THE SPORES OF LICHENS. 723 
he gives the forms of the spores, though not always accu- 
rately, and their measurements. While the Italian and Ger- 
man writers on the one hand tend to too great a subdivision 
of genera and species, Nylander, on the-other, is frequently 
too comprehensive, though this is perhaps the safer error of 
the two. 
Professor Tuckerman of Amherst, has expressed briefly 
his views on the value of spore characters, in his “ Lich- 
ens of California," 1866, and has laid the foundation of a 
more sound and instructive doctrine on this subject than 
previous writers. In his opinion, which has been followed 
-in what precedes, "less weight than has often been assumed 
should be given to spore differences of a merely gradal 
character, or such others as depend only on mensuration, 
and more to those that seem typical.” He considers that 
there are "two well defined kinds of lichen-spores, comple- 
mented in the highest tribe only by a well-defined inter- 
mediate one. In one of these (typically colorless) the 
originally simple spore, passing through a series of moditi- 
cations, always in one direction, and tending constantly to 
elongation, affords at length the acicular type. To this is 
opposed (most frequently but not exclusively in the lower 
tribes, and even possibly anticipated by the polar-bilocular 
sub-type in Parmeliacei), a second (typically colored) in 
which the simple spore, completing another series of changes, 
tending rather to distension and to division in one direction, 
exhibits finally the muriform type.” Tn accordance with this 
view Rinodina is distinguished from Lecanora, and Buellia . 
from Lecidea. Theloschistes parietinus is separated from 
Physcia, a genus with colored spores, and placed in a distinct 
genus, the type of whose spore is the polar-bilocular. On the 
other hand Biatora rubella would not be separated from that 
genus, which includes species with simple spores, merely on 
account of its septate spores, nor Buellia petrea placed in a 
distinct genus, Rhizocarpon, on account of its muriform 
spores, nor Lecanora cervina on account of its polysporous 
