Fink: Composition of a Desert Lichen Flora 97 



i and 2 of the list. The eastward-facing tuff at the bottom of 

 the exposure is doubtless often moist, so that Endocarpiscum 

 finds a' favorable habitat. It is not a little remarkable that the 

 tuff, even at the base of the exposure, gave only two lichens ; and, 

 while it can hardly be possible that the absence of other lichens 

 from the formation is more than a singular accident in distribu- 

 tion, if indeed the collecting was in this instance carefully done, 

 the data at hand tend strongly to prove that tuff is a very poor 

 substratum for lichens. 



Station V is a northward-facing basalt cliff on the north side of 

 Tumamoc Hill, just west of its summit, altitude 914 m. Mr. 

 Blumer thinks that this is perhaps as moist and cool a place as 

 can be found about the Desert Laboratory, but he writes that even 

 here lichens are absent from certain rock crevices and faces that 

 are perennially dry, and are for the most part limited to such sur- 

 faces as are frequently wet or moist. The lichens found in this 

 station are numbers 6, 7, 8, 10, 13, 15, 16, 19, 23, 26, 27, 30 and 33. 

 The general similarity of this list of lichens and that of station I 

 is apparent enough from the numbers, and it need only be stated 

 that the gelatinous lichens which commonly grow in moist and 

 shaded habitats are represented on the rocks here by numbers 6 

 and 7. This comparatively moist and cool station is the only one 

 at which these species were found upon the rocks. The failure to 

 get Endocarpiscum from this station is doubtless due to an over- 

 sight in the collecting. 



Station VI is at the bottom of the gulch west of the Desert 

 Laboratory, on the north slope, about an old tuff quarry, altitude 

 747 m. The rocks are tuff, with a few basaltic boulders, and the 

 station is drier than station V. The lichens found in this station 

 are numbers 3, 4, 8, 9, 10, 11, 13, 15, 16, 17, 18, 19, 23, 26 and 33. 

 The general similarity of this lichen assemblage to that in station 

 I is readily seen. The present station shows a larger number of 

 species of Acarospora than did station I, and the individuals of 

 this genus predominate more plainly in the present station, if one 

 may judge by the material sent. The tuff is doubtless a drier 

 rock than the basalt, not holding water so well, and it would seem 

 that is supports a fairly well developed lichen flora on the north- 

 ward-facing exposures, but not on the southward-facing. How- 



