LICHEN COMMUNITIES 363 



I. ARBOREAL 



Arboreal communities may be held to comprise those lichens that grow 

 on wood, bark or leaves. They are usually the dominant and often the sole 

 vegetation, but in some localities there may be a considerable development 

 of mosses, etc., or a mantle of protococcaceous algae may cover the bark. 

 Certain lichens that are normally corticolous may also be found on dead 

 wood or may be erratic on neighbouring rocks : Usnea florida for instance 

 is a true corticolous species, but it grows occasionally on rocks or boulders 

 generally in crowded association with other foliose or fruticose lichens. 



Most of the larger lichens are arboreal, though there are many excep- 

 tions : Parmelia perlaia develops to a large size on boulders as well as on 

 trees ; some species of Rantalinae are constantly saxicolous while there are 

 only rare instances of Roccellae that grow on trees. The purely tropical or 

 subtropical genera are corticolous rather than saxicolous, but species that 

 have appeared in colder regions may have acquired the saxicolous habit : 

 thus Coenogonium in the tropics grows on trees, but the European species, 

 C. ebeneum, grows on stone. 



a. Epiphyllous. These grow on Ferns or on the coriaceous leaves of 

 evergreens in the tropics. Many of them are associated with Phycopeltis, 

 Phyllactidium or Mycoidea, and follow in the wake of these algae. Obser- 

 vations are lacking as to the associations or societies of these lichens whether 

 they grow singly or in companies. The best known are the Strigulaceae : 

 there are six genera in that family, and some of the species have a wide 

 distribution. The most frequent genus is Strigula associated with Phyco- 

 peltis which forms round grey spots on leaves, and is almost entirely confined 

 to tropical regions. Chodat' records a sterile species, 5. Buxi, on box leaves 

 from the neighbourhood of Geneva. 



Other genera, such as those of Ectolechiaceae, which inhabit fern scales 

 and evergreen leaves, are associated with Protococcaceae. Pilocarpon leuco- 

 blepharum with similar gonidia grows round the base of pine-needles. It is 

 found in the Caucasus. In our own woods, along the outer edges, the lower 

 spreading branches of the fir-trees are often decked with numerous plants 

 of Parmelia physodes, a true " plant society," but that lichen is a confirmed 

 "wanderer." Biatorina Bouteillei, on box leaves, is a British and Continental 

 lichen. 



b. Corticolous. In this series are to be found many varying groups, 

 the type of lichen depending more on the physical nature of the bark than 

 on the kind of trees. Those with a smooth bark such as hazel, beech, lime, 

 etc., and younger trees in general, bear only crustaceous species, many of 

 them with a very thin thallus, often partly immersed below the surface. 



'■ Chodat 1912. 



