THE CONTINENTAL FLORA OF SOUTH SWEDEN 249 



A sinkint^ of the summer temperature may be imagined in a continental cli- 

 mate to be to a certain degree compensated by a greater supply of light. The 

 position of the northern distribution limits of species would thus appear to be 

 influenced even b\' the supply of light and so depend on the cloudiness and the 

 measure of shade in the vegetation. 



For temperate regions, therefore, the direct influence of the continental climate 

 on the plant world may be supposed to appear in the following biological 

 characteristics in the plants: 



(i) Xerophil)-; 



(2) A high »ecological temperature optimum* for the vegetative season; 



(3) The demand for a yearly season of rest of a certain duration; 



(4) The demand for a comparatively low temperature at the time of the origin 

 of the constituent parts of the flowers; 



(5) Heliophily. 



The nature of the continental soil and the ecology of continental species. 



The nature of the soil is intimately connected with the climate. Modern pedo- 

 logy distinguishes between different types of soil, above all with regard to the 

 connection with the climate. In the first place a distinction is drawn between 

 avid and hiiinid types of soil, of which the former are connected with a dry 

 climate, the latter with a damp one. 



According to Ramann (1911) and Wiegner (19 18), the characteristics oi arid 

 types of soil are, above all, inconsiderable weathering, the massing in the upper 

 layers of the soil of the electrolytes formed in the weathering, and an inconsi- 

 derable proportion of humus substance, depending on the scanty vegetation. 



Htitnid soil-types are inter alia characterized by strong weathering; a removal 

 from the upper layers of soluble electrolytes and certain colloids: ferric oxide, 

 aluminium oxide, silica etc.; also by a considerable massing in the uppermost 

 layer of plant remains (»forna>^ Sernander, 1918), which are only slowly de- 

 •molished and then appear as colloids in the condition which forms what is called 

 »ra\v humus ». 



Between these extremities many stages may be discriminated. What are known 

 as the semi-arid and the sevii-lnimid soil-types are characterized by abundance 

 of humus substance and at the same time by a good supply of electrolytes in 

 the upper layers. Aluminium-oxide and silica remain in the upper layers, giving 

 rise to clays. The colloidal humus substances absorb the electrolytes, during 

 which process they are transformed into absorptively saturated humus. Through the 

 agency of animals (earthworms) and water the humus is thoroughly mixed with 

 sand and clay. The mixture is a neutral or alkaline, granulary soil, the y>}nouldy> 



