1899 | DUNE FLORAS OF LAKE MICHIGAN 115 
were it not for the wind which gathers much of its dune material 
from the lower beach. 
Thus the lower beach is a barren zone between two zones of 
life. Below it there exist alge and_ other hydrophytic forms 
which flourish in the fury of the breakers; above it there exists 
the flora of the middle beach, a flora adapted to the most 
intense xerophytic conditions. At no particular time, perhaps, 
are the conditions too severe for some type of life; vegetation 
is excluded because of the alternation of opposite extremes. 
2. The middle beach. 
The middle beach is situated between the upper limits of the 
summer and winter waves, comparatively dry in summer but 
washed by the high storms of winter. It may also be defined 
as the zone of succulent annuals. The upper limit of this beach 
is commonly marked by a line of driftwood and débris. The 
instability of the beach conditions is often shown by the pres- 
ence of a number of such lines, marking wave limits for different 
seasons. “A very heavy storm will carry the débris line far up 
on the upper beach, to all intents and purposes carrying the 
middle beach just so much farther inland, as the flora of the 
next season testifies. Another season may be without the visita- 
tion of heavy storms and the middle beach will encroach upon 
the territory of the lower beach. The limits of the middle 
beach are altered more permanently by changes in the lower 
cach. In many places the lower beach is growing outwards, 
reclaiming land from the lake, while at other points the lake 
€ncroaches upon the land. Speaking broadly, the middle beach 
advances or recedes pari passu with the advance or recession of 
eat beach. To some extent the débris lines register these 
ae as their notable departure from persistent parallelism 
cy eee however, there is a considerable lack of paraller- 
the et débris lines of a single season, owing to variations in 
aon of the wind and other factors. 
aan beg in this zone are exceedingly severe, and 
The tu: a Hora of the most pronounced xerophytic characters. 
y of the winter storms as they wash over the middle 
