JULY, 1902. 
BRUXCKEX- 
— STUDIES IN PLANT DISTRIBUTION. 
141 
pear so rapidly as in southerly and westerly exposures. Conse- 
quently the mesophytic herbs, the asters, sunflowers, golden rods, 
hold their ground much longer ; the grasses do not make so much 
headway, and often other species of the family, which do not 
make so dense a sward as Poa, are more strongly represented than 
that genus. The hawthorns, hazelnuts and sumachs have strong 
competitors in the witch hazel, choke-cherry, prickly ash and 
others. Where there is a small spring on the hillside, as is very 
often the case, so that the soil for a few square rods is kept in 
swampy condition, Sauihiiciis nigra is sure to be on hand, while 
willows and dogwood are very apt to stray up from the valley. 
The trees from the neighboring forest are sure to join in the race. 
Generally speaking, the conditions on such northerly slopes are 
so much more favorable to plant life than those on southerly ex- 
posures, that the mesophytic nature of the locality is really never 
lost. After the trees are cut down, there come a few years during 
which the ground is mostly bare except for the forest litter and 
such herbs and grasses as may have occupied the forest floor and 
continue to flourish in the clearing. Some of the latter will not 
survive more than a season or two under the changed conditions, 
but others which delight in the rich, loose soil, but could not toler- 
ate the shade of the trees, will come in. Various species of Riihns 
are sure to grow luxuriantly, and their tangled vines are about the 
only impediment to the growth of the shrub and tree seedlings. 
Compared to the many years it takes the forest to reconquer a Poa 
heath, the progress of an area of this sort towards the forest 
stage is very rapid. The years before a thicket is formed are so 
few that one can hardly speak of a heath stage. In thickets of 
this kind the witch hazel is usually so much more abundant than 
the other shrubs mentioned, that one may properly call this the 
witch hazel, in distinction from the hawthorn association. 
Either of the two species of Rhus mentioned above is likely to 
be met with in both of the two associations just described. But in 
a few places near the lake shore there are thickets in which Rhus 
typhina is dominant to such an extent that one must properly dis- 
tinguish them as the Sumach Association. The staghorn sumach 
seems to have a peculiar liking for the steep, barren clay banks or 
bluffs along Lake Michigan. It invades them from above, es- 
tablishing itself first on the upper edge of the bluff and sending 
root suckers down the almost vertical bank. The beginnings of 
this operation can be seen in manv places all along the Lake. 
But in several places near Fox Point this enterprising shrub 
has in this manner conquered the whole bank from top to bottom 
for many rods. 
Another remarkable occurrence of the sumach association is 
