Chapter IV 



DISPERSAL AND MIGRATION: 

 AIDS AND BARRIERS 



Having stated our objectives and familiarized ourselves with the 

 main groups of plants, we must consider the methods by which 

 different plants increase their areas, at least potentiallv, by special 

 ' adaptations ' of the reproductive bodies and bv seizing such 

 opportunities for their transport as may be offered. These adapta- 

 tions are of the nature of beneficial structural modifications (see 

 Chapter III). The areas attained are the mainstav of our plant 

 geographical studies, and although they are liable to be profoundly 

 affected by past history (as we shall see in the next two chapters) 

 and are further greatly limited by the physiology of the plants 

 themselves (as we have already seen in Chapter III), these areas 

 must to a large extent be a function of the plants' own aptitudes. 

 In the final analysis, areal spreading is often limited by the ecological 

 reactions of the plant to a new environment which may, for example, 

 be too cold or too dry for its successful establishment. Such 

 reactions are primarily physiological, and, though their outcome is 

 often capable of modification, as we have already seen, they com- 

 monly determine the potential or ultimate area which a species can 

 occupy when there is fully effecti^'e dispersal. The actual areas 

 within the physiologically circumscribed potential ones are largely 

 determined by barriers to successful migration. 



It should be noted that dispersal and migration, although closely 

 connected, are different activities. Dispersal merely involves dis- 

 semination from the parent and distribution (in the dynamic sense) 

 to a new spot, whereas migration implies also successful growth and 

 establishment (ecesis). Thus dispersal is a necessary forerunner of 

 migration, which is actually accomplished only on establishment in 

 a new place. In nature only a small proportion of the plant bodies 

 which become dispersed, and which may conveniently be termed 

 disseminules (diaspores), actually become established and effect 

 migration. Not only do many of them die prematurely or fall on 



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