JANUARY, 1902. BRUNCKEN — STUDIES IN PLANT DISTRIBUTION. 
25 
Various suggestions have been made to explain this phenomenon, 
which is so surprising when first encountered (7), but hitherto 
these suggestions have not ripened into more than plausible hy- 
potheses. The following observations may furnish some material 
towards an ultimate solution of the problem : 
The genus Viola is represented by seven species in the near 
neighborhood of Milwaukee, and by at least four others in other 
parts of Eastern Wisconsin. These species form an illustrative 
series in which xerophytic characters increase as one departs from 
those growing in typically mesophytic places to either those species 
inhabiting dry hillsides or wet and boggy situations. The species 
forming the mesophytic center is V, scahriuscula. It has hardly 
anything suggestive of a protection against excessive transpira- 
tion, with its broad leaves, juicy stems, light green color and al- 
most complete absence of pubescence. This species flourishes in 
the shadiest portions of our mesophytic forests, under the hard 
maples and beeches, where the humus is deepest and yet the drain- 
age sufficient to prevent the formation of deleterious acids. Very 
similar to this species both in habit and habitat is V. Canadensis, 
which does not occur in Milwaukee Cotfnty but is fairly common 
farther North. The next species in regard to slight development 
of xerophytic characters is V. pubescens. The name of this species 
is by no means suggestive of its general appearance, for while one 
can find individuals with a fairly conspicuous coat of hair, there 
are others almost as naked as V. scahriuscula. The seed pod is 
an exception, for this organ is invariably covered with a very close 
coat of hair. This species is also found principally in the meso- 
phytic forest, but it is not so closely confined to the beech and 
maple groups as scahriuscula. It is quite satisfied with the ordi- 
nary type, where oak, hickory, cherry and other trees compete with 
the maple, where the shade is less dense and the humus does not 
reach the same thickness and richness. Viola ohliqua and the 
rather rare V. ovata come next. They are not absent from the 
forests where V. puhescens has its favorite stations, but far more 
characteristic of thickets of shrubs and saplings, where there is 
still a reasonable amount of humus. They are also found in the 
vigorous grass of meadows, and in the willow brush along the 
rivers. In at least one place I have found Viola ohliqua growing 
copiously in a small sedge marsh. Both of these species have a few 
xerophytic characters, the most conspicuous of which is the rolling 
in of the leaf edges. (This also occurs sometimes in V. puhes- 
cens). By this the stomata of the upper leaf surface are protected 
from the wind. 
(7) See Warming, Oekologische Pflanzen-Geographie, German ed., page 69. 
