714 MINNESOTA BOTANICAL STUDIES 



A comparison of the figures for St. Paul and Fayette for the 

 eight years since the record has been kept for the latter place, 

 shows a difference in annual precipitation of 6.77 inches in 

 favor of Fayette. Comparison of St. Paul and Dubuque for the 

 whole twenty-three years shows a difference of 9.31 inches per 

 annum in favor of Dubuque. Now, comparison of Fayette and 

 Dubuque for the eight years shows a difference of .50 inches 

 per annum in favor of Fayette. Thus these last two places, 

 only about fifty five miles apart, show so little difference in 

 amount of precipitation that the Dubuque figures may be sub- 

 stituted for Fayette without great error. Also, a glance at the 

 table shows that 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 Minne- 

 apolis can, of course, give rise to no appreciable error, and 

 this difference of abuut 9.31 inches, with the accompanying dif- 

 ference of humidity 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. 



Though dealing with a small area, it will be seen that this 

 paper may be regarded as preliminary to a study of the dis- 

 tribution of lichens all along the upper Mississippi river. 

 Some of the questions that would arise in such a study have 

 been briefly stated in this paper, and others have been sug- 

 gested to me which I have not stated. 



The principal conclusions as to distribution and habitat of 

 the lichens about Minneapolis may be given as follows: 



(1) The lichen flora of Minneapolis and vicinity is not a rich 



one, being about 25 per cent poorer than some portions 

 of northern Iowa, and doubtless also than southeastern 

 Minnesota. 



(2) The difference in number of lichens for the two areas com- 



pared affects species, but scarcely extends to genera to 

 any appreciable extent. It also affects individuals, giv- 

 ing a large number of rare or infrequent species for 

 Minneapolis. 



(3) The cause of the comparatively smaller number of species 



and individuals for Minneapolis and vicinity is its dryer 

 climate. Other causes act, as stated elsewhere, to a very 

 mited extent. 



