rgtt] HERRE—DESERT LICHENS 293 
ber only three dwell on wood; Buellia triphragmia occurs sparingly 
on the trunks and limbs of sage brush, while Xanthoria polycar pus 
is rather common on the same habitat, usually near the base of the 
gnarled and twisted trunk, where it is well sheltered by the densely 
branching shrub above. Candelariella cerinella is best developed 
and most abundant on rocks, but likewise occurs on clumps of dead 
moss in the shadow about the base of decumbent sage brush 
trunks, and to some extent on the trunks themselves; but it is, 
‘properly speaking, a rock lichen. 
Comparing our 55 saxicolous lichens with the 25 obtained near 
the Carnegie Desert Laboratory at Tucson, Arizona, we find 22 
genera represented at Reno, while only 11 were collected at Tucson. 
Apparently but 9 species are identical in the two localities. 
Three genera are especially well represented about Reno: 
Lecanora with 9 forms, Acarospora with 7, and Lecidea with 5, 
these three comprising over 38 per cent of the species of rock lichens 
of the region. If we include the highly successful Caloplaca elegans, 
we have 4 genera of the 22 which probably cover three-fourths of 
the rock surface devoted to lichens; while the addition of Rinodina 
and Gyrophora would give us at least nine-tenths of the individuals 
found on the desert rocks. The remaining genera are not only poor 
in number of species but are also poorly represented by numbers of 
individuals. As we pass from desert to alpine conditions, we find 
that Gyrophora becomes the dominant genus, followed by Acaro- 
Spora, Caloplaca, and Lecidea. 
The rainfall at Tucson is 1 3.53 inches, or 42.87 per cent greater 
than at Reno. Yet no species of Lecidea were collected at Tucson, 
while at Reno the genus is well represented, relatively speaking. 
This shows that their absence at Tucson is not due to dryness, but 
to other causes. It will be noted that the chief representative of 
the genus at Reno is the alpine or subalpine Lecidea atrobrunnea. 
In like manner we may account for the prominence of the Lecanora 
rubina group at Reno and its entire absence at Tucson by the 
higher altitude and prevailing low temperature for a large part of 
the year at Reno, these species of Lecanora being characteristically 
subalpine or alpine plants. 
The coloring characteristic of the rock ledges of the desert and 
