1899] DUNE FLORAS OF LAKE MICHIGAN 393 
So long as the crest is unoccupied by plants, the advance will 
continue almost without hindrance. The life conditions at the 
crest are so much more severe than on the slope that vegetation 
is almost certain to be excluded until the advance is checked by 
physical agencies. For the capture of a rapidly moving dune, 
aplant species should have the power of rapid germination pos- 
sessed by the bugseed, the power of vegetative propagation pos- 
sessed by the willows, the capacity for growth in height possessed 
by the cottonwood, or even more than that. The growth of the 
young plant during the first season should be greater than any 
of the above, so as to more than counterbalance the vertical 
“omponent of advance during the period of rest. The life cycle 
should be of very great length. The plant should be able to 
‘ndure all extremes of heat, cold, and drought, and all degrees 
poets by the sand. No plant species in the Lake Michi- 
sa region begins to meet all of these requirements, and, as a 
“ensequence, the dunes would advance indefinitely so far as veg- 
ttation is concerned. : 
Various physical conditions tend to check the progress of many 
“nes. “As a dune advances farther and farther from the lake, the 
Ns power of the wind which moves it becomes reduced. 
ey is largely spent before the crest of the pany 
me “ae because of inequalities in its path. The win 
ar. pede up other dunes between the lake and the ee 
advance a dunes serve as barriers, and of course check t e 
dunes oa Occasionally there are hills in front of the es 
Mary ag check the advance temporarily, at least. sie: 
iS the ro a ga manent decrease or cessation of ee e 
Wanderin on or entire loss of available wind sips’ gn 
lifechisto a oe advance rapidly at any perip = pens 
direction ‘ his is because their mGveMeHt is in mae a 
force of x sex that of the prevailing wind, or because pected 
Cause “8 mg wind is not directed toward their a : 
Te much Physical reasons. Dunes of this slowly gate e 
almost an 26 common than the other, and may De : 
Y point along the entire southern and eastern s ore. 
