90 



Mycologia 



jene heisser, regenarmer Gegenden und Wiisten den gemeinsamen 

 Charakter der ausserordentlich verdickten Aussenrinde."§ The 

 studies of the present writer in agreement with those of Zukal 

 show that lichen formations of horizontally exposed rocks in 

 regions of average rainfall, as well as those of perpendicular or 

 inclined, southward-facing rocks, may show the same structure 

 as the lichens of the desert rocks. 



In connection with the observations of Zukal and related to 

 other statements above and to problems to be considered below, 

 statements of the writer in a paper recently published are of 

 special interest.. In the study of the lichen formations of sand- 

 stone ripraps, 1 1 it was found that the northward-sloping rip- 

 rap supported a lichen formation containing quite a proportion 

 of fruticose and foliose species, while the formation of a south- 

 ward-facing riprap a few feet away was composed almost ex- 

 clusively of closely crustose and strongly corticate species. The 

 latter formation, like the lichen aggregations of the rocks in 

 the vicinity of the Carnegie Desert Laboratory, shows Acaro- 

 spora commonly present, while the members of this genus were 

 extremely rare on the northward-facing riprap. This northward- 

 facing riprap supported an abundance of Biatora myriocarpoides, 

 which was replaced on the southward- facing riprap by Buellia 

 myriocarpa, a lichen whose structure protects it better against 

 the drier habitat through the greater tendency toward disappear- 

 ance of thallus and the better development of such structures as 

 exciple, hypothecium and paraphyses. It is significant that 

 Biatora, with its poorly developed exciples, hypothecia and para- 

 physes, is entirely wanting in the lichens sent from the area about 

 the Desert Laboratory. 



General Consideration of Structure 



It is well known that the more a thallus is branched or lobed, 

 the more young, tender, growing points are exposed and the 

 greater the amount of transpiration of moisture, other things 



§ Zukal, H. Morphologische und biologische Untersuchungen iiber die 

 Flechten. Sitzungsbericht. kaiserl. Akad. Wien. 14: 1308. O 1895. 



II Fink, Bruce. A Lichen Society of a Sandstone Riprap. Bot. Gaz. 38 : 

 269-279. O 1904. 



