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 to pay attention to the ecological grouping of corticolous 

 lichens was A. L. F^e', 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 Cinchonae was the one known as " Loxa," the bark of which was 

 covered with species of Parmelia, 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 Trype- 

 thelium, Chiodecton, Pyrenula and Verrucaria were also represented. On each 

 species of tree some particular lichen ws^ generally dominant : 

 A species of Thelotrema on Cinchona oblongifolia. 

 A species of Chiodecton onjc. cordifolia. 

 A species of Sarcographa on C. condaminea. 



Fries ^ in his geography of lichens, distinguished as arboreal and "hypo- 

 phloeodal" 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' 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, Parmelia 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 CoUemaceae 

 such as Leptogium Hildenbrandii, Collema nigrescens, etc. 



Arnold* 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, 



» Fee 1824. 2 Fries 1831. ^ Krempelhuber 1861. * Arnold 1891, etc. . 



