1014 MINNESOTA BOTANICAL STUDIES. 



distinct type. Thus, to this extent, there is zonal distribution 

 bat the irregularity of surface is so considerable that the pre- 

 ponderance of characters are azonal. Where the water in its 

 fluctuations of level exerts an influence it is commonly to wash 

 out the organic debris from among the boulders and thus to 

 decrease the nitrogenous per cent. This is not entirely a one- 

 sided interchange, for where aquatic algae, as for example, 

 Aplianizomenon are developed in enormous quantities they are 

 often delivered by the waves among such boulder- shore inter- 

 stices to an extent that can only be appreciated when one has 

 seen the great drifts of decaying algae that sometimes occur 

 in mid-summer, upon such banks as are able to retain them 

 through irregularity of surface. The region next the water's 

 edge does not usually maintain large trees, although occasion- 

 ally willows or poplars may stand on the extreme outer border 

 of the shore, while behind are shrubs and herbs. More 

 ordinarily the edge of such boulder slopes is occupied by 

 plants of Spiraea, Amehinchier, Salix, Campanula, Alnus, 

 Heuchera and a general mixture of psammophy tes, lithophytes, 

 nitrophytes and hydrophytes that indicate by their irregular 

 juxtaposition the complexity of the substratum as an ecologic 

 area. Farther back forest growths of Populus, Betula, Pinus, 

 Quercus, Fraxinus or Acer may be met with and the general 

 slope maintains a greater variety of plants than is often found 

 on areas of similar size, in the region of description. In Plate 

 LXXVII, a typical boulder-slope is shown with a front slope 

 group of azonally distributed shrubs and herbs, and a back- 

 slope formation of mingled Betula and Populus. This view is 

 taken in MacPherson's bay and shows also an Indian village 

 with the birch-bark canoes of the aborigines drawn up upon 

 the shore. 



Boulder slopes having been established in the Glacial period 

 are not at all comparable to new talus, but only to the oldest. 

 Upon the boulders themselves, lithophytic lichens and mosses 

 are commonly abundant and shores consisting of boulders 

 buried in humus or the sand have their distinctive characters 

 of plant population as was described for the shores of buried 

 talus. 



Fine boulder shores. As in the case of talus shores a dis- 

 tinction arises when the rock fragments are small. In such 

 instances they are easily buried by humus or by sand and the 

 characteristic lithophytic vegetation does not appear. This 

 type of shore occurs either as strand or as slope. The fine 



