Notes and Comment. 



191 



The second paper which appeared in Mycologia, for May 

 1909, is on "The Composition of a Desert Lichen Flora." Thirty- 

 three species are enumerated which were collected in the vicinit y 

 of the Desert Laboratory at Tucson, Arizona, and although the 

 author was hampered by having only material collected by 

 others, his discussion of certain problems suggested by this 

 relatively meager collection is indicative of the new and hopeful 

 methods that are coming into vogue in Lichenology. Instruc- 

 tive comparisons are made between the lichen flora of the Desert 

 Laboratory domain and that of a region previously studied by 

 the writer in Minnesota. The lichen flora of the Arizona area, 

 taken as a whole, is very different from that of the area in Minne 

 sota, with which it is compared, but the rock species, twenty-five 

 in number, show a striking resemblance to those growing on ex- 

 posed granite in the latter state. Thus, on the one hand, the 

 differences of composition correspond with climatic differences 

 obtaining in the two regions, while the likeness of the rock 

 species may be referred to similarity of edaphic conditions, the 

 studies of the author going to 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. 

 The lichens occurring on the rocks in the vicinity of the Desert 

 Laboratory are noticably lacking in species with conspicuously 

 lobed thalli, a fact which may correspond to a demand for de- 

 crease of surface in contact with the drying environment, and 

 also, quite possibly, to the advantage offered by a structure not 

 easily torn away from the substratum by desert gales. 



Protection is afforded as a rule, by a pseudo-parenchyma- 

 tous cortex, which protects the living algal cells and the hyphae 

 of the medullary layer against the drying effects of high wind 

 and the direct rays of the sun. What may be protective color- 

 ation, manifest in the development of black lines or spots on the 

 upper surface, was observed in every species having a light- 

 colored thallus, but the observations thus far made are not 

 wholly conclusive, notwithstanding the opinion of Zukal that 

 such lines of black occur on the younger or injured portions of 

 thalli to protect the algal cells from the intense rays of sunlight 

 in hot regions. 



