60 COMMUNITY FUNCTIONS— DYNAMICS OF BIOTIC FORMATION 



Migration in Plants. Although migration devices have long been 

 a subject of interest to botanists, the actual process itself has received 

 little attention, especially in terms of experimental study. Such study 

 is peculiarly difficult in nature, and this fact explains in large part 

 the persistence of the idea that the movement of migrules is as effec- 

 tive as it is universal. This view has received much support from 

 the general abundance of weeds and the readiness with which they 

 take possession of disturbed places. However, it finds little warrant 

 in natural communities, in which not only is movement itself much 





Fig. 13. — Migration of a dune pioneer {Ahronia maritima) by means of creeping 

 stems; seashore, Santa Barbara, California. (Photo by Edith Clements.) 



more restricted than commonly supposed, but also the establishment 

 of invaders is rendered almost wholly impossible by the competition 

 of the dominants. Transport to a great distance has long possessed 

 dramatic interest, but is rare in fact and even much rarer in effect. 

 When the embryo is not destroyed by the agent of distribution, as 

 regularly happens with water and birds, its establishment becomes 

 possible only in disturbed or bare areas. In consequence, dissemina- 

 tion in nature is of little import to the community, unless a change 

 of climate intervenes. Its chief significance is in the colonization of 

 primary bare areas or secondary disturbed ones and in supplying the 

 newcomers for the stages in the ensuing succession. The mainte- 



