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146 EDVARD VON KRUSENSTJERNAFig . 3. Erratic boulder with thecommon species Dicranum seapariumand Hypnum cupressiformeon the vertical side, forminga widespread union in the Plagiothecionfederation. On the uppersurface dominant Pleurozium. Alsikewood. Aug. 1956. PhotoH. Jahn.this stage Tetraplodon mnioides sometimes occurs,and on naked earth or on soil covered by the lichensBaeomyces rufus and/or Lecidea humosa, Buxbaumiaaphylla may often be found.In moister places Aulacomnium palustre, Polytrichumcommune and Sphagnum nemoreum willoften occur and in wet depressions Sphagnum spp.and Drepanocladus fluitans.In the outer Archipelago of Stockholm, mainlyin the birch zone outside the conifer limit, the mossand lichen communities are-according to DuRIETZ (opp.cc.)-rather different owing to such conditionsas strong wind, salt spray, bird dunging, etc.THE PLAGIOTHECIUM DENTICULATUM FEDERA­TION (PLAGIOTHECION).-The federation is composedof acidophilous, photophobous communities,occurring on more or less vertical surfaces of rockor big boulders (Fig. 3).Dicranum scopariumParaleucob1·yum longifoliumCynodontium strumiferumHypnum cupressiformeLophozia longidensCrocynea membranaceaLepraria glaucellaParmelia omphalodesP. physodesP. saxatilisSphenolobus saxicolusOn dry surfacesIn less dry placesTritomaria quinquedentataPlagiothecium denticulatumI sothecium myosuroidesCetraria glaucaSphaerophorus fragilisS. globosusDiplophyllum albicansH ypnum imponensRhacomitrium aquaticumScapania nemorosaIn somewhat moist placesDo., above all in southwesternSwedenLichens, like the precedingmosses growing inrather shady placesA more western union ofacidicolous species onrather wet rockFinally, two strongly photophobous unions maybe mentioned, the first one containing the fairlycommon species Bartramia pomiformis and Pohliacruda, sometimes, too, Isopterygium elegans. Thisunion can be found in crevices, etc. The latterunion contains only a single society, that of Schistostegapennata. The protonema of this rare moss isvisible on the bottom of deep dark crevices orcaves with some soil, owing to its ability to reflectthe light.Communities on rock with reduced acidityA reduction of the acidity of silicious substratescan be effected either by litter, in particular that ofash, elm, or lime trees or hazel shrubs, and by metalActa Phytogeog1·. Suec. 50