584 METASPERMAE OF THE MINNESOTA VALLEY. 



By the assistance of this fact of organised and stratified com- 

 petition in the realm of plant society the dynamic relations of 

 plants to one another are, in general, to be explained. We no 

 longer permit ourselves to look at a grove standing in the midst 

 of the prairie as stable or even as quiescent, but we picture to 

 ourselves the complex condition of strain which exists in vary- 

 ing degree and under different degrees of organisation, between 

 the different plants, species and groups of species. Such a dy- 

 namic condition should perhaps be recognised in terminalogy 

 more than it is and instead of speaking of the northern group 

 of plants one should name such a floral element the south-bound 

 group and instead of calling by the name of "southern" those 

 plants which occupy a southern range one should refer to them 

 as north-bound. For certainly the exigency of existence is 

 such for every creature that it welcomes an expansion of op- 

 portunity for development. Room for growth is an important 

 factor of such an opportunity, and for the plant already estab- 

 lished in high northern latitudes this room for growth is to be 

 found only by a southward extension. 



General features of plant-distribution. The total number 

 of flowering-plant species at present occupying the crust of the 

 earth is estimated by De Candolle to be in the vicinity of 

 250,000. The mean area of each species is about y|^ of the 

 surface of the globe or 45,500 square leagues. Of this number 

 the valley of the Minnesota with its 16,600 square miles of 

 country contains 1,174 species or about ^^^ of the whole num- 

 ber. The relationships of this flora can be understood only 

 after a general survey of the distribution of plants over the 

 whole earth. 



If one should follow any parallel of latitude that might be 

 selected until he returns to his starting point he would pass 

 through regions characterised by diversities of plant-popula- 

 tion. As he crossed rivers, mountain ranges and oceans, the 

 familiar plants of one region would become fewer in the 

 adjacent region and very likely disappear. When half way 

 around the earth from the point of departure our hypothetical 

 traveler would find himself in a floral region distinctly different 

 from the one of his starting point. This difference would in 

 general increase in amount and distinctness inversely with the 

 height of the latitude. At the equator or near it the difference 

 would be great, while on the parallel of 70° N. lat. or 70° S 

 lat. the differences boih in amount and distinctness would be 

 less. The increase in differences as the latitude decreased 



