184 BOTANICAL GAZETTE [mance 
a number of years old is shown in jig. 5. It is possible to see, 
even from the photograph, that the lower portions of the trees are 
covered by the sand. The growth of a cottonwood dune, there 
fore, is of a symbiotic nature, exactly as is the growth of an 
Ammophila dune, in spite of the great difference between the 
life habits of these two dune-formers. | 
~The controlling part which plants play in dune formation 
still further shown by the variations among the embryonic dunes _ 
as to area, shape, and height. Dunes that are formed by Agi 
pyrum, Ammophila, or Salix are capable of indefinite areal expal- 
sion, since these plant types have extensive vegetative propapt 
tion. Populus or Elymus dunes, on the other hand, always retail 
essentially the same area, since there is little or no vegetative 
reproduction, and since the opportunity for any further develop: 
ment of seedlings is removed. Young seedlings are often found 
in the moist sand of the depressions, but never in the dry 
of the dunes. The Agropyrum dunes are always very low, sch | 
dom if ever a meter in altitude. The Ammophila dunes ® 
higher because this latter plant type has a greater power de 
upward growth than has the former. Both the Agropyna 
and Ammophila dunes are alike in being relatively large 
low, with slopes almost as gentle as those of the beach. The 
Salix dunes are equally large, but higher and steeper because 
the greater power of upward growth. The Prunus du 
very small because of the slight vegetative propagation and ye 
they are relatively high and steep because of the pron 
power of upward growth. 
The cottonwood dunes are the highest of all, oft 
an altitude of several meters, because the cottonwoods 
higher than any other dune-former; they are also the ste 
because there is no vegetative propagation. The doi per 
a cottonwood dune is shown in fig. 5; the marks in the aii 
the right are the paths made by the movement of the ~ ras, 2 
the steep slope, and indicate a gradient of about 30°: ~ 
and 3 indicate the gentle slope of the Ammophila meee take 
cottonwood dunes are commonly long ridges parallel tot 
en havidg 
