220 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



are relatively few of the latter unless it be among weeds. For 

 acclimatization, except in unusually hardy and tolerant plants, 

 involves adaptation to different habitat conditions which is so very 

 gradual that the time-span required would be likely to exceed that 

 during which Man has been a potent factor in plant distribution. 

 It may be expected to involve the natural selection of suitable biotypes 

 or more extreme mutants, towards which naturalization is no more 

 than a step. 



That such naturalization, at least, is going on widely in the world 

 today, is a further indication, if any were needed, that the various 

 regions of the globe do not support by any means all of the species 

 which could thrive there — at least in the absence of competition. 

 However, special studies indicate that, quite apart from the eifects 

 of competition, plants transferred to regions of seemingly comparable 

 habitat may have serious obstacles to overcome before they can be 

 considered fully naturalized. These obstacles may be introduced by 

 climatic or other environmental conditions which, although they 

 appeared similar, are actually significantly different from those of 

 the plants' original habitats (as in minor variations of soil com- 

 position), or are not commonly recognized as important (as in the 

 case of some light and temperature effects). This frequent difficulty 

 of naturalization is one reason for the rather small percentage of 

 alien species that actually enter into the composition of most wild 

 floras in undisturbed tracts. Even of the numbers that may be able 

 to propagate successfully and remain year after year in one spot, 

 or sometimes increase their area by aggressive extension, few are 

 known definitely to be permanent and able to persist in the absence 

 of Man. Most lengthening of the floristic lists by aliens is probably 

 only temporary. 



This brings us to the other main reason, namely the need for Man's 

 continued protection, behind the rarity of fully naturalized alien 

 plants even relatively to the number of aspirants. A plant which 

 has escaped from cultivation or a cultivated area on to some adjoining 

 rubbish dump or otherwise disturbed tract, even if it manages to 

 perpetuate itself there for years as many do, is far from attaining 

 the status of full naturalization. In between mere escape and 

 complete naturalization lie the various stages of success — including 

 capabilities of spread, colonization, and possibly even the attainment 

 of a dominant position. This last is often accomplished locally or 

 sometimes extensively by plants which are effectively dispersed and 

 rank in growth, as in the case of Fireweed [Epilobium angustifolium 



