ms 
peer 
1899] DUNE FLORAS OF LAKE MICHIGAN 377 
gravity. ig. 22 shows a plant society that is being destroyed 
mainly by gravity. The view is taken looking at the side of a 
deep gorge-like wind-sweep which the wind has cut. As the 
wind blows along, its energy increased by concentration, a large 
amount of sand is picked up along the base of the steep sides. 
The sand is as steep as it will lie, so that each removal causes a 
movement of the sand down the slope. The fallen trees shown 
in the photograph have been overturned and carried down the 
slope in just this way. That the direct action of the wind is also 
powerful enough to destroy without the assistance of gravity is 
proven by the dead but standing trees at the left, where the 
action of gravity happens to be much less. 
Many plant species resist the process of dune rejuvenation to 
‘surprisingly successful extent. Fig. 23 shows the last remnant 
ofa plant society that may have been somewhat extensive. The 
ee at the center is a basswood, a tree which could never 
develop in such an exposed situation. In all probability this 
mound is a fragment of a protected lee slope, on which the bass- 
Wood grew and flourished foratime. The grass at the left is 
Glamagrostis : the tenacity with which it holds its ground has 
fady been mentioned. Sometimes a group of cedars, JSunipe- 
fee Virginiana, remain at the apex of a conical mound of sand, 
their associates having been swept away with the sand in which 
they Stew. On the beach at Charlevoix there is a stranded 
“lump of stunted trees of Thuya; they are probably the remnant 
ae Society which has been otherwise destroyed. =. 
Asa wind-sweep is developed, and the evergreen VOR 
(destroyed, Many plants that have been previously mentioned a 
tharacteristic of bare and exposed situations again make their 
“pearance. ~The most prominent of these are Artemisia Cana- 
denis (or 4. caudata), Elymus Canadensis, Solidago humilis Gill- 
i Asclepias Cornuti, CEnothera biennis, Rosa sc, oa cea 
lemagrosts longifolia, Prunus Virginiana. In addition to these 
‘ihe Come in, of course, the annuals and biennials mentioned in 
“anection with the wind-sweeps on the dune-complex. 
Wh: ire 
hile rejuvenated dunes are to be found along the enti 
