Il8 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



chance of effective survival. But when it is recalled that pollen 

 grains are formed each year in trillions of trillions, and that a single 

 pollen ' bullet ' finding its stigmatic billet in a millennium might 

 suffice to carry thither the genes of any subspecific characters it may 

 possess, the possibility can scarcely be denied of what has facetiously 

 been termed ' absent-treatment hybridization '. Wind and insects 

 are the chief transmitting agents of pollen, though carriage is also 

 effected by some other animals (especially small Birds) and by 

 water. Most of the strikingly beautiful features of flowers, as well 

 as their possession of nectar and scent, are adaptations to attract 

 insects to gather pollen for the purpose of cross-fertilization, and so 

 it is to be expected that this is very commonly effected, though 

 chiefly over rather short distances. 



Dispersal by Human Agency 



There can scarcely be any question that Man is the most active 

 agent of vegetational change — including plant dispersal — of modern 

 times. He is the greatest despoiler of forests and causer of erosion, 

 dispersing weeds as well as growing crops. As he travels about the 

 world in greater and greater numbers and with ever-increasing speed 

 and ease, he is always transporting the disseminules (or sometimes 

 transplanting whole individuals or groups) of plants either intention- 

 ally or unwittingly. Also of vast importance is his indirect effect, 

 through the pasturing of his domestic animals or his disturbance 

 of natural communities of herbivorous animals. As a result, there 

 are few parts of the world where the vegetation and its component 

 flora do not bear the stamp of Man's interference, and quite a few 

 areas, for example in Hawaii and Ceylon, where the native plants 

 have been largely ousted by alien ones. In general, however, unless 

 there is some drastic disturbance of the natural vegetation, recently 

 introduced plants fail to compete successfully with the native 

 dominants and, consequently, take only a minor part in the con- 

 stitution of most plant communities. Often these aliens are limited 

 almost entirely to burned-over or otherwise cleared areas — such as 

 waysides and abandoned fields, which characteristically support 

 hosts of weeds. 



Between the extremes of those plants, such as many horticultural 

 strains, which are restricted to gardens and need constant tending, 

 and those which, following introduction, have so thoroughly estab- 

 lished themselves by natural agency that they are distinguishable 



