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 earch 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. Daring Mesozoic time 

 they had a 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 evolution 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. 



