Ig2 BOTANICAL GAZETTE [MARCH 
sand is deposited by the wind and it rolls down the steep slope, 
spreading itself quite evenly. ig. 77 shows an average gentle 
windward slope, figs. 13-15 steep leeward slopes. 
The Prunus dunes are particularly favorable for a study of 
the origin of an active dune, since their form is most at variance 
with that of the wind-shaped dune, as described above. A 
Prunus dune is commonly a low cone more or less rounded at 
the top. As soon as the plants are dead, and sometimes long 
before that event occurs, the wind endeavors to reduce the wind- 
ward gradient by removing the sand toward the top and blowing 
it over on the other side. In this way the roots are exposed and 
existence made less endurable, if the plant is still living. Prunus 
dunes with roots exposed on the windward side are common at 
Dune Park, Beaver island, and elsewhere. On the terrace at 
Glen Haven, some dunes have been carried beyond their former 
resting place, leaving the scraggy clumps of Prunus roots at the 
rear. 
What has been said of the Prunus dunes holds more ot less 
for the other types. The cottonwood dunes especially are pect” 
liarly subject to the destructive attacks of the wind, since their 
shape also notably fails to correspond with that of a normal 
wind-made topographic form. The lower Ammophila, Salix 
and Agropyrum dunes are less likely to suffer destruction, and 
yet small Ammophila dunes were seen on the dune-complex # 
Glen Haven that had been blown away from their first abode, 
leaving the Ammophila stranded at the rear. Even while living: 
these dune-formers were unable to hold the dunes which they 
had helped the wind to build; much more when dead are they 
likely to have the dune swept on beyond them. 
The destructive action of the wind and the transforma fe 
a stationary into an active dune are very much retarded byt 
tenacity with which the stems and roots retain their place, i 
when dead. A plant which thus has the power to hold its ie 
tion and keep the sand from being blown away is ge 
called a sand-binder. In this connection it may also be ¢ a 
a dune-holder, as it has been already called a dune-for” 
tion of 
