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. 



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 diflBcult. In the northern hemisphere then, the 

 Monocotyledones form a large percentage of the northern, and 

 the MetachlamydesD a large percentage of the southern species. 

 For the Monocotyledones as a group are lower in the scale of 

 organisation than the Archichlamydeag or Metachlamydese. 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 Archichlamydeai 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, Oymnocladns, Qleditsia, 

 Sophora or Lysilonia. But the compound leaf is a tropical char- 

 acter, as indicated by Grisebach, and marks a development 

 from, and improvement over the simple leaf. It is important 



