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MINNESOTA BOTANICAL STUDIES. 



The Saint Peter sandstone along the Mississippi river near 

 Minneapolis, and that along the same river in northeast- 

 ern Iowa near McGregor, may be compared as to lichen-floras 

 by use of the following table, which gives the species charac- 

 teristic of these rocks in both places, and also those found on 

 them in each place and not in the other. 



This table affects comparisons thus far made in no way since 

 the Iowa locality, now under consideration, is a different one 

 than that previously used. In all the comparisons thus far 

 made, the lichens on the Saint Peter sandstone have been elim- 

 inated with those of other substrata occurring in only one of 

 the localities. Now, in the above table it will be noticed that 

 the Iowa region has the advantage in the number of species not 

 common to both. Knowing what occurs in Iowa, I examined 

 the Minnesota locality very carefully, and the advantage is ap- 

 parently due to the more favorable conditions for lichen growth 

 in northern Iowa. The four species common to both regions 

 are doubtless distributed along the river between the two local- 

 ities wherever these rocks are exposed. How far north the 

 four species found only in the Iowa locality extend, and how 

 far south that found only in the Minnesota locality extends, are 

 questions of interest. Knowledge on this point might lead to a 

 modification of views just stated. 



Usnea barbata (L.) Fr. var. liirta Fr., is also confined to the 

 Saint Peter sandstone at the Minnesota locality, but not at the 

 Iowa one, though occurring on this formation there also. This 

 rock is apparently its most natural habitat in the regions con- 

 sidered, to which habitat it is confined in the one less favorable 

 to lichen development. Though, as in this instance, I have 

 omitted from the last table the species found on these rocks 

 and also on other substrata near by in one or both regions, ab- 

 ruptness in floral change due to stress caused by change in sub- 

 strata is seldom better illustrated than in comparing the lichens 



