184 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



usually absent inland, and accordingly in the floras of many individual 

 regions it is either lacking or actually of disjunct distribution. 

 Again, a ' continuous ' area may have ribbon-like prolongations 

 extending beyond its main boundaries and even lack continuity in 

 these prolongations, especially when they are narrow, as for example 

 along river valleys which are interrupted by narrow gorges. 



Of continuous intercontinental ranges we may consider four main 

 types : the cosmopolitan, the circumpolar, the circumboreal (or, 

 alternatively, circumaustral), and the pantropic. 



(i) Cosmopolitan — distributed all over the globe. In reality no 

 species is truly so, or, probably, found in all edaphically similar and 

 hence potentially suitable habitats. Thus even without the funda- 

 mental effect of climate and the common interference of other living 

 organisms, it seems unlikely that any one kind of plant can be really 

 cosmopolitan. Those which most closely approach being so are 

 the ones which are least exacting in their habitat requirements, 

 tending to be ubiquitous. These wide-ranging plants which tend 

 to be indifferent to environmental conditions are called ' cosmo- 

 polites ' or ' pan-endemics ' ; in view of the merely relative nature 

 of the condition, the newer term ' semi-cosmopolite ' seems more 

 accurately descriptive of them. They should at least occur in all 

 of the six widely inhabited continents. Actually, outside of weeds 

 of cultivation that have followed Man, they seem to be confined to 

 the lower groups of cryptogams. 



(2) Circumpolar — distributed around the North or South Pole. 

 This term, again, has been used far too commonly and loosely. It 

 seems desirable to apply it only to plants which reach the arctic or 

 antarctic ' polar ' regions, wherever else they may occur, and pre- 

 ferable, at least in the present writer's opinion, to accept as ' arctic 

 circumpolar ' only those plants which occur at least in all of the 

 ten sectors into which he has divided the Arctic for such purposes.^ 

 For these plants are truly ranged around the North Pole. Whether 

 such criteria can be used in the case of the Antarctic has not yet 

 become clear. Even if the limits of the Arctic are rather narrowly 

 set, so as to exclude for example the whole of continental Scandinavia 

 and Iceland, there are rather numerous arctic circumpolar species 

 already known among the higher plants, and many more among the 

 lower cryptogams which tend to be relatively easily dispersed and 

 less exacting in their habitat requirements. Still others will, clearly, 



^ Cf. Circumpolar Arctic Flora (Clarendon Press, Oxford, pp. xxviii + 514, 

 1959)- 



