RELATIONSHIPS OF METASPERMAE. 595 
to notice that this state of tension which has been described, 
while of a purely biological nature, serves to produce results 
quite analogous with similar physical tensions. In the mutual 
pressure of solids the liquid that may be contained in their 
pores is crowded to the surface of the mass. In the same way 
we may figure to ourselves the weaker plants of a formation 
crowded to its periphery where they meet and struggle with 
the weaker plants of an adjacent formation. This is excel- 
lently seen in the line between forest and prairie in such a dis- 
trict as the Minnesotavalley. It is not the characteristic grass 
ofthe prairie that grows close up to the characteristic tree of 
the forest, but between the two there is a zone of plants not 
perfectly. established in either forest or prairie. This transi- 
tional formation between forest and prairie is generally com- 
posed of species weaker than the characteristic plants of either 
formation. 
Movement of tensions. Again it is apparent that under the 
present climatological conditions of the earth the equatorial 
pressure must increase and that the polar pressure must di- 
minish. Under such a generalisation of plant-dynamics it 
becomes apparent that with all the complex interdependences 
and competition of individuals with individuals, species with 
species, formations with formations there is, more fundamental 
and more general, a competition between the centrally and the 
distally located individuals, species and formations. Further 
it is apparent that the line of tension as it has been termed will 
progressively move to higher and to higher latitude. Thus as 
the cumulative equatorial pressure increases while the cumula- 
tive polar pressure at the same time decreases, the line of ten- 
sion, other things being equal, will manifest progressive 
acceleration in its movement from lower to higher latitudes. A 
number of conditions intervene to retard this movement of the 
line of tension and in consequence it is less rapid, actually 
than hypothetically. Among these retarding conditions are 
the increased difficulty of acclimatisation of north-bound plants 
as they extend further northward and the increased solidarity 
and consequently increased resistance of northern plants. And 
beside these two general factors in the retardation are the 
factors in the special cases as they might be named—the 
various conditions, topographical, nutrimental, biological, which 
confront each individual or species as it increases its range in 
any direction. The most. important visible results of these 
retarding influences are to be looked for in the changes of 
