Fink: THE LICHENS OF MINNESOTA. 



707 



cents which these numbers represent of the whole number of 

 lichens found in each locality, on the substrata considered. 



The table shows very little difference in the per cents of 

 species on different substrata for the two localities, and this 

 would seem to indicate that, though the dryer climate of the 

 Minnesota region has caused a poorer development of lichens 

 than is found at the Iowa locality, it has not caused these plants 

 to seek substrata especially favorable for their development. 

 Other factors enter in to compensate differences which would 

 otherwise occur to such an extent that the table shows in this 

 respect just what it would not show were it not for these fac- 

 tors, — similarity as to number of lichens on different substrata 

 for the two regions. 



These other factors have prevented the development of a 

 larger per cent of earth and rock lichens. As climate becomes 

 dryer the relative per cent of these lichens should increase 

 because nearer the earth there is more moisture. First as 

 to the lichens on calcareous rocks, the per cent for Fayette is 

 a little' higher than that for Minneapolis, while the opposite 

 condition should follow the difference in climatic conditions 

 between the two places. This apparent difficulty is easily 

 explained since the calcareous rocks outcrop at the surface 

 much more frequently at the former locality. The lichen-spe- 

 cies on calcareous rocks at Minneapolis are those confined in 

 both places to perpendicular exposures, while one-third of the 

 species found at Fayette are characteristic of surface outcrops. 

 Deducting one- third of the 19.33 per cent given in the table for 

 Fayette leaves less than 12.7 per cent and gives Minneapolis 

 an advantage of more than 3 per cent for conditions as to sub- 

 strata existing at both places. This is given as the true rela- 

 tion so far as influenced by the difference in hygrometric con- 

 ditions. 



