1905] BROWN—THE PLANT SOCIETIES AT YPSILANTI 275 
the dry, loose sand would take by gravity alone. Andropogon 
scoparius, A. furcatus, Chrysopogon avenaceus, and Stipa spartea 
are the most common of these soil binders. 
(3) Juniper heath—The conifers are restricted to open or unfa- 
vorable situations, and thrive where other arborescent forms do not. 
Only isolated individuals of juniper and cedar are met with at the 
bayou; but southeast from the city are found slopes covered by them. 
On the brink of a bluff which has been undermined by the stream 
so as to leave the slopes bare of any vegetation, Juniperus communis 
makes an early appearance. From this point it throws out decum- 
bent rooting stems, which, partly directed by the movement of the 
unstable soil, come to extend down the slope, and the growth is con- 
tinued from above downward. A covering is soon made, in which 
also occur J. virginiana, Lepargyraea canadensis, Rhus aromatica, 
and Stipa spartea. 
None of the other societies occur under conditions equally xero- 
phytic. The juniper and cedar are most common (fig. 4), but not 
“a characteristic of such situations as other less frequent members, 
like buffalo berry or porcupine grass. If we compare slopes of differ- 
ent ages, evidence will be found showing that the juniper slope is 
transformed into an oak slope as conditions become more favorable 
for the growth of vegetation. In the heath appear Populus grandi- 
dentata and P. tremuloides and seedlings of black locust, basswood, 
and oak. Cedar Bend, Ann Arbor, has gone a step farther. Here 
the oaks are dominant, but there is still much juniper, cedar, and 
aspen. Even the final black oak stage is not without decayed logs, 
ees, and isolated individuals, suggesting the former prevalence 
of the Juniper and other members of that society. 
(4) Thicket societies.—One of the most characteristic features of 
_ Ape as a whole is the large proportion and variety of 
: ns ich it contains. Along the bluffs these shrubs occur either 
scattered individuals or mixed together as thickets or undergrowth. 
oe species of thorn and bramble, Ampelopsis, Smilax, 
ni lis ing thi and other vines, occur most frequently in dense 
i os. where an open ledge or slope affords a chance. Rhus 
. - glabra, however, do not tend to mix with other shrubs, 
ict? a each other, but form patches of pure growth, which 
hguish at once the sumac thicket from all others. 
