176 IOWA ACADEMY OF SCIENCES. 



113 species found at Minneapolis are about 72 per cent of the 

 157 species of Fayette lichens, and it will be an approximately 

 correct estimate to say that lichens are one-fourth more numer- 

 ous at the latter place than at the former. 



The cause of the difference in number of lichens in the two 

 places is evidently to be sought mainly in climatic differences. 

 Several considerations have conspired to cause me to arrive at 

 this conclusion. 



First. — Most species of lichens are more disposed to confine 

 themselves to moist situations at Minneapolis, as about the 

 bodies of water mentioned above, in heavy woods, or when in 

 dry places near the ground. The last tendency is noticeable 

 in Graphis scripta (L) Ach., which in dry places most frequently 

 grows low down on the trunks of the trees. In passing up 

 from the Mississippi river banks fifty to one hundred feet to 

 the level ground just above the bluffs the decrease in number 

 of species and individuals, whether on rocks, earth or trees, is 

 very striking. In one place, within or near the city limits, the 

 granitic boulders just above the bluffs are well covered with 

 lichens, while twenty rods back from the river in open ground 

 the rocks of the same kind are nearly bare of them. The 

 decrease is not so marked in lichens growing on trees as in 

 those growing on rocks, but is noticeable. I am not referring 

 now to change in species in passing to the drier locality, which 

 also occurs here as elsewhere, and is due to stress caused by 

 environment. Further, it may be said that a decrease would 

 occur in numbers in other regions, but observation shows it to 

 be more marked in dry climates. In parts of northern Iowa 

 no such noticeable decrease occurs. Here fifteen or more spe- 

 cies of lichens may easily be found on a single tree in moder- 

 ately dry situations, and nearly all the species commonly 

 occurring on the boulders in the vicinity of Fayette may be 

 found on a single one in an open, dry field far removed from 

 any stream. 



Second, — The gelatinous lichens, which thrive in moist places, 

 are much more common at Fayette. The first table will show 

 that the genera Collema and Leptogium show twelve species at 

 Fayette and only two at Minneapolis. I took special pains to 

 investigate this peculiarity of distribution at the latter place, 

 searching deep wooded ravines where these species should 

 abound. It may be added that two of the three species given 

 for the one locality are much rarer than any one of the twelve 



