360 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



the great land-masses, the main examples in principally cool regions 

 being the North American ' prairies ' and the Russian and adjacent 

 ' steppes ', with, in somewhat warmer regions, the South African 

 ' veld ', South American ' pampas ', and southern Australian and 

 New Zealand grasslands. These last three regional types frequently 

 bear isolated or sometimes aggregated low trees or shrubs and accord- 

 ingly constitute savannas ; indeed much of southern Australasia, 

 particularly, is often indicated as savanna on world vegetation-maps. 

 Nevertheless the real dominants are usually Grasses, so such types 

 seem best classed here. In addition, meadows and other grasslands 

 occur widely in temperate and allied regions as biotic plagioclimaxes. 



All grasslands have this in common, that they are dominated over 

 at least most of their often vast area by Grasses — usually by various 

 and often mixed perennial species which are narrow-leafed hemi- 

 cryptophytes, many of them being gregarious and extremely hardy. 

 Characteristically, the rooting is shallow and the underground parts 

 form a matted turf which holds rainwater when and where it falls, 

 checking penetration to deeper layers and accordingly aidmg the 

 grassland plants to prevail in their competition with trees in transi- 

 tional areas where the rainfall is barely sufficient for arborescent 

 growth. The turf and sod of old dead leaves, etc., may also help 

 to check the successful establishment of tree seedlings. The winter 

 in the so-called climatic grassland areas is often severe and dry, as 

 may be the later summer; but recurrent spring and early summer 

 rains that more than compensate for evaporation will favour the 

 Grasses, which can then vegetate actively. 



The main types of grasslands occurring in temperate regions may 

 now be briefly described. 



I. The prairies of central and western North America, ranging 

 from well north in Canada southwards into Mexico. The dominant 

 Grasses mostly form clumps (' bunch Grasses ') in the drier regions 

 or more extensive sodded swards in the less arid ones, being inter- 

 spersed with a large variety of subdominant forbs (herbs of other 

 than grass habit). These often give the prairie a distinctive tone 

 locally and fall into mixed societies flourishing for example at different 

 times of the year. In general, however, various greens prevail in 

 the early part of the growing-season, and yellows or greys later on, 

 sometimes followed by darker orange or other autumn tints. Woody 

 plants are usually few and unimportant, except in depressions or 

 as a response to overgrazing, which may also favour Cacti. Yet 

 differences from spot to spot tend to be so marked and unstable 



