MICHIGAN ACADEMY OF SCIENCE. 91 



in the pools consists of algae, and microscopic plant-life. Around the edge 

 occur such northern forms as Carex aurea, Eleocharis palustris glaucescens, 

 Scirpus cfespitosus with Pinquicula vulgaris, and Carex viridula, with Primula 

 mistassinica forming tufts, in which the dust and sand carried by the wind 

 is retained, enabling the plants gradually to increase and retain more soil. 



Where the rocky beach is still beyond the reach of the unfavorable fac- 

 tors of the environment, and the soil increases in amount and in depth, and 

 is richer in humus, the flora becomes an interesting assembly of forms. How- 

 ever, even without the .aid of inorganic processes of weathering, plant so- 

 cieties seem capable of invading and of establishing a soil supporting a tree 

 vegetation. The differences in the habitat are nowhere sharply defined, and 

 hence such plants as those just mentioned are found together with a crevice 

 vegetation of Vaccinium Pennsylvanicum, V. nigrum, Amelanchier Cana- 

 densis, Limifea Americana, Phegopteris phegopteris, and with Fragaria Vir- 

 giniana, Euthamia graminifolia, young white pine, mountain ash, hemlock, 

 cedar and birch forming a transitional zone leading into the mixed meso- 

 phytic forest beyond. 



In other places the fluctuations and differences of flora! constituents cor- 

 respond with the changes in the habitat. With a rocky beach retreating 

 inland and becoming steep and precipitous, or broken up into a shingle beach 

 the flora shows no transition zone. The adjacent forest type is directly suc- 

 ceeded by a lichen society. 



Dunes. — On the other hand, the formation of a wave deposited terrace 

 below the water-level, and a sandy beach, and the consec[uent shallowing of 

 the water, is soon folloAved with a gradual downward movement of the inland 

 vegetation. The sand composing such beach dunes was, no doubt, washed 

 up and worked over by the waves, and later by the winds. In the wave- 

 washed area, the sandy beach is barren of vegetation. Only in places fairly 

 well-protected from winds and shifting sands, a condition is maintained en- 

 abling plants to encroach rapidly. In exposed situations, however, the in- 

 vasion and succession of plants is greatly or entirely retarded. Along Mid- 

 dle and Partridge Bay, west of Presque Isle, are several wind blown sand- 

 hills, confined to a narrow belt fringing the lake shore, and presenting quite 

 a contrast to the habitat described above and swamps and bogs back of the 

 dunes. The contour of the dunes, varying from 10 to 15 feet in height, 

 shows that they were shaped by northeast winds. Whatever the origin of 

 these ridges, within them are visible old soil lines, and several scarred trunks 

 of jjine trees, long ago buried by the dunes and now again uncovered by wave 

 action. 



The life-conditions are exceedingly severe. The intensity of direct sum- 

 mer rays is increased l^y the low specific heat of the sand, and by reflection. 

 Though slightly below the surface the sand is usually moist, yet the ease 

 with which the surface layers are heated by day and cooled by night gives 

 rise to extreme temperatiu'e differences. Nowhere are the Avincls more se- 

 vere, dessicating and cutting, and the soil more porous, poorer in nutrient 

 food material, and the eva]Doration of water so rapid. All of these features 

 combine to furnish conditions excluding almost entirely the possibilit}^ of 

 plant life. Nevertheless, at many points a number of plants are able to 

 withstand such conditions. No vegetation is found on the lower or the steep 

 middle beach, but on the upper beach, about 10 feet above the lake level, 

 and sloping gradually into the swamp beyond, the flora is composed of Hud- 

 sonia tomentosa, forming clumps of densely tufted growth, as the dominant 

 -species, and of the equally efficient dune holders, Vaccinium nigrum, Arcto- 



