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The vegetation of Swedish lakes 43plants. These communities have their best developmentwithin the lower parts of the rivercourses,perhaps richest on the Dalalven before that riverwas rigorously controlled. Among perennial plantsoccur Eleocharis acicularis, I soetes echinospora andscattered specimens of Alopecurus ,(lequalis. Thereare many more annual plants including Subularia,Limosella, Crassula, Peplis, Elatine hydropiper, E.triandra, Callitriche verna and occasionally Polygonumfoliosum (cf. Figs. 15 and 16).The kind of substrate and the annual rhythm inwater level fluctuation are prerequisites for thispeculiar vegetation. During the period of lowwater in winter, frost penetrates down into thesebottoms and horizontal layers of ice cleave thebottom material' and cause it to expand and heave(LoHAMMAR 1938). On such an unstable substrate·Phragmites and other tall rhizomatose perennialplants never have the possibilities of forming closedcommunities, leaving the space free for small andweak competitors.ECOLOGICAL DIFFERENCES DUE TO CLIMATE.­Many aquatic plants are killed if exposed to frost(op. c.). The risky zones have a different verticalextension within the various parts of Sweden. Insouthern and south-western Sweden late autumnalrains and occasional thaws in winter produce ahigh water .level during the relatively short andmild winter, the lowest water here occurring duringsummer. Within the northern coniferous region theextreme low water appears during the later partof winter and it is followed abruptly by the highwater of spring, resulting from the thaw of the snowcover. The hard winters in northern Sweden, withthick ice formed at low water, evidently reducethose parts of the lake bottoms which many aquaticplants would have been able to occupy undermilder climatic conditions. These winter conditionscertainly exert an influence on the geographicaldistributions of a number of aquatic species.After winters with extremely low water levelsand frost penetrating deeply into the lake bottoms,even extensive communities of water plants maybecome eliminated. The death of water plantsthrough frost is also a common consequence afterlakes have been drained (LoHAMMAR 1949, plates).4- 652151 APhs 50Inventories of lakes in the vicinity of the mountainshave shown that the number of helophytesdecreases more rapidly with elevation than thatof elodeids. It is possible that this is due to the factthat many helophytes have their wintering organswithin the levels that are exposed to frost in NorthSweden.On its most elevated localities as well as in thefar north Scirpus lacustris generally grows atabout a depth of 1-1.5 m (or in running water)but not in very shallow lake water as it frequentlydoes in the southern part of the country.In South Sweden Butomus grows mostly inwater so shallow that it can expand the greaterpart of its foliage in the air. In the Torne river(cf. PEKKARI's cntribution) and in some lakes inLappland it grows exclusively at frost-free depths(1 to 2 m) and it never flowers. The explanation ofthis abnormal behaviour is uncertain, for experimentshave shown that it is able to stand at leastmoderate winter cold.In the oligotrophic lakes in southern Sweden oneoften observes during a period of low water insummer the exposed, flowering mats of Litorellaand in the shallow water close to the shore, largestands of pleasantly flowering Lobelia not rarelyintermingled with Isoetes lacustris. All three speciesare sensitive to frost and their upper fringes in thelakes of interior North Sweden are at greaterdepths (1 m or deeper at normal late summer level).If flowering at all, Lobelia then has cleistogamousflowers, andLitorella remains sterile. Juncus bulbosusin southern and central Sweden inhabits woodlandbrooks, shallow bays, etc., and is frequently fertile.It easily freezes to death, and in interior NorthSweden where it is rare and always sterile one mayfind it growing as an elongated elodeid at a depthof 1 or 2 m, usually in the vicinity of the thresholdof the outlet or outside one of the affluents, wherethe mobility of the water in winter prevents thefreezing of the bottom.On the other hand the colder climate of the northmay reduce the possibility of penetrating to greatdepths. · In Uppland Phragmites goes down to morethan 2m (in Lake Erken 2.3 m). Growth to this depthis probably not found in the north or in the interioruplands. In Vettasjarvi (altitude 352 m) in TorneActa Phytogeog.r. Suec. 50