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24 MATS WlERNparticular species of trickle-water on rock, of springsand mires, including vascular plants, bryophytes andalgae-here easily pass over the boundary betweenland and water.To obtain an idea of the downward process it isnecessary to disregard the many variations in thedistribution and composition of algal belts that arecaused by subordinate gradients such as the differencein exposure and other respects between thefringe and the interior of an archipelago, not tospeak of quite local conditions such as types ofbottom configuration and substrates and the orientationof the rocks in relation to the light.Not all species take part in the downward process.Some are obligate of the surface (e.g. Urosporapenicilliformis, several species of Enteromorpha,Dictyosiphon chordaria) or cannot exist in theabsence of considerable tide (Pelvetia; Fucus ceranoidesat shifting salinity). Of the littoral species ofthe Swedish West Coast, . Fucus spiralis sticks to· its niche in a locked position. Ascophyllum, thougha littoral plant, reduced in size, at its interiorboundary at Kullen (see above), goes down to 3.3 min the Gullmar Fjord (GISLEN 1930).Among species showing evident dislocation, welist the following under headings referring to whatcould be considered as the habitat they usually areconne