sierra ranges as in the Appallachian. 
3 
PRESSURES AND TENSIONS. 
General considerations of equatorial pressure. We have 
already seen that the plants of tropical regions may be con- 
sidered as striving to migrate to higher latitudes. In this way 
a general pressure of plant-population is set up along the cen- 
tral regions of the earth’s surface. This pressure diminishes 
as one approaches the equator, but becomes greater through 
cumulative additions as one passes into extra-tropical regions. 
A similar north and south polar pressure of population is set 
up by the plants of northern and southern regions. It thus 
happens that two lines of tension might be run around the earth 
in northern and southern extra-tropical regions, and these lines 
would be marked by transitional floras and by more or less or- 
ganised competition between the northern and southern forms. 
Under the positive equatorial pressure opposed by the negative 
polar pressures a segregation of metaspermic plants would take 
place in such a way that gradually the weaker and older forms 
of plants would find themselves pushed out between the inter- 
stices, as it were, of the stronger, and would thus be compelled 
to content themselves with conditions of existence progres- 
sively more difficult. In the northern hemisphere then, the 
Monocotyledones form a large percentage of the northern, and 
the Metachlamydee a large percentage of the southern species. 
For the Monocotyledones as a group are lower in the scale of 
organisation than the Archichlamydeze or Metachlamydex. The 
result of what I have named here equatorial pressure has this 
peculiar effect upon the construction of plant-zones—or to em- 
ploy a different comparison, plant-armies—that the weaker are 
always forced to fight in the front. In the case of the trees of 
the Archichlamydez in North America, those with undivided 
leaves are more northern in general than those with divided 
leaves. The range of Populus, Betula, Salix, Acer is in general 
more northern than that of Fraxinus, Gymnocladus, Gleditsia, 
Sophora or Lysiloma. But the compound leaf isa tropical char- 
acter, as indicated by Grisebach, and marks a development 
from, and improvement over the simple leaf. It is important 
