RELATIONSHIPS OF METASPERMAE. 599 
as well as from parallel to parallel. As there was before to 
be distinguished a progressive movement, with attendant ac- 
celerations and retardations, to higher latitudes, so here there 
is a similar movement towards higher altitudes, and as fluctua- 
tions arose before in the rate of progression, so, too, similar 
fluctuations will here arise from similar conditions. In general 
these longitudinal tensions are to be studied under the laws of 
the lateral tensions. 
So far as concerns the Minnesota valley the central tension- 
line lies far to the west of it and this fact will be seen to have 
an evident effect upon its floral population when, in the next 
chapter, more particular and detailed attention is given to the 
character of that population. Andso, too, the Appallachian 
tension-line lies far to the east of the valley. Its influence like 
that of the central tension-line is slight. Indeed the influence 
of these two longitudinal tensions is felt only indirectly in a 
region so remote from either as is the valley of the Minnesota. 
Such indirect influence is however appreciable, and is apparent 
on the one hand in the presence of plants like Collomia and on 
the other by the presence of the different species of Rhus. 
Minor tensions. In an area, considerable in extent and 
diversified in topography, as is the valley of the Minnesota, 
there are to be distinguished what I may be permitted to term 
minor tensions. By this there is not meant the forest and 
prairie delimitation, for that is to be referred in large part to 
the principal lateral tension, developed by equatorial pressure. 
The various topographical features of the Minnesota valley, 
with its gorges, glens, vales, meadows, hills and headlands, 
bring about slight but distinguishable segregations of floral el- 
ements. Between meadow and bluff there exists a minor ten- 
sion-line, between swale and knoll on the prairie, between hill 
and ravine in the forest there are to be discovered such minor 
tensions. But just as these minor tensions are due to slight 
differences, so too their progressions, accelerations, retarda- 
tions and fluctuations are so variaole that their very existence 
becomes a matter principally of averages. Nevertheless their 
presence may be determined in the field or a priori. The influ- 
ence of these minor tensions on habitat is great, but it is after 
all an influence transmitted from the more general continental 
tension and may as properly be referred to the latter. Upon 
the physiognomy of the district these minor tensions have a 
conspicuous effect and to their presence may be ascribed much 
