414 Fixx: Some Common Types oF LICHEN FORMATIONS 
renula are most representative. These plants have very rudimen- 
tary thalli, consisting mostly of a network of hypophloeodal hyphae 
in which are entangled the algal cells. In gaining a foothold, the 
hyphae seem to work their way between the decaying cells of the 
bark, and, once firmly established below the surface, the bark itself 
furnishes both protection and support, as well as moisture. After 
Pyrenula the species of Graphis and Arthonia contain the next 
most typical members of smooth bark lichen communities. In 
these two genera, the thalli are of essentially the same structure as 
in Pyrenula, 4 
The crustose species of Biatora, Buellia and Lecidea all have 
small thalli devoid of cellular cortices. But these plants have, as 
is commonly true of the more rudimentary epiphloeodal thalli, an 
upper colorless network of hyphae devoid of living algal cells and 
tending to lie in a horizontal direction. These hyphae, together 
with a number of entangled dead algal cells, form more or less of 
a protection to the living algae below. The lichens having such 
thalli may be looked for in either the smooth or the rough bark 
formations, and the same is true of certain species of Lecavora, 
Placodium and Pertusaria which have larger thalli, possessing at 
least some indication of upper cortex. 
In the above statements I have had in mind the bark forma- 
tions of our common deciduous trees, and such lichen assemblages 
may be looked for wherever these trees exist in considerable num- 
bers in forests. Yet no American has seriously studied the rela- 
tions of epiphyte or parasite to host, though statements may be 
found in scattered writings regarding the occurrence of certain 
lichens upon a given species of tree. In Europe, F. Arnold, in 
his systematic studies of the lichen flora of Munich, Germany, 
and of that of the Jura Mountains, has exhaustively recorded the 
lichens growing upon each species of tree and has extended the 
study to other substrata than trees. This is interesting, especially 
regarding those lichens that are confined to a particular host oF 
substratum, but Dr. Arnold has left practically untouched the 
more interesting and obscure problem of the adaptations of the 
lichens to these substrata. 
Passing from these lichen formations of the trees, let us next 
give attention to what appear to be the next most easily observed 
