250 RIKARD STERNER 



or the >•> ordinary h2imus>>. This is an excellent nutritive substratum for plants: 

 there is a good supply of nitrogen in the mould which is a ver\- important 

 factor (e. g. Hesselman 19 17 a and b). Thanks to its granulary structure, the 

 soil gets a great capacity of absorbing heat (compare Kraus, 191 1) and gets 

 better drained. 



From a biological point of view, this soil-type is to be sharply discriminated 

 from the more markedly humid ones. The upper layer in these, the »raw iiumus 

 layer», is in a high degree devoid of the (|ualities characteristic of the mould 

 and favourable to plants. 



Hence the chief characteristics of more continental soil-types are as follows: a 

 great amount of electrolytes in the upper la\-ers; and, when there is a vegetation cover, 

 a topmost layer formed by granulary humus, which gives the soil a high porosity, 

 good drainage, a high power of heat absorption, a good supply of nitrogen 

 and so on. 



The assumption would seem greath' justified that continental species arc in 

 one way or other through their ecology confined to continental soil-types. Con- 

 sequently the climate should indirectly in a high degree determine the distribu- 

 tion of these species. 



Climatic soil-types may, however, have cdapJiic ones subordinated to them. 

 The latter are above all caused by the changes in the composition of the mineral 

 soil, and in its amount of electrolytes. 



In more continental districts this is of small importance, the amount of electro- 

 lytes being always sufficient at the weak leaching to satisfy the humus substan- 

 ces. In more maritime districts, on the other hand, the composition of the 

 mineral-soil may in a high degree determine the soil-type. The supply of electro- 

 lytes may be so great that, in spite of the great leaching, the humus substances 

 can be satisfied and give the soil a granulary structure. Hence in maritime 

 districts with a large electrolytic content in the soil there may be a comparati- 

 vely continental type of soil, which should naturally be of immense nnportance 

 to the distribution of continental species.' 



It is to a great extent in its activity as an electrolyte that liuic is of such 

 fundamental importance to the distribution of plants. The amount of lime influ- 

 ences in a high degree the physical structure of the soil. The easily demon- 

 strable close connection between lime and the distribution of many species, how- 

 ever, has long been, and still is, one of the main problems of phytogeograpln'. 

 To the indirect importance of lime now mentioned, in fact, must be added its 

 role as a more directly active factor, concerning which research has not yet 

 attained any definite result. The current opinion would seem to be as follows: 



1 



lli're also attention niav he ilrawn lo the fact that the nature of tlie hunuislaver is, to a certain 

 Icgrce, dependent on the nature ot the vegetati\e cover. 



