758 METASPERMAE OF THE MINNESOTA VALLEY. 



genus Carex is one of the apparent causes of the variation from 

 the general percentages, but it is interesting to see that it is 

 the "mean" taxonomic group that of the Archichlamydeae 

 which suffers by reduction, to the greatest degree. While multi- 

 plicity of species is often a sign of comparative newness in a 

 genus this multiplication may arise either in older or newer 

 families. The monocotyledonous and metachlamydeous herbs, 

 in their relation to the general and special tensions, have been 

 explained in outline above. A similar explanation must be 

 offered of the reduction of the Archichlamydeae from the domin- 

 ant family position. At once in the oldest and in the youngest of 

 the three taxonomic groups have been working the causes which 

 tend to multiciplicity of genera in excess of reduction, for these 

 two groups are peculiarly exposed under the law of ejection. 

 Hence they become relatively plastic and specific modifications 

 are frequent. The third group, however, undergoes the series 

 of changes which tend to reduction of species in excess of mul- 

 tiplication, and, with this reduction, the tendency is towards 

 greater solidarity of formations and movement toward the shrub- 

 by or arboreal habit. Thus in the percentages of the dominant 

 families further evidence concerning the difference in meaning 

 of Archichlamydeae, Metachlamydeae and Monocotyledones, in 

 the distribution over a limited area, is discovered. 



VI. CONCLUSION. 



The statistical investigation of the Metaspermae indigenous 

 to the valley of the Minnesota having now been completed as 

 far as the limits of this work may permit, it remains to offer 

 some brief summary and explanation of the more important 

 facts believed to have been determined in the preceding pages. 

 It has been shown that while the valley of the Minnesota is 

 geographically central in the North American continent, it is 

 by no means botanically central, but on the contrary, strongly 

 southern and eastern, This particularly important fact needs 

 explanation. Upon examination it would appear that two sets 

 of factors must be conceived as having interacted to bring 

 about this result. These factors may be grouped as physical 

 (in the narrow sense) and biological. Under the first head it 

 must be observed that while geographically central, the Min- 

 nesota valley is not central in point of elevation, climate, pre- 

 vailing winds, and drainage. The line of mean elevation lies 

 to the west of the valley, the continental climatic mean, so far 

 as concerns temperature, lies to the north of it. The winds of 



