1899] ~ DUNE FLORAS OF LAKE MICHIGAN 109g 
of the wind, since the latter agent is unable to pick up and carry 
for any distance the gravel or large sand particles of the beach. 
As is well known, soil made up chiefly of quartz sand has 
certain marked peculiarities which strongly influence the vegeta- 
tion. The particles are relatively very large; hence the soil is 
extremely porous and almost devoid of cohesion between the 
grains. These features are of especial importance in their effect 
upon the water and heat relations as shown elsewhere. As a 
rule, sandy soils are poor in nutrient food materials, nor do they 
rapidly develop a rich humus soil because of the rapid oxidation 
of the organic matter. 
WATER. 
A factor of great importance, here as everywhere, is the water 
relation. Nothing need be said of atmospheric moisture, since 
that is sufficient to develop a rich vegetation if properly con- 
served, as is shown by the luxuriance of neighboring floras. 
Because of the peculiar physical properties of quartz sand, pre- 
cipitated water quickly percolates to the water level and becomes 
unavailable: to plants with short roots. The water capacity of 
sand is also slight, nor is there such pronounced capillarity as is 
characteristic of many other soils. Again, the evaporation from 
@ sandy surface is commonly quite rapid. All of these features 
combine to furnish a scanty supply of water to the tenants of 
ey a ie rapid cooling of sand on summer nights may, 
however, result in a considerable condensation of dew, and thus, 
Deine; way, compensate for the other disadvantages. 
ecological factors thus far mentioned act together har- 
Pane ay. and produce a striking composite effect upon the 
we A flora which is subjected to periods of drought is 
monl ao flora and its component species have com- 
sort é a out various xerophytic structural adaptations of one 
cold, es < “a Again, a flora which is subjected to extreme 
varius es y when accompanied by severe winds, takes on 
istic of a] Spies adaptations similar to those that cw character- 
flora aay and arctic floras. The dune flora is a composite 
ing both xerophytic and arctic structures. In those 
