990 MINNESOTA BOTANICAL STUDIES. 



In many respects this is a remarkable group of plants. The 

 presence of Juniperus sabina, Hudson ia tomentosa, Campanula 

 rotund if olia and the distribution also of some of the grasses sug- 

 gest the aspect of rocky shores where, on high barren ledges, 

 these plants are characteristic. The most important generali- 

 zation that is to be derived from a study of these dunes as a 

 whole, is that their population resembles that of rock shores 

 rather than of beaches. The meaning of this appears to be 

 that texture of the soil is less important here in determining 

 the types of vegetation that shall become established, than are 

 the other factors — e. g. sterility, elevation, exposure to wind, 

 withdrawal from spray and diminution of moisture. Indeed, 

 except for its texture, the dune is chemically and topograph- 

 ically similar to the rounded quartzyte and gneissic masses 

 with which the whole region abounds. I take it that the pre- 

 valence upon dunes of Juniperus sabina, Hudsonia tomentosa, 

 Campanula rotundifolia and the rest indicates an essential eco- 

 logic similarity between dunes and rock masses, and the vege- 

 tation of the dune which at first sight would naturally be con- 

 nected with beach vegetation must really be regarded as of 

 lithophytic rather than as of ordinary psammophytic, limnetic 

 types. 



It is possible instructively to classify the dune formations as 

 developed upon dune slopes, dune summits and in dune pools. 

 These three areas may be noted briefly in their sequence. 



Dune slopes. Owing to the shifting, loose, sterile condition 

 of the substratum, only a small group of plants characterizes 

 this area. Plate LXXIII gives a view of an altogether typical 

 slope on Isle aux Sables. The vegetation of the foreground 

 shows Prunus pumila, Populus tremuloides and Juniperus commu- 

 nis as the dominant species, while farther back Artemisia and 

 Elymus may be discerned and to the right a dune summit bear- 

 ing dwarfed trees of Celtis and Cerasus. Prunus pumila a pro- 

 nounced psammophyte tinds such slopes a congenial home, and 

 the species is developed in such areas quite as abundantly as 

 upon the strand. Juniperus communis, rather rare upon beach 

 areas except where it enters the back strand from a coniferous 

 formation inland, is abundant, upon the dune slopes, but not 

 more so than its congener J. sabina, an omnipresent crevice 

 plant over all the rocky islands of the region. It must be noted 

 that the station of a shrub like Juniperus upon a dune produces 

 around its roots somewhat of the conditions of a crevice. The 

 prostrate habit enables it to hold organic substances, and the 



