Chap. I] CHARACTERS OF THE TEMPERATE CLIMATE 425 



Dicotyledones. 



The group of Dicotyledones includes a much smaller number of tree- 

 species in the temperate zones than it does in the tropics, and the forests 

 composed of Dicotyledones are much less mixed. This is especially the 

 case in the cold temperate belts, where broad-leaved forests usually 

 present almost pure woods of certain Amentaceae, whilst in approaching 

 the tropics the number of tree-species becomes greater and their ad- 

 mixture more uniform. Dicotyledonous shrubs are very rich in forms 

 in the warm temperate belts ; on the contrary, woody lianes are very 

 feebly represented. The distribution of herbaceous Dicotyledones on 

 grassland and in the shade-flora of woodland is about the same as in 

 the tropics. 



Next to the Coniferae, Amentaceae, in particular species of Fagus and 

 Ouercus, in the southern zone species of Nothofagus, to a less extent 

 a few other species (Castanea, Carpinus, Betula, Juglans), are the most 

 important constituents of the forests in temperate zones. In stations 

 where edaphic conditions arc the controlling factors, woods of other 

 Amentaceae appear, especially species of Salix and Alnus on wet soil, 

 species of Betula on sandy soil and on sphagnum-moor. 



In the temperate zones as compared with the tropics, the Urticineae 

 are of subordinate importance as trees (Ulmus, Celtis, Morus) ; herbaceous 

 forms are commoner (Urtica, Parietaria, Humulus). 



Of the two families of the Polygoninae, the Piperaceae are almost 

 completely absent, whilst the Polygonaceae are much more numerous than 

 in the tropics, and appear particularly in grassland and open situations. 



Centrospermae : the Chenopodiaceae, as inconspicuous shrubs and herbs, 

 rarely as small trees, are very common and rich in forms on saline 

 soil, particularly on steppe and desert, but also on manured land. The 

 Caryophyllaceae occur in both zones, in the southern only as Alsinoideae; 

 they form important constituents of grassy tracts and of the herbaceous 

 ground-flora of woodland. The Nyctaginaceae occur in the warm temperate 

 belts of America, and the Aizoaceae are in particular important con- 

 stituents of the South African flora. 



Among the Polycarpicae, the purely mesothermic Ranunculaceae take 

 the foremost place in temperate zones, chiefly in the northern zone. The 

 Magnoliaceae are represented by a few forest trees in Japan, Himalaya> 

 and North America ; only the widely distributed Drimys Winteri reaches 

 South America. The Lauraceae overstep both the tropical limits and 

 form important constituents of warm temperate woodland, but in the 

 cold temperate zones they are represented by only a few species (Laurus 

 Sassafras in North America). 



The Rhoeadinae, with the exception of the Capparidaceae, are mostly 



