182 BOTANICAL GAZETTE [aren 
typical Elymus dune, as found on the South Chicago upper 
beach, is represented at the center of fig. g. These grasses 
grow in tufts and do not have any extensive vegetative propa 
gation, but the tufts may be so close together as to act likea 
large social clump of Ammophila. a 
Certain shrubs are of almost equal importance as dune 4 
builders with the grasses. Among these, the willows, Salis : 
adenophyllaand S. glaucophylla, and the sand cherry, Prunus pumilt, 
deserve especial mention. All of these shrubs have socid 
habits, and rapidly increase their area of control in all directions. — 
The willows are particularly well fitted to build up a perennial 
beach dune. They are about as well adapted to a xerophytic 
environment as are any of the grasses, and their vegetative 
increase is about as rapid. The fact that they are shrubs and 
have a greater power of vertical stem elongation makes them 
even better fitted to rise above the sand. When a shoot of om 
of these willows is buried, roots are sent out from all the buried 4 
portions, even up as far as the floral axis. The willows, too, cal 
be uncovered without suffering serious injury. In short, the : 
species of Salix are able to adapt their stems to a root envinee 
ment, or vice versa, better than any other plants found along the 
coast. Hence the willows stand abreast of Ammophila as dune- 
formers. Another shrub that seems to have all the necessaly 
requirements for a dune-builder is Cornus stolonifera \e a 
Baileyi), though it is probably less fitted for a xerophytic ae 
owever, Cornus dunes are by no means rare on the beach. 
The only trees which make any significant contribution © 
dune formation are the poplars, Populus monilifera and P. 4 ee an 
fera. Of these, the former is the more important, especia 
southward. These trees have little or no vegetative propag : 
of the willow type. Every year great numbers of cotton 
seeds germinate in protected depressions on the uppe ee 
As the young plants grow rapidly, it is not long before they me 
groups dense enough to retard the sand-laden winds. a the 
cottonwood dune is formed, a type which characte 
Lake Michigan shore at very many places. A cottonwood 
