178 



IOWA ACADEMY OP SCIENCES. 



these numbers represent of the whole number of lichens found 

 in each locality on the substrata considered: 



The table shows very Jittle difference in the per cents of 

 species on different substrata for the two localities, and this 

 would seem to indicate that, though the drier 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 at Minneapolis. As 

 climate becomes drier the relative per cent of these lichens 

 should increase because nearer the earth there is more mois- 

 ture. First as to the lichens on calcareous rocks, the per cent 

 for Fayette isalittlehigher than that for Minneapolis, while the 

 opposite condition should follow the difference in climatic condi- 

 tions 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 

 substrata existing at both places. This is given as the true 

 relation so far as influenced by the difference in hygrometric 

 conditions. 



