Fink : Composition of a Desert Lichen Flora 89 



lichens, but only a single loosely foliose species was sent and not 

 a single fruticose one. Moreover, the loosely foliose lichen sent is 

 especially adapted structurally, as will be noted below. Num- 

 bers 3, 5, 6, 7, 28, 30 and 31 were found on the ground and 

 number 24 was collected on the base of a tree trunk in a moist 

 place. These numbers may be omitted from the considerations to 

 follow immediately. 



Comparisons with Lichen Formations of Other Regions 



The other twenty-five lichens of the list were found on rocks 

 and bear a striking resemblance to those of a " Lecanora for- 

 mation of exposed granite."* The lichens of this formation of 

 exposed granite in Minnesota, and those of several other similar 

 formations studied by the writer in the same state, show barely a 

 larger proportion of foliose species than do the lichens of the 

 rocks at Tumamoc Hill. Not only is there this general struc- 

 tural likeness; but when we take into account the difference in 

 latitude and in moisture conditions, it is remarkable that the 

 genera of the list for Tumamoc Hill are largely represented in 

 the formations of the exposed rocks in Minnesota, while there is 

 also a very considerable likeness in the species. Coville and Mac- 

 Dougal give 11.74 inches as the average annual precipitation of 

 moisture at Tucson, during fifteen years of observation^ while 

 the writer found the record for Granite Falls, Minnesota, where 

 the Minnesota lichen formation used in the comparison above 

 occurs, to be 21.83 inches.^ This difference is doubtless the main 

 one of the factors which give the Arizona region a lichen flora 

 as a whole very different from that of the Minnesota area, but 

 which are not sufficient to produce striking differences between 

 the lichen floras of rocks in the former region and those of the 

 exposed rocks in the latter place. H. Zukal says : " Auch zeigen 

 die an der Siidseite an nackten Felsen wachsenden Flechten und 



* Fink, Bruce. Contributions to a Knowledge of the Lichens of Min- 

 nesota. — V. Lichens of the Minnesota Valley and Southwestern Minnesota. 

 Minn. Bot. Stud. 2: 286-288. D 1899. 



f Coville, F. V., and MacDougal, D. T. Desert Botanical Laboratory of 

 the Carnegie Institution. Pub. Carnegie Institution of Washington 26. N 

 1903. 



t Fink, Bruce. Op. c. 279. 



