AGGREGATION AND MIGRATION 249 



agents, though it varies much for the same agent. The distance 

 of migration is variable. It is often greatest in the case of man, 

 other animals, ocean-currents, and wind, and small or scarcely 

 perceptible when the movement is due to gravity, growth, or 

 propulsion. Seeds may be carried half-way across a continent in 

 a week by strong-flying birds, while migration by growth or expul- 

 sion is limited to a few centimeters or at most a few meters per 

 year. This slowness is partly counterbalanced by the greater 

 number of disseminules, and the much greater chance of becoming 

 established. 



Experiment 64. Modifications for migration. List a number of 

 species of the flora according to formations and arrange them in a table. 

 Divide the table into five columns, and record the behavior of each 

 species with respect to the part modified, the kind of modification, the 

 seed production, position of disseminule, and the agent of migration. 



262. The direction of migration. The direction in which a 

 migrant moves is determined by the agent concerned. The general 

 movement is forward or outward, the lines of travel radiating in 

 all directions from the parent area. This is well illustrated by 

 the action of winds which blow from any quarter. In the case of 

 constant winds, migration is more or less definite, the exact direc- 

 tion being determined largely by the fruiting period of the species 

 concerned. The position of invaders with reference to the original 

 home does not necessarily indicate the only direction of migration, 

 since seeds are regularly carried to places in which they can not 

 obtain a foothold. 



The local movement of plants carried by animals takes place 

 in all directions, while their distant migration follows the path- 

 ways of the migratory birds or mammals. Distribution by man 

 is determinate when it takes place along commercial routes or 

 along highways. In ponds, lakes, and other bodies of standing 

 water, migration usually occurs in all directions, but in ocean- 

 currents and streams it is determinate except for motile species. 

 Dissemination by gravity, slopes, and glaciers is local and definite, 

 while propulsion is entirely indeterminate. ]\Iigration by growth 

 is equally indefinite, but it produces a radiate movement away 

 from the parent mass, while propulsion throws seetls into the 

 mass as readily as away from it. From the preceding it is e\-ident 

 that distant migration may take place by means of water, wind, 

 animals, or man, and that it is in some degree determinate, since 



