8 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 
DISPOSITION OF LICHENS ADOPTED IN THIS WORK. 
It is here accepted as an established fact that lichens arose through 
associations of alga and fungus in a symbiotic relationship. Once a 
lichen was established, it began to vary, so that we now have abundant 
evidence that many genera and species of lichens belong to the same 
phylogenetic tree. 
The vegetative tract is commonly much reduced in fungi as a 
result of parasitism or saprophytism, since the food is either prepared 
by the host plant or found ready made in the substratum. How- 
ever, in the fungi which have entered into associations with alge to 
form lichens, the vegetative tract or thallus has assumed the new 
function of protecting the alge so that they can increase in number 
and perform more effectively their duty in the partnership. In 
response to this change in function, the highly varied and complex 
foliose and fruticose thalli of the higher lichens have arisen, in their 
evolved forms departing widely from the most closely related fungal 
ancestors. . 
Add to this the peculiarities of reproduction among lichens, a 
problem still needing careful study, and we shall have some of the 
facts which cause many botanists to maintain that lichens, after all,. 
constitute a distinct class of fungi, or perhaps two distinct classes. 
However, others hold that lichens should no longer be treated sepa- 
rately except for special purposes in much the same way as parasitic 
fungi or poisonous plants, not necessarily all closely related, are 
often grouped together. Further phylogenetic studies may lead to 
the distribution of lichens among fungi, to the exclusion of the class 
Lichenes. This method would then be used in the general systematic 
treatment of fungi, though there would still remain students who 
would treat lichens separately. The present work is the result of 
many years of study of the lichen flora of a particular region, and 
therefore a separate treatment is the only method available. Taking 
into account, then, the unsettled state of opinion regarding the classi- 
fication of lichens and the purpose of the present treatise, it seems 
best to follow Engler and Prantl in recognizing the class Lichenes. 
THE FUNGAL SYMBIONTS AND THEIR RELATED FUNGI, 
In all of the common lichens of temperate regions the fungal 
symbionts are Ascomycetes. The fungus has become so modified, in 
many instances, since entering into the symbiotic relationship, that 
there is little resemblance between it and the ancestral form. There 
is sufficient likeness, however, to lead to a general conviction that 
the fungal symbionts have been evolved in part from the Discomycetes 
and in part from the Pyrenomycetes. Of these two groups such 
genera as Peziza, Patellaria, Hysterographium, Melaspilea, Phacidium, 
