a 
1899] DUNE FLORAS OF LAKE MICHIGAN 367 
inthe lake region. It may be regarded as the culmination of 
the series which began with the lower beach. 
2. The evergreen series. 
a The heath.—The life-history of a windward slope is 
vastly different from that which has just been described. If one 
Views a region of established dunes from the lake side, he sees 
a landscape in which evergreens predominate, whereas a view 
fom the land side often shows a decided dominance of decidu- 
ous trees. Not only the windward slopes but the summits have 
Ml evergreen flora. The key to these facts is exposure to desic- 
cating factors, especially heat, cold, and winds. So far as the 
soil is concerned, there is but little difference between the con- 
ditions on the windward and leeward slopes. In both cases 
there is a vegetation carpet and a covering of humus. The more 
gentle slope favors the retention of moisture, though this factor 
S counterbalanced by the desiccating influence of the wind on 
| Soll. At the south end of the lake the soil of the leeward 
Slopes is drier than that of the windward slopes, because of the 
“outhern €xposure and consequent drying influences of the sun. 
‘ven in this latter instance, the contrast between the floras 
of the two slopes is tremendous. It is the condition in the air, 
‘ot the condition in the soil, which determines the difference 
“te found. Both slopes have a mesophytic soil ; the leeward 
Pealso has a mesophytic air, but the windward slope has a 
“tophytic air. 
a heath has several origins but one cea It ass 
n the slow-growing embryonic dunes, in depressions © 
"Per beach, on the fossil beach, on gravel terraces, OF in 
Pastures, Ip all cases the dominant plant species come to be 
_ of the following : Arctostaphylos Uva-urst, ele 
‘long a poectus Sabina procumbens. The first ae a ane 
ath: © entire coast, the latter only northward. she 
_S been but little used in America, perhaps beca a 
i. have the peculiar Calluna heaths of Europe. dae ae 
tic flora in 
De mere used, may be defined as a xerophytic ona 
