LICHEN COMMUNITIES 373 



Most calcicolous lichens are almost completely dependent on the lime 

 substratum which evidently supplies some constituent that has become 

 necessary to their healthy growth. Calcareous rocks are usually of softer 

 texture than those mainly composed of silica, and not only the rhizoidal 

 hyphae but the whole thallus — both hyphae and gonidia— may be deeply 

 embedded. Only the fruits are visible and they are, in some species, lodged 

 in tiny depressions (foveolae) scooped out of the surface by the lichen-acids 

 acting on the easily dissolved lime. 



Those obligate lime species may be found in associations on almost any 

 calcareous rock. Watson' has given us a list of species that inhabit carboni- 

 ferous limestone in Britain. Wheldon and Wilson^ have described in West 

 Lancashire the "grey calcareous rocks blotched with black patches of Pan- 

 narias {Placynthium nigruiii) and Verrucarias, or dark gelatinous rosettes of 

 Collemas. White and grey Lecanorae and Verrncariae spread extensively, 

 some of them deeply pitting the surface. These more sombre or colourless 

 species are enlivened by an intermixture of orange-yellow Physciae {Xan- 

 thoriae) and Placodii by the ochrey films of Lecanora ochracea and lemon- 

 yellow oi Lecmiora xantholyta. Amongst the greenish scaly crusts oi Lecanora 

 crassa may be seen the bluish cushions of Lecidea coeruleo-nigricans, the 

 whole forming an exquisite blend of tints.'' 



The flora recorded by Flagey^ on the cretaceous rocks of Algeria in the 

 Province of Constantine does not greatly differ, some of the species being 

 identical with those of our own country. Placodiums and Rinodinas were 

 abundant, as also Lecanora calcarea, Acat'ospora percaenoides and Urceolaria 

 actinostoma var. calcarea. Also a few Lecideae along with Verrucaria 

 lecideoides, V. fuscella, V. calciseda and Endocarpon moiistrostim. The rocks 

 of that region are sometimes so covered with lichens that the stone is no 

 longer visible. 



Bruce Fink' gives a typical community on limestone bluffs in Minnesota: 



Pannaria {Placynthium) Jiigra. Placodium citrinuni. 



Crocynia lanuginosa. Bacidia inundata. 



Omphalaria pulvinata. Rhizocarpon alboatruni var. 



Collema plicatile. Dermatocarpon miniatU7n. 



Collema pustulattim. Staurothele umbrinum. 

 Leptogium lacerum. 



ForsselH pointed out an interesting selective quality in the Gloeolichens 

 which are associated with the gelatinous algae, CltJ'oococcus, Gloeocapsa and 

 Xanthocapsa. The genera containing the two former grow on siliceous rocks 

 with the exception of Synalissa. The genera Omphalaria, Peccania, A nema, 

 Psorotichia and Enchylium, in which Xanthocapsa is the gonidium, grow on 



1 Watson 1 918^. 2 Wheldon and Wilson 1907. ^ Flagey 1901. 



^ Bruce Fink 19022. ^ Forssell 188,:;. 



