Fink : Classification of Lichens 
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outer, loose layers of periderm. They never bore between firm 
layers of periderm as do the crustose epiphloeodal and the hypo- 
phloeodal lichens. 
Evernia prunastri (L.) Ach. is attached to the substratum by a 
broad, basal holdfast, which becomes larger as the lichen grows. 
Lindau (8i) found the plant often attached to lenticels, through 
whose loose tissues the rhizoids penetrate and also extend a short 
distance into the surrounding periderm. When this plant grows 
on older branches, the holdfast penetrates into the periderm and 
spreads about between the layers, which are finally separated so 
that masses of hyphae come to lie between them. The rhizoids 
may penetrate at least to the tenth layer of periderm. The sepa- 
rated layers become thinner as they are pushed outward by the 
penetrating hyphae, until they are finally only about half their 
original thickness. The thinning does not indicate that the 
hyphae have dissolved material in the walls of periderm cells, for 
if this were true, the hyphae would be able to penetrate through 
the cell walls. As the periderm layers are carried upward, they 
come in contact with the atmosphere, which doubtless dissolves 
the walls. However, it is reasonable to suppose that the air pro- 
duces some change in the composition of the periderm fragments 
such that the lichen hyphae may be able to dissolve and appro- 
priate the transformed product, thus securing food and aiding in 
the thinning process. Lindau observed the same thinning of 
periderm fragments in which some crustose lichens were growing. 
Hypophloeodal lichens, especially those that live a long time 
outside the relationship with an algal host, must secure a large 
amount of nourishment from the periderm, with which their 
whole thalli come in contact. The epiphloeodal crustose species 
are in closer contact with their algal hosts and less closely 
attached to the bark. Hence, they must secure more nourish- 
ment from the alga and less from the substratum. The foliose 
lichens enclose numerous algal host cells and are much less 
closely attached to the substratum than any crustose forms, being 
above it, while their rhizoids penetrate but a short distance into 
it, so these must depend still more upon the algal host and less 
upon the substratum. Fruticose forms are still less closely 
