430 ZONES AND REGIONS 



numerous and sometimes very common species. They are richly 

 developed, especially in Mediterranean countries. 



Riihiinac : the family of Rubiaceae, which is strongly developed in the 

 tropics, becomes quite subordinate in the temperate zones and by far 

 the majoritj' of its forms are herbaceous. The much smaller families of 

 Caprifoliaceae and Valerianaceae are almost entirely mesothermic and 

 boreal, but never form important constituents of the vegetation. 



The Compositae play at least as important a part in temperate floras, 

 as in the tropics ; here also they prefer grassland districts. Their sub- 

 families to some extent inhabit both zones, but are in part exclusively 

 or chiefly attached to one of them. Thus the Liguliflorae and the Cynareae 

 are mainly boreal, the Labiatiflorae are austral and almost exclusively 

 American. The two other families of the Ag^regatac are mesothermic, 

 the Dipsaceae being mainly north temperate, the Calyceraceae South 

 American. 



LITERATURE. 



The climatic data are chiefly taken from Hann's Handbuch der Klimatologie, 

 2nd ed., 1S97, and his Atlas der Meteorologie, 1S87 ; also from Woeikof, Die KHmate 

 der Erde, Jena, 1S87. 



The data regarding geographical distribution of mesothermic groups of forms 

 are taken from the Natiirliche Pflanzenfamilien of Engler and Prantl. 



