MacMillan: shores at lake of the woods. 991 



sand around its base will contain more nitrogenous material 

 than is ordinary on the general dune slope. 



By the shifting of the sand, propagation is rendered difficult, 

 the number of species is cut down so that dune slopes come to 

 be monotonous beds of Primus, Juniperus, Ehjmus and Arte- 

 misia, with occasional dwarfed shrubs of Populus, or even of 

 Sambucus. By the constant shifting about of the sand, the soil 

 is turned over and oxidized, and this process is unfavorable 

 both for the storage of moisture near the surface, and for the 

 development of a general nitrogenous richness that would favor 

 the growth of herbs. It is only on the dune summits, and there 

 sparingly, that such plants as need humus-content in more 

 marked quantities, can very well establish themselves. 



The characters of the dune slope, then, are these: sterility, 

 exposure to wind, low moisture content, low temperature, con- 

 stant oxidation of organic waste — in short much the same 

 characters that one would discover io the study of a ledge of 

 light colored quartzy te. The vegetation, by its types, its mon- 

 otony, its dwarfed and prostrate habit, and its extreme loose- 

 ness of aggregation responds to these conditions in much the 

 same way that it does when establishing itself upon a rock 

 surface. 



Dune summits. The surface of the dunes is generally alto- 

 gether irregular, a succession of slopes, hummocks and hol- 

 lows. The tops of the hummocks are commonly tenanted by a 

 distinctive group of small trees or shrubs with a limited inter- 

 stitial vegetation somewhat like that of back strand areas 

 along shore. Such a dune summit is shown in Plate LXXIV, 

 and a somewhat zonal arrangement of plants may be observed- 

 Surrounding the clump of shrubs that occupies the summit is a 

 growth of Elymus, Artemisia and Agrostis with Juniperus seen 

 on the right. The common trees of these dune summits on Isle 

 aux Sables are Quercus, Fraximis, Celtis, Cerasus and Populus, 

 together with numerous individuals of Sorbus sambucifolia. 

 Mingled with the dwarfed trees are shrubs of Rubus, Rhus, 

 Spiraea — a plant with great catholicity of habitat — and Betula. 

 Depending somewhat upon the size and age of the summit 

 formation, there are added low herbs including Vagnera, Hy- 

 pericum, Potent ilia, Onagra, and others, but the variety is 

 always small, and dune summits, like dune slopes, show a great 

 monotony of specific types. 



The conditions at the dune summit differ from those of the 

 slopes in one or two important ways. The formation is much 



