642 



METASPERMAE OF THE MINNESOTA VALLEY. 



metachlamydeous genera 12.5 per cent, are limited in their 

 range to the western hemisphere. This percentage does not 

 include, however, the results of the succeeding table, for in 

 every case distinctive not total range, is given. This was 

 explained above for the families. The last table of the series 

 follows : 



The figures of the last table indicate two facts concerning 

 distribution, both of which are important. By the slight varia- 

 tion in the last column from the mean of 13. 2 per cent, it will 

 be seen that, so far as the valley of the Minnesota and its Meta- 

 spermae can be placed in evidence, there is about an equal 

 tendency in each of the three taxonomic groups to develop 

 endemic genera. The last column of figures serves to strengthen 

 our belief that the Metachlamydeae are the youngest of the 

 three groups, for while the opportunity for developing endemic 

 genera is no better in this group than in the other two, we find 

 upon examining the figures of the second column that the 

 Metachlamydeae include by far the larger per cent, of the en- 

 demic genera. The apparent explanation of this fact by the 

 diiferent length of time that has sufficed for distribution in and 

 out of the continent, in the case of the three groups of un- 

 equal age, is even more clearly developed by the evident inad- 

 missibility of attributing this difference of range to any in- 

 herent quality of the groups in question. 



An examination of the genera may also be conducted to de- 

 termine the North American development of each. In the next 

 table the genera are classified as northern, southern, eastern 

 and western. Some explanation of this grouping may be 



