248 RIKARD STERNER 



sterile. The long warm autumns and mild winters in maritime districts may 

 perhaps have an unfavourable influence on the creation of flowers in continen- 

 tal species. 



In tropical deserts the division into periods of the year is occasioned by the 

 rainfall conditions. Even in the continental regions of the temperate zone the 

 rainfall may cause periodicity in plants. The ineffective rainfall during the height 

 of summer forces many species to rest. The main characteristic of the vegeta- 

 tion of steppes and prairies is, indeed, the great number of species that flower 

 in spring or autumn. 



In his well-known biological system Raunkiaer has given expression to an 

 important side of the connection between the character of climate and the eco- 

 logy of plants, viz. the various ways in which plants manage to survive the 

 unfavourable period (periods) (Raunkiaer, for instance, 1907 and 1909). RaurV- 

 kiaer has given » biological spectra » from certain desert districts. They are 

 characterized by a high percentage of »therophytes». Unfortunately there are no 

 spectra available from continental districts in the temperate zone. It is evident, 

 however, that the vegetation on the more rigorous steppes and prairies is cha- 

 racterized by a high percentage of therophytes, but also a high percentage of 

 geophytes. The geophytes are equipped for utilizing the short period of vegetation 

 between winter and the dry height of summer. 



As the vegetation in more extremely continental districts, such as steppes, 

 pampas, deserts etc. is only developed in one layer, and consequently no species 

 are shaded by others in any degree worth mentioning, the flora should be greatly 

 heliophilous, composed of species demanding (or at least preferring) full light. 

 Even in continental forest districts the ground flora would probably seem to be 

 comparatively heliophilous because of the scanty cloudiness and sparse wood 

 layer. 



As regards the influence of the supply of light on plants the following things 

 may be especially pointed out in this connection. The origin and development 

 of the constituent parts of the flowers are greatly furthered by a rich supply of 

 light (Jost, 1. c , p. 488). The demand of plants for light depends on the tem- 

 perature in which they live. With sinking temperature the demand for light is 

 increased. Light and heat may to a certain extent compensate each other. Thus, 

 for instance, it has been found that the demand of plants for light increases 

 with the geographical latitude and height above the sea (Wiesner, for instance, 

 1895, p. 704 ff., and 1907, p. 31; compare also Brockmann-Jerosch, 1913).^ The 

 latter fact may especially be of importance to continental species. 



' The sources of error in Wiesner's method of measuring the demand for light, viz. the fact that the 

 thermic effect of light cannot be directly measured, would not seem in this case to be able to decrease 

 the value of the results to any appreciable e.vtent (compare Riibel, 1912, pp. 44 and 45.) 



