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 Appallachian 
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 Metachlamyceae—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 386.6 per cent. 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. Itis 
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. Asin 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- 
