708 MINNESOTA BOTANICAL STUDIES. 



Next as to the lichens on granitic rocks, the difference of 1.3 

 per cent in favor of Minneapolis is not so great as might be 

 expected. This is due to the fact that so many of the granitic 

 boulders are in dry open places. The few in moist or shaded 

 places are reasonably well covered with lichens, but those not 

 thus protected are not, as has been stated elsewhere. The lime- 

 stone exposures are usually shaded along the wooded river 

 banks, hence the advantage for these rocks would be greater 

 than for the granitic rocks were it not for the lack of surface 

 outcrops of the limestone rocks. 



As to the earth lichens, the region including Minneapolis 

 lacks the calcareous- earth lichens of the Iowa region, because 

 the calcareous rocks are more deeply covered by drift and have 

 not been so frequently exposed to help in soil formation. Of 

 the 22 earth lichens found at Fayette 7 occur on calcareous 

 earth, while of the 12 found at Minneapolis only 2 occur on 

 calcareous earth. Reducing the first number to 15 and the 

 third to 10 gives Minneapolis an advantage of .3 per cent. 

 This .3 per cent does not show the effect of atmospheric differ- 

 ences between the two places compared, because of the Minne 

 apolis Cladonias only one-third occur on the earth, while of. 

 those at Fayette about two-thirds grow on the ground. Since 

 I shall be able to present no very satisfactory explanation of 

 this difference in distribution of the Cladonias, it might be fair 

 to throw the earth Cladonias out of the calculation; and, if this 

 were done, the advantage in favor of Minneapolis in the per 

 cents would be about 2.5. 



As to the wood lichens about Minneapolis, it will be seen that 

 if the per cents of rock and earth lichens were what we should 

 expect from hygrometric conditions alone, the per cent of these 

 would rise and that of the wood lichens would fall in compara- 

 tive proportion. In other words. conditioDS other than atmos- 

 pheric have tended to decrease the rock and the earth lichens, 

 but not the wood lichens. Scarcity of lichens on trees removed 

 from the large bodies of water and not in heavy forests has 

 been noted elsewhere in this paper. As to lichens on dead 

 wood, especially old boards, the Minneapolis region furnishes 

 8 and Fayette 14. The per cent of the whole lichen-floras in 

 favor of the latter region is about one. In the Minneapolis 

 region the lichens on old boards are common enough in damp 

 places, but in dry ones old boards are frequently quite bare of 

 them. In the Iowa region the old boards are abundantly sup- 

 plied with lichens, even in dry places. 



