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212 SV ANTE PEKKARIposition. Potamogeton gramineus is a frequentdominant, and in a moderate current becomes aboutfour metres long. Other dominant or eo-dominantspecies include P. perfoliatus, Myriophyllum spicatumand M. '!'lterniflorum, as well as Ranunculuspeltatus, Hippuris vulgaris and Sparganium spp. inunder-water forms. Some elodeids, like Callitricheautumnalis and Ranunculus confervoides, grow exclusivelyor almost so in the rivers, being rare orabsent in the lakes. In the Bothnian Bay, however,they are both common.CRYPTOGAMIC VEGETATION OF THE RAPIDS.-Inthe submerged vegetation in the rapids epilithicspecies prevail completely with bryophytes in themore moderate current and algae predominant invery speedy water. There is no counterpart to thisvegetation in other types of large waters in thearea, except for some similarities to the epilithicbryophyte vegetation in the wave-exposed littoralof the archipelago.In addition to several smaller ones, there aretwo large rapids, one in each river. Both belong tothe type prevailing in the north, having a broadriver-bed with a floor of mostly large boulders overand between which the water attains locally highbut very variable speed, the general inclination ofthe rapid however being quite modest. For therapid Kukkolankoski in the Torne river R. MELIN(1930, p. 16) gives 14 m in 3.8 km. As indicated,the environmental conditions are highly variable,and in consequence, the vegetation develops acomplicated mosaic pattern.Below the cryptogamic vegetation of the Kamlungerapid in the Kalix river will be described, theKukkolankoski being in most respect similar.Above the high-water line of the early summer,lichens and mosses prevail completely. Furtherdown, the epilithic lichens are reduced both inquantity and number, and close to the normalautumnal low-water line the black Verrucariapachyderma is dominant, reaching down to thelatter line. There are small, firmly attached tuftsof the moss Schistidium Agassizii in small crevicesor other somewhat sheltered stone surfaces. Onlyone or two dm above the autumnal low-water linereach the amphibious mosses Fontinalis antipy-retica, F. dalecarlica and Hygrohypnum alpestre, andin better shelter, also Bryum spp. (e.g. B. pseudotriquetrum),Fissidens osmundoides, F. viridulus,Hygrohypnum ochraceum and H. dilatatum. Furtherdown, only slightly above the mentioned line, afelty mat of Campylium helodes begins, and on thedownstream side of boulders grows the only fairlycommon hepatic of this rapid, Plectocolea obovata.The algal vegetation includes a bright greenupper belt of filamentous Zygnemales, one foot orso in vertical extension and having a rather sharpupper limit, which is found higher up on the upstreamside of the boulders and is also adjusted tothe variable shape of the boulders in other exposure.Four genera are represented, viz. Zygnema, Spirogyra,M ougeotia and M ougeotiopsis, the former threevery likely by several spp. However, fertile materialhas only been observed in Spirogyra lapponica andZygnema melanosporum, and other species mustremain unidentified. Conjugation in the two mentionedspecies is evidently abundant when thethreads are laid bare (cf. IsRAELSON 1949, p. 325),and is accompanied by a shift in colour from greento chocolate or dark bluish, respectively. Thesinking of the water level occurs at different times,and therefore fertile algae can be collected fromvarying dates, in different years. In 1963, wellfertile material was collected as early as July 2nd,but in next year only at the end of July when thedelayed exposure to the air took place.Other algae typically forming a zone in swiftcurrent are Lemanea fluviatilis and L. condensata(lSRAELSON 1942, p. 20). The latter forms a dense,shaggy coat on the boulders, about 1 to It cmthick. Its upper part is laid bare at low waterduring late summer only. Usually below this latterlevel we find Sirodotia suecica, A udouinella ( Chantransia)H ermanii and Didymosphenia geminata.The algae of somewhat slower water-wheremosses begin to prevail-include beside the Zygnemalesalso cyanophytes and Batrachospermum moniliforme,Draparnaldia glomerata, Coleochaete divergensvar. catharactarum, Oedogonium spp., Bulbochaetespp., etc.Winter conditions are interesting. Whereas thecalmer stretches of the rivers are icebound for halfthe year or even more, the large rapids run partly.Acta Phytogeogr. Suec. 50