130 PLANT GEOGRAPHY 



siliceous soil, and the Eng forest, on the ferruginous loam 

 known as " laterite," with Dipterocarpus tuber culatus 

 Roxb. Behind shelving sea-shores with what is still 

 generally known as Ipomcea Pes-caprce may be thickets 

 of Barringtonia / or, especially in tidal waters, the prop- 

 roots and pneumatophores, or aerating roots, of the 

 oxylophytic Mangroves ; or, in the Old World, Nipa and 

 Pandanus ; or, on drier shores, Spinifex squarrosus L. 



WARM TEMPERATE FORMATIONS. Among the meso- 

 therm plants of the Sub-tropical and Warmer Temperate 

 Zones the most noticeable formations are, perhaps, the 

 various sclerophyllous communities to which some 

 reference has before been made. In the maquis of the 

 Landes of Bordeaux Erica is dominant; in Spain, the 

 switch-like Retama, or, in the " jar ales," Cistus ; whilst 

 on the higher plateaux are the " tomillares," or scattered 

 patches of " Tomillo " (Thymus). Prosopis~is dominant 

 in the Calif ornian " chaparral," and Colletia sometimes 

 in the " espinal " of Chile, whilst in the Australian 

 " scrub," " MaJlee " (Eucalyptus), " Brigalow " (Acacia 

 harpophylla F. v. M.), or other species of " Mulga " 

 predominate locally. Whilst most xerophilous associa- 

 tions are " open," or sparse, the forests of evergreen oaks 

 and of Cedrus atlantica Manetti in the Mediterranean 

 area are " close." 



COOL TEMPERATE FORMATIONS. In no regions have 

 the formations been so thoroughly studied as in the 

 Cooler Temperate Zone in Europe and North America. 

 Among interesting points which have arisen are the 

 relations of lime and acid humus to the woodland, where 

 Oak and Birch and the Heaths characterise soils without 

 lime which have acid humus (Fig. 15), and Beech woods 

 and short-growing pasture grasses distinguish the chalk 

 downs (Fig. 16). 



In the Cold or Arctic Zone, trees dwindle to dwarf 

 bushes, and the principal formations are the varieties of 

 tundra. 



Whether we study the geographical relations between 

 the floras of wide regions or analyse with the minutest 

 . topographical detail the vegetation of the smallest areas, 

 we are faced by an infinity of problems of climate, 

 geology, soil, physiology, dispersal, evolution, and 

 structural modification which demand painstaking ob- 

 servation and experiment. 



