THE CONTINENTAL FLORA OF SOUTH SWEDEN 



259 



exactitude naturally involves j^reat difficulties. Values of a certain usability 

 may be obtained by calculating the number of days during which the mean 

 temperature exceeds o° Centigrade. On the basis of the mean-temperatures for 

 pentades published by Hamberg (igo8) I have computed the values given in 

 Table 4, which may be left to speak for itself. 



b. Atmospheric precipitations and Cloudiness. Precipitations exercise a great 

 influence on the distribution of plant species, above all by their effect on the 

 quality of soil, the heat of the 

 soil and its physical and ehe- 

 mical structure. 



When we try to find a rela- 

 tion between the distribution 

 of atmospheric precipitations 

 and of species, the meteoro- 

 logical data can give us a safer 

 basis of discussion than in the 

 matter of the distribution of 

 temperature. Fig. 3 shows, 

 according to Hamberg (191 1), 

 the distribution of rainfall in 

 South Sweden during the sum- 

 mer half-year (May— October). 

 Among the general features 

 the following might be speci- 

 ally emphasized: 



The importance of topo- 

 graphy to the distribution of 

 rainfall is conspicuous in the 

 considerable outward bulge to 

 the east of the isohyetals over 

 the South Swedish highland, 

 over Tiveden and Kolmarden. 

 The western slope of the South Swedish highland has the largest rainfall within 

 this region, and the south-eastern coast regions — situated as they are in »rain 

 shadow* — have the smallest amount. The large flattish districts, especially the 

 Malar and Ostgota plain, show small amounts of rainfall. 



The consequence of the small coast rainfall, however, is, as far as plants are 

 concerned, counterbalanced by a smaller saturation-deficit of the air, which de- 

 presses the intensity of transpiration. 



The precipitations during the winter half year (November— April) are naturally 



Fig. 3. The average rainfall in South .Sweden tluring the 

 mer half-year (May— October), after Hamberg 1911. 



