1899 ] DUNE FLORAS OF LAKE MICHIGAN 107 
LIGHT AND HEAT. 
Nearly all of the dune societies are characterized by a high 
degree of exposure to “ight. Particularly is this true of the beach 
and the active dunes. The intensity of direct illumination is 
greatly increased by reflection; the glare of the white sand is 
almost intolerable on a bright summer day. The semperature 
relation is even more marked in its influence upon plant life. 
Because of the absence of vegetation and the general exposure 
of sand dunes the temperature is higher in summer and lower in 
winter than in most localities. This great divergence between 
the temperature extremes is still further increased by the low 
specific heat of sand. On sandy slopes protected from cold 
winds the vegetation renews its activity very early in the spring, 
because of the strong sunlight and the ease with which the sur- 
face layers of sand are heated. Willow shoots half-buried in 
the sand frequently develop fully a week in advance of similar 
shoots a few centimeters above the surface. Similarly in the 
autumn the activity of plant life ceases early largely because of 
the rapid cooling of the superficial layers of sand, as well as 
because of direct exposure to the cold. 
WIND. 
; The wind is one of the most potent of all factors in determin- 
ing the character of the dune vegetation. The winds constantly 
gather force as they sweep across the lake, and when they reach 
the shore quantities of sand are frequently picked up and carried 
on. The force with which this sand is hurled against all obsta- 
a in its path may be realized if one stoops down and faces it. 
he carving of the dead and living trees which are exposed to 
these natural sand-blasts is another evidence of their power. 
Fleshy fungi have been found growing on the windward side of 
logs and stumps completely petrified, as it were, by sand-blast 
action ; sand grains are imbedded in the soft plant body and as 
it rows the imbedding is continued, so that finally the structure 
appears like a mass of sand cemented firmly together by the 
meee | the bark of the common osier dogwood is red on the 
