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 Minnesota valley. It is not the characteristic grass 

 of the 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 



