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Botanical indications of air pollution 287tion no doubt has something to do with this, it doesnot necessarily provide the whole explanation. Twoother species with a more or less similar behaviourare Parmelia physodes and Cetraria chlorophylla.Somewhat farther from the city centre than theones mentioned above these two species can togethercover practically the whole trunk of treessuch as Fraxinus excelsior and Que reus robur. Innormal vegetation they are not at all so predominanton the same substrate.Some types of pollution favour certain speciesof plants. An example of this is found at Koping,Vastmanland, where a chemical plant releasingcompounds containing nitrogen has greatly stimulatedXanthoria parietina and other lichen species.The vascular vegetation is also reacting to thisnutrient supplement. In certain exposed areas thedeciduous trees have giant leaves, the tissue isloose and in winter the peripheral parts of the crownsare easily damaged by frost. The conifers are evenmore sensitive in this respect. In the field layerof the natural vegetation nitrophytes such as Urticaand Aegopodium are conspicuous.Swedish research has so far singled out only afew sources of pollution-especially S02-and almost all attention has been devoted to the epiphyticlichens. The reason for the first-namedrestriction is simply that our project is not yetcompleted. Amongst other items we intend tostudy hydrogen fluoride damage and hope to startthese investigations some time in 1965. The factthat research has been limited to epiphytes hastwo reasons. Firstly the present studies are to alarge extent based on experiences from K varntorp,where the epiphytes seemed much more sensitivethan the epilithic and terrestrial species. Thisimpression is gained in Stockholm as well. Secondlyepiphytes are easy to collect. In towns lichens whenpresent can usually be found in cemeteries andparks. The species on the tree trunks come offwithout much effort, but to wrestle in public withthose firmly attached to grave-stones, the pedestalsof statues etc. can hardly be expected of the lichenologist!Why epiphytic lichens are more sensitive thanothers is still unknown. The substrate may play apart, but until we learn more about this factoritself we cannot say how it is involved. An interestingexception to the rule that the terrestriallichens tolerate pollution better than epiphytic onesexists in the surroundings of Ornskoldsvik, wherenegligible quantities of S02 released by a pulp factoryhave caused considerable damage to the lichensof a neighbouring hill. The importance of topographyand weather conditions is in this case unmistakable,as it is also in the town of ,Linz, inAustria (BORTENSCHLAGER & ScHMIDT 1963). Luckily,howeyer, we have so far been spared the typeof grave soil damage reported from, e.g. Canada(GoRHAM & GoRDON 1960a and b, etc.).The question why lichens happen to be so sensitivealso still awaits an answer. Perhaps the slowmetabolism of the lichen as exemplified by itsslow growth has something to do with it. Lichensare assumed to grow most in spring and autumn.In the Stockholm area the maximum air pollutionoccurs in spring. Its sluggish metabolism probablyprevents the lichen from getting rid quickly enoughof the harmful substances it receives. Indeed, itseems more likely that the thallus actually storesthese substances. SMITH (1960) points out that thethallus easily absorbs nutrition from solutions itcomes in contact with and adds the following, "Itis possible that the marked sensitivity of lichensand other epiphytes to atmospheric pollution maybe due partly to the possession of highly efficientmechanisms for accumulating substances from verydilute solutions, and partly to the fact that theseplants have to rely almost entirely for nutrientson substances carried down in the rainfall fromabove. A similar explanation could be advancedfor the observation of GoRHAM (1959) that mossesand lichens show greater accumulation of radioactivesubstances in areas of fallout than do angiosperms."In Sweden we should take note of thelatter point, since reindeer exist on lichens to aconsiderable extent. SMITH's suggestions are alsosupported by LOUNAMAA's (1956) investigations ontrace elements in certain plants.Acta PhytogeogT. Suec. 50