* 
338 The American Naturalist. [April, 
somewhat in detail. The great works of DeCandolle®, Grise- l 
bach‘, Engler’, Drude’, and the numerous special brochures of 
these and of many other writers on geographical botany, have 
pointed out with much distinctness the existence of plant-for- 
mations. These formations are of different degrees. In the 
United States there are, in the eastern portion, two very distinct 
formations that belong to the highest division—namely, the 
forest and the prairie. These two great formations, in the 
broadest sense, are scarcely anywhere in contact with each 
other, but are separated by what may be a very narrow strip 
of plants imperfectly developed in either forest or prairie. 
The region occupied by such a transitional strip of plants I 
have named in the work cited above, a tension-line. Wherever 
two formations of different sorts, such as hardwood forest and 
coniferous forest, meadow and hillside-grassland, or any other 
two, come together, there is such a tension-line developed along 
the line of contact. 
The character of the tension-line between formations has 
received as yet very little study, and it is not possible to speak 
of it as exactly as one would wish. My own observations are 
confined to the tension-lines between forest and prairie in 
the Minnesota valley, a region of special study for the past two’ 
years. Certain interesting facts have been deduced. These 
may be noted as follows: . 
The plants of the tension-line consist of such species as are ` 
sparsely developed in the more central areas of the two adja- 
cent formations, or in some cases of species very poorly devel- 
oped in the solid parts of the two formations, 
The number of individuals of the first group is generally 
greater over a given area along the tension-line than in the 
solid portions of the formations. 
*Géographie Botanique Raisonnée, on exposition des Faits Principaux et des Lois 
papers la Distribution Geographique des Plantes de Pepoque actuelle. Tom. I, 
‘La Vegetation du Globe, d’apres sa Disposition suivant les Climats, esquisse d’une 
Geographie Comparée des Plantes, trad. par P. de Tchihatchef. Tom. I, II. (1877). 
