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132 BENGT PETTERSSONFig. l. The great alvar on Oland has variable soil conditions. Over large parts the Orthoceras Limestone comes to the surfaceand is but slightly weathered. The rooted vegetation is then confined to fissures which are arranged in a rhomboida1pattern. In the background the fissures are widened to crevices in which spinous shrubs grow, especially Juniperus communis,Prunus spinosa, Rosa spp., and Rhamnus cathartica. June 14, 1947. Photo Bengt Pettersson.lichen flora and vegetation. He believes that dispersalfactors are primarily responsible for thisdifference. However, the lichen flora and vegetationare richest along the coasts on Gotlandespecially along the eastern coast. Illustratingexamples are provided by Ramalina polymorphaand R. siliquosa.The epiphytic and epilithic cryptogamous vegetationas a whole is much more impoverished onGotland than on Oland. SJOGREN (1961, 1964) hasstudied these kinds of bryophyte vegetation indeciduous woods on Oland, and it has to be pointedout that on Gotland there is no parallel to this asto richness. As there is plenty of similar grove vegetationon Gotland, the explanation must be lookedfor among the climatic conditions.Oxytropis campestris which is characteristic ofthe Great Alvar of Oland is only found in onecoastal loality on South-East Gotland (perhapsintroduced here) and does not show any trend tospread after its discovery a century ago.The distribution of this species on Gotland istypical of several other disjunct and mostly southernspecies. They show a preference for the coastaldistricts and some of them are restricted to thesouthernmost part of the island, e.g. Adonisvernal is, Aster linosyris and H elichrysum arenarium.Many southern species have only a trend towardsa coastal distribution on Gotland, e.g. Orchissambucina which in the coastal localities shows aricher variability of the colour of the flowers {BENGTPETTERSSON 1958, p. 83-89), Oephalanthera longi-Acta Phytogeogr. Suec. 50