1899] DUNE FLORAS OF LAKE MICHIGAN IgI 
(. THE ACTIVE OR WANDERING DUNES. THE DUNE-COMPLEX. 
1. Transformation of stationary into wandering dunes. 
The symbiotic growth of the beach dunes and their builders 
may go on for years, but a prolonged existence of these relations 
is unlikely. Asthe mounds get larger and higher the conditions 
for further accumulation of sand become, if anything, more 
favorable. It is not so with the plant tenants, for each year 
they are raised farther from their chief base of supplies, the 
water level. It is probable, also, that the desiccating influence 
of radiant energy and wind upon the aerial organs becomes 
greater and greater as the years go on, because of greater expo- 
sure at the higher levels. Again, the dune-formers, although 
perennials, do not have an indefinite duration. The life cycle of 
the cottonwood is relatively short for atree. All of the other 
prominent dune-formers spread more or less by vegetative prop- 
Soni 5 it is difficult to determine a definite life cycle. 
thiaas tg an existence of indefinite duration might be secured 
in th re propagation, many individual shoots must die 
€ course of time. 
aa ts ss another, then, dune formation ceases and 
the a... . dune history begins. There is here an end of 
bee the 2 Soe dunes, a beginning of the wandering dunes. 
first eeu are no longer able. to oppose as progress, the 
a tendency to eet action of the wind is seen in 
heretofore ee ape the topographic forms. The beach dunes 
tendency ey ed are more or less symmetrical, because af the 
entirely by a symmetrical plant growth. A dune fashioned 
SyMmetrica] Sela without the assistance of plants 1s never 
Usual] : € windward slope has a very gentle gradient, 
_aY about 5°, and because of the destructive action of the 
Wind, this slope is t e destruc ser 
Ward slope, how opographically rough and SHAE YER: ie e : 
is Very smooth a is much steeper, averaging about 30 sun 
of wind, The a e even, because determined by gravity instea 
‘Ng or rolling “hy sweeps up the gentle windward slopes, blow- 
€ sand along until the crest is reached; here the 
