RELATIONSHIPS OF METASPERMAE. 611 
different portions of the earth’s land-surface in point of vege- 
tation. These differences resolve themselves into distance, 
humidity and history differences. It happens then that the 
earth can be divided into floral regions. The northern hemis- 
phere is more composite than the southern and its several re- 
gions are more affected by each other than are those of the 
southern. The Minnesota valley is found to bear closer rela- 
tionship with certain portions of the earth than with others. 
The geological history of the country is needed for the explan- 
ation of all these phenomena. There is a general tension be- 
tween plants centrally and distally located on the earth’s land 
surface. More special tensions, between areas less and less, 
arise from this general tension and contribute to the general 
tension. The tension-lines are not constant, but variable 
under a complicated series of modifying laws. By means of 
these tensions, habitat, physiology, evolution, have been 
altered in their character. The origin of metaspermic 
plants was probably in the Jurassic. During Mesozoic time 
they hada very slight foot-hold on the periphery of stronger 
formations. In Tertiary time they underwent various migra- 
tions and became more strongly established. After Tertiary 
time the movements of glaciers had a profound influence upon 
the evoJution and distribution of plants. The results of this 
glacial period are to be discovered in the conditions of the 
present. To-day, under various forces, the modification of the 
flora still continues. 
