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1899] DUNE FLORAS OF LAKE MICHIGAN 185 
shore. This is partly due to the fact that the depressions in 
which the seeds germinate are parallel to the shore, because of 
wave action. The prevalence of lake winds perpendicular to 
these ridges also helps these topographic forms to retain their 
original shape. Summing up this matter, then, the area of a 
beach dune is determined chiefly by the amount of the vegeta- 
tive propagation of its tenants, the steepness is determined by 
the rapidity of this propagation, and the altitude by the power 
of the dune plants to increase in height. 
Of the dune-forming species, Ammophila arundinacea and 
Salix adenophylla are the most abundant, occurring almost every- 
Where that beach dunes exist. These two species are most at 
home when half-buried in the wind-blown sand, and occur much 
*s frequently in other associations. Prunus pumila and Cala- 
"agrestis longifolia occur throughout the region, but are less 
ie of the beach dunes than are the first-named species. 
Paes a ae on the upper beach, as already shown, and 
es ag ay ; Calamagrostis is particularly characteristic of 
at Glen eas dunes, Agropyrum dasystachyum is very common 
home on Sa farther north, and, like Ammophila, is most at 
South of Glen ai dunes. Populus monilifera is very common 
farther inland aoa the cottonwood dunes are usually formed 
northward to ¢ an the other types. This Species is replaced 
candicans x... extent by Populus balsamifera and its variety 
ra ine 3, on the same tree !) , though this poplar 
a dune-former than the cottonwood. Salix 
frequent on the beach dunes, especially southward, 
aracteristic of the swamps; the same may be said 
mifera. Elymus Canadensis is not abundant any- 
beach dunes 
Interesting cor 
Paragraphs, § Corolla 
: ry may be deduced from the last three 
N the life oa various dune forms are caused by differences 
istribution of on A dune-forming species, it follows that the 
tibution of th _ topographic forms coincides with the dis- 
¢dune-formers, Low dunes of the Agropyrum type 
© not f 
Sund at the south end of the lake. The steep ridgelike 
