IOWA ACADEMY OF SCIENCES. 153 



little difference in amount of precipitation that the Dubuque 

 figures may be substituted for Fayette without great error. 

 Also, St. Paul suffered less from the drouth of recent years 

 than Dubuque, and hence than Fayette, so that the figures for 

 the smaller number of years cannot be relied on, and 9.31 

 inches per annum doubtless is nearer the average difference 

 between Minneapolis and Fayette in precipitation than is 6.77 

 inches. The use of St. Paul figures for Minneapolis can, of 

 course, give rise to no appreciable error, and this difference of 

 about 9.31 inches, with the accompanying difference of humid- 

 ity of the atmosphere, seems to account very largely for the 

 difference of 28 per cent in number of species of lichens. No 

 reliable figures as to relative or absolute humidity could be 

 obtained. 



The following conclusions may be drawn relative to lichen- 

 distribution in northeastern Iowa: 



1. The lichen-ttora of the region is richer than that of the 

 Minnesota region about 150 miles north, and doubtless 

 there is a gradual decrease in number of lichen species in 

 passing north from the Iowa region. 



2. The cause of the difference in favor of the Iowa region is 

 neither its more southern location nor advantage in sub- 

 strata, but mainly at least its moister climate. The proofs 

 given in the paper maybe briefly summarized as follows: 



{a) Lichens intse Iowa region are not so much inclined 

 to confine themselves to moist situations. 



(&) Gelatinous lichens, which thrive best in moist places, 

 are four times as numerous in the Iowa region. 



(c) Were it not for other than atmospheric conditions, 

 the number of tree lichens would be larger in compari- 

 son with earth and rock lichens in northeastern Iowa. 



{d) Even the Saint Peter sandstone, occurring in moist 

 places, has more lichens in the Iowa region. 



(e) The greater number of habitats of certain lichen 

 species in northeastern Iowa must be included. This 

 has been noted for Usnea harhata (L) Fr. , var. Mrta 

 Fr. , and a comparison of the number of species found 

 on both wood and rocks at Fayette and Minneapolis, 

 as shown in the second table, is further evidence. 

 Other instances could be drawn from a comparison of 

 the lists of sjDecies for the two places. 



