96 



Mycologia 



whether this meagerness is due to the presence of the tuff or to 

 the southern exposure. Mr. Blumer stated in a letter that the 

 lichens seem to grow better on the basalt, but only one of the 

 stations reported is composed entirely of tuff, so the question 

 of relative suitableness of the two kinds of rocks for lichen sub- 

 strata cannot be certainly solved from data at hand. Mr. Spal- 

 ding and Mr. Blumer both stated in letters that lichens are very 

 scarce on southward-facing rocks, the latter gentleman writing: 

 " Their place of best development is on the northerly faces of 

 basaltic rocks, where they are often beautifully conspicuous. On 

 sunny aspects of rock faces they must be looked for to be found." 

 However, the most remarkable thing about the short list of seven 

 lichens found in this station is that five of them belong to the 

 genus Acarospora, and that every species and subspecies of the 

 genus known to occur on Tumamoc Hill is found in this one 

 station. The writer must again refer to his work in Minnesota,* 

 where he has found the genus represented in every one of the six 

 lichen formations of exposed horizontal rocks studied, whether 

 on granite, quartzite or pipestone, and in all but one by the species 

 Acarospora xanthophana. Also, he must recur to his statement, 

 in the paper on " A Lichen Society of a Sandstone Riprap,"f 

 regarding the frequent occurrence of Acarospora on the south- 

 ward-facing riprap and its very rare occurrence on the north- 

 ward-facing riprap a few feet away. These data, with those of 

 station III, establish beyond doubt that species of Acarospora, 

 with their strong protective cortices and their cellular structure 

 throughout are the most characteristic xerophytes of all our 

 American lichens thus far studied from the ecologic point of 

 view. They occur in xerophytic associations as a small propor- 

 tion of the plants of lichen formations in exposed environments 

 in regions for the most part mesophytic, and are found at station 

 III making a very large proportion of a lichen aggregation on the 

 southward-facing, dry and often hot rocks of a desert region. 



Station IV is in the same locality as station II, but differs in 

 that it is an outcrop of tuff facing eastward at the bottom of the 

 exposure. The plants determined from this station are numbers 



* Fink, Bruce. Op. c. and other papers of the same series, 

 f Fink, Bruce. Op. c. 278. 



