206 BOTANICAL GAZETTE [APRIL 
ines the vast numbers of loose rocks lying upon air-dry soil, yet 
thickly covered with lichens. This fact is impressed still more 
strongly when one sees the luxuriant growth of certain lichens upon 
the loose slabs and fragments forming the talus about cliffs; often 
they are some feet above the soil or outcropping rock, and lie in such 
a position that the hot desert air blows all about them, under as well 
as over; yet the lichens seem to thrive as well on these fragments 
as on the cliff itself. In these cases it seems plain that the only 
source of water for the lichens thereon is from direct precipitation 
and from the atmosphere. 
During the summer months it is highly probable that a careful 
study would show that the great diurnal drop in temperature which 
comes in all arid regions with the advent of darkness is likewise 
accompanied by a deposition of moisture upon, or at least an 
increase in water content of, the desert lichens. : 
When one considers that the temperature drops 30° to 35 F. 
every night during the summer months, it is apparent that 
without any increase in the actual amount of water present there 
is a great increase in the percentage of moisture, and it is believed 
by some of us that the lichens are able to take advantage of this 
relative increase and absorb enough moisture to maintain their 
vitality during the long, hot, dry season. : 
Though the winters here are characterized by a comparatively 
low temperature, they are yet open and sunny, so that during the 
daylight hours they are usually quite mild. This mildness and the 
fact that the winter is the season when most moisture is available, 
especially that from melting snows, cause the lichens to have their 
period of most active growth during the months from November 
to June. Except under unusually favorable conditions, it 15 not 
likely that the Nevada desert lichens are more than able barely t 
keep alive during the remainder of the year. 
That desert conditions are, in the main, unfavorable to ye 
growth of lichens as a whole is shown by the limited number “ 
genera and species represented, while a considerable number 0 
favorable spots, such as under overhanging rocks on the nor 
of cliffs, or deep within crevices. But that some species are per 
