240 PLANT PHYSIOLOGY AND ECOLOGY 



to the gradual spreading of the pLants, neighboring famiUes ap- 

 proach each other and finally mingle more or less completely. 

 In both cases the mixing of the two or more species to form a 

 community is due to migration. The conversion of a family into 

 a community takes place usually through the invasion of mobile 

 species. The change occurs when one or more individuals of a 

 second species becomes established in the family group. The real 

 nature of the community becomes more evident when several 

 generations have brought about a considerable increase in number. 

 A community is a group of two or more families, regardless of the 

 number of individuals in each. The families may remain more 

 or less distinct from each other, or may become so intermingled 

 that their identity is completely lost. The first condition is 

 frequent in open formations, while the latter is the rule in closed 

 ones, in which sufficient time has elapsed for repeated migration 

 in all directions. 



Communities vary greatly in size and definiteness. They may 

 contain but two species, or they may consist of a large number. 

 They may be entirely distinct, as often happens in open forma- 

 tions when they are separated from each other or from families 

 by the bare soil. In denser vegetation they may be fairly dis- 

 tinct, or may blend into each other, and finally become indis- 

 tinguishable. 



'^O^ 



Experiment 63. Study of families and communities. Stake out a 

 permanent quadrat to include one or more distinct families, and one to 

 include a definite community, preferably in an open formation. Make 

 a chart of each quadrat. After the first year the development of family 

 and community may be traced by comparing the new charts with those 

 of previous years. 



254. Migration. Migration includes all movements by means 

 of which plants are carried from the original home, or away from 

 the parent individual. It is distinct from ecesis, the act of be- 

 coming adjusted or established, but together wdth it gives rise 

 to invasion, which contains the two ideas of movement and estab- 

 lishment. An analysis of migration shows that four factors enter 

 into it, viz., mobility, agent, distance, and topography. These 

 are not all present in each case of migration, but as a rule each 

 factor plays some part. Mobility is the capacity of a plant for 

 migration, as shown by the various modifications of fruit, seed, 



