174 BOTANICAL GAZETTE [MARCH 
heath appears to originate on fossil beaches or on secondary 
embryonic dunes or other places where the danger of burial is not 
great. It will be noted that both the heath and the pine forest 
are dominated by evergreens. These societies commonly occur 
near the lake or on lakeward slopes, which are northern slopes 
as well. On these coniferous dune slopes there is to be found 
another notable collection of northern plants, resembling eco- 
logically the peat bog plants already mentioned. Heaths and 
coniferous forests also occur on sterile barrens and in depressions 
where the conditions are unfavorable for deciduous forests. A 
slight change in the physical conditions may bring about the 
rejuvenation of the coniferous dunes, because of their exposed 
situation. This rejuvenation commonly begins by the formation 
of a wind sweep, and the vegetation on either hand is forced to 
succumb to sand-blast action and gravity. 
A third type of established dune is that in which the oaks 
predominate, and especially Quercus coccinea tinctoria. The oak 
dunes are more common inland and on southern slopes. Prob- 
ably the oaks follow the pines, but the evidence on which this 
is based is not voluminous. The pines certainly have a wider 
range of habitat than the oaks, occurring in wetter and in drier 
soil and also in more exposed situations. The mutual relations 
of the pines and oaks are certainly interesting and deserve some 
very careful study. Pine forests prevail on the north or lake- 
ward slopes and oak forests on the south or inland slopes. 
With the pines are other northern evergreen forms, such as 
Arctostaphylos, while with the oaks are Opuntia, Euphorbia, 
and other more southern types. The density of the vegetation 
on the north side is also in contrast with the sparser and more 
open vegetation of the south side. The cause for this radical 
difference on the two slopes is doubtless complex, but it is 
obvious that the north slope has greater moisture, shade, and 
cold, and probably more wind. Which of these is the more 
important is not certain, but the presence of the northern spe- 
cies seems in favor of cold or wind as the chief factor. 
There are a number of interesting sand hills and ridges at 
