98 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



' barren ground ', or come to rest where they cannot even start a 

 new Hfe, or fail to survive the struggle with stronger competitors, but 

 the ecological conditions and physiological reactions have to lie 

 within often narrow limits for ultimate success. In any case, there- 

 fore, the vast majority of disseminules are doomed. 



These disseminules, the actual bodies moved, are most often 

 reproductive structures such as spores, seeds, or fruits. In numer- 

 ous instances, however, they are special structures of a vegetative 

 nature, or unmodified parts of plants, whole plants, or even groups 

 of plants — though in the last instance usually effective only by 

 chance. An example of a whole plant being transported was the 

 Water-hyacinth mentioned in the last chapter and shown in Fig. 21, B. 



Often the same plant species or individual will produce more than 

 one type of disseminule, thereby increasing its chances of effective 

 migration. Thus, whereas the majority of our familiar north- 

 temperate forest trees, such as Oaks and Spruces, normally repro- 

 duce by seed, they may also do so by means of suckering, layering, 

 or other vegetative activity. Moreover, many of the plants that 

 are most successful in colonizing vast areas, resort to more than one 

 means of dispersal. Thus the Common Reed {Phragmites com- 

 munis agg.), which is often claimed to be the most widely distributed 

 vascular plant species in the world, has the multiple advantages of 

 a wind-dispersed, plumed fruit, and a water-dispersed, more or less 

 buoyant rhizome — besides considerable variability in form, and an 

 ability to occupy a wide range of moist to aquatic habitats. These 

 it colonizes so aggressively and holds so strongly that its ' beds ' 

 form a formidable barrier against immigration by other plants. On 

 the other hand, one of the numerous unsolved problems of plant 

 geography is that of why many plants with seemingly excellent 

 advantages in dispersal are not widely distributed. Yet another 

 major question is posed by the number of groups and even species 

 that are widespread without seeming to have any adequate means 

 of dispersal. That precisely the same type of plant should have 

 evolved separately in several different places is almost unthinkable 

 to most students, and so it is widely assumed that the areas currently 

 occupied by particular plants are due to dispersal and effective 

 migration. Now that we have explained the distinction between 

 these terms, they need not henceforth be separated. Rather will 

 we refer to dispersal when the question of establishment can be 

 ignored, and to migration when such establishment is to be 

 emphasized. 



