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10 HUGO SJORSYearly precipitation in mm160014001200100090080060'5 8°2'fSthlms o' mcrid, 2'200 kmFig. 11. Annual precipitation in Sweden. From Angstrom1958 (adapted from Atlas over Sverige).elevations in the mountains. In the northern mires(see the section on them) local permafrost occursin the form of palsas.GROWTH PERIOD.-For the regional features ofSwedish indigenous vegetation, the mentioned traitsof the climate may have considerable importance.Another important set of factors is related to thelength of the growth period. Unfortunately it iscustomary in Sweden to give the "vegetationActa Phytogeog1·. Suec. 504060'58 '56 'period" as the time with a mean diurnal temperatureat or above +3°0 (ATLAS OVER SvERIGE}or +4°0 (HULTEN 1950, p. 39*). Both temperaturesare chosen too low, because they will give iso-linesof duration that show up strongly oceanic areas astoo favourable. It is well known that at least formany crops and for forest vegetation, too great anoceanity of climate is unfavourable. With iso-linesof duration of a period above +6°0 (or perhapseven higher) we obtain a climatic picture (LANGLET1937, p. 373) that corresponds fairly well to thezonation of major vegetational regions althoughthe favourable autumnal effect of the Gulf ofBothnia is not fully accounted for; still, such acomparison cannot be made in sufficient detail atpresent due to lack of reliable data on local climateand vegetation.In the mountains, the actual vegetation periodis also checked by snow accumulation, and latesnowareas have a much shorter season of growththan the normal, which is anyway no longer thanabout three months at the timberline in Lappland,and still shorter at higher altitude. The adaptationof some mountain plants to grow at very low airtemperatures (but heated internally from solarirradiation) will cause some growth even at altitudeswhere there is no growth period at all in the meteorologicalsense.PRECIPITATION AND RUN-OFF.-Precipitation inSweden is not so unevenly distributed as in Norway,but still there are great differences. Low figures(about 400 to 500 mm) are found in the archipelagos,along some parts of the coastline itself, andon part of the Baltic islands; further near the largelakes, in sheltered valleys in the northern uplandand, locally, the eastern Scandes; finally in a largearea in the north-north-east. Medium precipitation(about 500 to 700 mm) prevails for instance inmany eastern and central parts, and in most ofthe northern uplands, with considerable increasewith altitude. High figures (about 700 to 1000 mm)are typical of the. western half of south Swedenand of the western elevated upland from Varmlandand Narke to northern Dalarna; still higher amountsof precipitation (frequently 900 mm to about 2000mm) evidently fall within the Scandes, at least in