RELATIONSHIPS OF METASPERMA.E. 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. And so, 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 variable 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 



