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364
ECOLOGY
As the trees become older and the bark takes on a more rugged character,
other types of lichens gain a foothold, such as the thicker crustaceous forms
like Pertusaria, or the larger foliose and fruticose species. The moisture that
is collected and retained by the rough bark is probably the important factor
in the establishment of the thicker crusts, and, as regards the larger lichens,
both rhizinae and hold-fasts are able to gain a secure grip of the broken-up
unequal surface, such as would be quite impossible on trees with smooth bark.
Among the first t6 pay attention to the ecological grouping of corticolous
lichens was A. L. Fee 1
a Professor of Natural Science and an ,
Army doctor,
who wrote on many literary and botanical subjects. In his account of the
Cryptogams that grow on "officinal bark," he states that the most lichenized
of all the Cimhotiae was the one known as " Loxa," the bark of which was
covered with species of Parmelta, Sticta and Usnea along with crustaceous
forms of Lecanora, Lecidea, Graphis and Verrucaria. Another species, Cin-
chona cordifolia, was completely covered, but with crustaceous forms only :
species of Graphidaceae, Lecanora and Lecidea were abundant, but Trypethelium,
Chiodecton, Pyrenula and Verrucaria were also represented. On each
species of tree some particular lichen was generally dominant:
A species of Thelotrema on Cinchona oblongifolia.
A species of Chiodecton on C. cordifolia.
A species of Sarcographa on C. condaminea.
Fries 2
in his , geography of lichens, distinguished as arboreal and "hypophloeodal"
species of Verrucariaceae, while the Graphideae, which also grew
on bark, were erumpent. Usnea barbata, Evernia prunastri, etc., though grow-
ing normally on trees might, he says, be associated with rock species.
More extensive studies of habitat were made by Krempelhuber 3 in his
Bavarian Lichens. In summing up the various "formations" of lichens, he
gives lists of those that grow, in that district, exclusively on either coniferous
or deciduous trees, with added lists of those that grow on either type of tree
indifferently. Among those found always on conifers or on coniferous wood
are : Letharia vulpina, Cetraria Laureri, Pannelia aleurites and a number of
crustaceous species. Those that are restricted to the trunks and branches of
leafy trees are crustaceous with the exception of some foliose Collemaceae
such as Leptogium Hildenbrandii, Collema nigrescens, etc.
Arnold 4 carried to its furthest limit the method of arranging lichens
ecologically, in his account of those plants from the neighbourhood of
Munich. He gives " formation "
lists, not only for particular substrata and
in special situations, but he recapitulates the species that he found on the
several different trees. It is not possible to reproduce such a detailed survey,
which indeed only emphasizes the fact that the physical characters of the
bark are the most important factors in lichen ecology: that on smooth bark,
1 Fee 1824.
2 Fries 1831.
3
Krempelhuber 1861.
4 Arnold 1891, etc.