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The growth on rock 147Fig. 4. Antitrichia cu1·tipendulacovering a big boulder in the shadypark of Linne's Hammarby nearUppsala. Other species listed fromthe same union include Homalotheciumsericeum, Hypnum cupressiforme,Leucodon sciuroides(originating from fragments droppedfrom the trees), Thuidiumabietinum, Th. Philiberti and Tortularuralis. Aug. 1956. Photo H.Jahn.ions taken up from raindrops that have passedthrough the canopy, or by any kind of dust impregnation.THE GRIMMIA COMMUTATA FEDERATION (GRIM­MION COMMUTATAE).-A strongly photophilouscommunity, occurring especially on comparativelysmall surfaces of ice-scoured Archaean rocks inagricultural districts, near farms etc., where fertilizingdust is available. This community has onlybeen studied in Uppland and Sodermanland. I havelooked for it, e.g. in northern Smaland, but in vain.(Cf. map showing the distribution of Grimmiacampestris in v. KRusENSTJERNA 1945, p. 164.)The lichens belonging to this federation havebeen studied by SERNANDER (1901, 1912d) and DuRrETZ (1932 b). SERNANDER's (1901) term "nitrophilouslichens'' refers to the common occurrenceof some species, e.g. Lecanora saxicola ( =muralis),Physcia stellaris, Ramalina polymorpha, Xanthoriaparietina, etc., on rocks, exposed to fertilizing dustor bird dung. The following constellations of speciesbelong to this group:The Lecanora saxicola SERNANDER 1912d, p. 823formationThe Physcia stellaris-adscendensformationDo.The Ramalina polymorpha-XanthorialychneaformationParmelia pullaP. stenophylla ( = P.molliuscula)P. isidiotylaH edwigia ciliataGrimmia campestrisGr. commutataH edwigia ciliataGrimmia montanaGyrophora hirsutaH edwigia ciliataHypnum cupressiformeThuidium abietinumTortula ruralisDo.Du RrETZ 1932 b, p. 98V. KRUSENSTJERNA 1945,p. 70Do.This union (op. c., p. 72)often occurs in smallcrevices or depressionswith some soil. Stonecropspecies (Sedum) arecommon hereA very variable, partly therophytic, partly perennialvascular vegetation is associated where somesoil occurs on these rocks. According to LOHAMMAR(1964), this vegetation is highly unstable in compositionand dominance from year to year.THE ANTITRICHIA CURTIPENDULA FEDERATION(ANTITRICHION).-This plant community (v. KRu-.Acta Phytogeogr. Suec. 50