386 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



Northern Wood-rush {Liizula confusa agg.), and various forbs (such 

 as the same Viviparous Knotweed), in addition to Mountain and 

 Arctic Avens {Dryas spp.), which are somewhat woody, and which 

 may dominate considerable areas. But ahhough scattered heathy 

 plants occur in them, these areas are scarcely heaths, any more than 

 are the lichen-rich ones dominated by xerophilous Sedges that 

 characterize dry and exposed situations {see p. 392). Moreover 

 their vegetation is usually rather poor, often barely covering the 



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Fig. III. — Dry tundra on raised area overlooking Hudson Bay, composed princi- 

 pally of an intricate mixture of xerophilous Lichens, Grasses, Sedges, and other 

 herbs. Dwarf woody plants also occur, and the surface is interrupted by pro- 

 jecting lichen-covered boulders. 



ground in spite of a plentiful admixture of Lichens and sometimes 

 also of Bryophytes. Fig. iii shows such an area in which boulders 

 project through the thin and somewhat heathy, lichen-rich vegetation. 

 Especially on limestone or porous sandy substrata is growth often 

 poor and the vegetation relatively sparse, although the component 

 flora particularly in calcareous areas may be very various. 



It would accordingly seem that the major variations in the precise 

 type of tundra take place chiefly, but by no means solely, with 

 local water conditions working through exposure or edaphic factors, 



