142 MISSOURI BOTANICAL GARDEN, 
Life Zones of which a map is given. This is due, no 
doubt, to the fact that they are not (or little) dependent on 
soil conditions. In giving the distribution I have, however, 
not only stated political and altitudinal limits as far as pos- 
sible, but a diagram accompanies each species. 
SUBSTRATA. 
Though in my own herbarium labels I have indicated 
whenever possible the exact character of the substratum, and 
this method has also been followed in many of the modern 
herbaria examined, yet it does not seem either practical or 
particularly valuable to list minutely the substrata for lichen 
species. I have therefore employed the following general 
terms: coniferous or deciduous trees; dead timber—fallen 
trees, etc.; old wood—fence rails, roofs, etc.; stones, boulders 
and ground, including actual soil. The question of exposure 
is also important. My observations go to prove that 
strongly lighted or sunny exposures generally produce 
the fruited (apothecial) conditions, whereas sorediate or 
soraliate phases are found most commonly in shaded or 
humid places. Yellow lichens are almost invariably sun- 
light species, which their color may explain (I c. 26). Rigid 
types are frequently maritime. 
HERBARIA. 
Below is given a list of Herbaria I have examined with the 
names of the curators, to whom I am gratefully indebted. It 
has been absolutely necessary in such a variable family to 
examine an enormous amount of material. It is only by so 
doing that one can judge the true value of species and 
varieties. In two of my former papers on the genera, I 
gave lists of all material examined; such a procedure, 
though of distinct value is, nevertheless, impractical in the 
present extended work. 
U. 8S. National Museum, Washington, D. C.—Drs. J. N. Rose and 
W. R. Maxon. 
Academy Natural Sciences, Philadelphia, Pa.—Dr. 8. Brown. 
New York Botanical Gardens, New York, N. Y.—Dr. N. L. Britton. 
