722 METASPERMAE OF THE MINNESOTA VALLEY. 



of the continent, for the general movement has been both north 

 and south and the lake- region trench with the Appalachian 

 system has favored their movements in the east rather than 

 the west. Between Monocotyledones and Archichlamydeae on 

 the one hand and Metachlamydeae on the other, the topog- 

 raphy of the continent, considering the points of greatest dis- 

 tinctive pressure for each group, would seem, perhaps, to tend 

 towards the establishment of a general diagonal tension-line 

 running in a direction north-west by south-east. On the south 

 and west the upward movement of north-bound plants would 

 be easier than on the north and east, while on the north and 

 east the downward movement of south-bound plants would be 

 easier than on the south and west. Under such conditions the 

 preponderance of the Metachlamydeae which are distinctively 

 central and north-bound would be rather to the west than to the 

 east, while the preponderance of Archichlamydeae which are 

 distinctively distal and south bound would be rather to the 

 east than to the west. In the Monocotyledones, since they are 

 the most generally distributed, if this were a just explanation, 

 we should expect to find such a difference strongly marked 

 and such is actually the case, for of the NSE. element the 

 Monocotyledones form 36.6 percent, while of the NSW. ele- 

 ment they form but 12.8 per cent. Evidently in the regions of 

 the secondary longitudinal tensions their percentages would 

 vary to the extent of complete reversal, in many cases. It is 

 only in a region comparatively uninfluenced by longitudinal 

 tensions that the diagonal tension could be recognised. 



Whether or not the above paragraph indicates the true ex- 

 planation of the different composition of the NSE. and 

 NSW. range-elements, it is an interesting fact to note that in 

 this central basin of the continent the NSE. species are pre- 

 ponderantly Monocotyledones and Archichlamydeae while the 

 NSW. species are preponderantly Metachlamydeae. 



The NSEW. element with its preponderance of Monoco- 

 tyledones and slight representation of Metachlamydeae, offers 

 no difficulties. By it, a further indication of the general ranges 

 of Monocotyledones, as a group, and the special ranges of 

 Metachlamydeae, as a group, is given. As in other similar 

 cases the Archichlamydeae are seen to occupy the intermediate 

 position. 



The four binary elements, NE., NW., SE., SW., are 

 similarly simple of explanation, with the exception, possibly 

 of the NW. element. In the NE. element the Monocotyle- 



