12] VEGETATIONAL TYPES OF TEMPERATE LANDS 361 



that the whole has been claimed to constitute ' an open system ' in 

 which repeated readjustment to disturbance overrules any tendency 

 to equilibrium. In general correspondence with decreasing rainfall, 

 the Grasses fall into three groups based upon stature and known 

 respectively as ' tall ', ' mid ', and ' short ' Grasses. Thev range 

 from the height of a Man, or taller, down to a few centimetres in the 

 cases of the most drought- orpasturing-resistant types. Fig. 106, A, 

 shows an area of true prairie in Nebraska, dominated principally 

 by mid Grasses, but with shrubs and even trees in some damp 

 depressions. Fig. 106, B, illustrates an area of short-grass prairie in 

 Colorado, while Fig. 106, C, shows a similar area that has been badly 

 overgrazed. In the prairies there is a long resting stage each year, 

 which is due to low temperatures in the North — the melting of winter 

 snow affording plentiful water in early spring — and to low rainfall 

 in the South and West. The general unity of this grassland climax 

 is evidenced by some of the grass species occurring in nearly all 

 of the component associations, and by the large number of grass 

 genera — such as Agropyron, Boiiteloiia, Elymiis, Poa, Sporobolus, and 

 Stipa — that afford dominants more or less throughout its great 

 range. Nevertheless, differing local conditions lead to preclimax 

 and postclimax communities and different treatments to biotic 

 plagioclimaxes, while near the forest border is an ecotone with trees 

 which elsewhere persist chiefly along watercourses. In such zones 

 of tension, each life-form takes advantage of the slightest variation 

 of edaphic or biotic impress which may be in its favour. 



2. The steppes of the U.S.S.R. and adjacent lands, covering vast 

 areas south of the northern forests and north of the central deserts, 

 etc., and ranging from eastern Europe to eastern Asia, with outposts 

 farther west and south. These steppes are similar to the North 

 American prairies in all essential respects, though they differ con- 

 siderably in the component genera and much more in the species. 

 The Grasses tend to be highly xerophilous and to form dense tufts 

 or cushions composed of the remnants of previous years' growth 

 seated on stools of superficial but much-branched fibrous roots, 

 while the associated forbs are mostly hardy perennials or bulbous 

 geophytes. These associates and any annuals or woody plants are 

 mostly small, few exceeding half-a-metre in height. The growth 

 of trees is almost everywhere prevented by the scarcity of water 

 and by the extremely severe winters, when strong and dry winds 

 blow over the frost-bound soil on which snow may afford protection 

 only for ground- vegetation. Between the northern forests and open 



