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INVASION 

 By invasion (Goeze I882:J09) is understood the movement 

 of plants from an area of a certain character into one of a 

 different character, and their colonization in the latter. 

 This movement may concern an individual, a species or a 

 group of species. From the nature of invasion, which con- 

 tains the double idea of going into and taking possession of, 

 it usually operates between contiguous formations, but it 

 also takes place between formational zones and patches. 

 More rarely and less noticeably, there may be invasion into 

 a remote vegetation, as a result of long carriage by wind, 

 water, birds, railroads or vessels. Movement or migration, 

 however, represents but one of the two ideas involved in in- 

 vasion. Migration merely carries the spore, seed or pro- 

 pagule into the area to be invaded. In ecesis, ' the spores 

 or seeds germinate and grow, after more or less adjustment 

 and, in case the latter becomes sufficiently complete, the new 

 plants reproduce and finally become established. With all 

 terrestrial plants, invasion is possible only when migration 

 is followed by ecesis, because of the inherent differences of 

 formations or of areas of the same formation. In the case 

 of surface floating forms, such as Lemnaceae, and of the 

 plancton, ecesis is of much less importance, on account of 

 the uniformity of the medium and the lack of attachment, 

 and migration is often practically synonymous with invasion. 



MIGRATION 



Migration has been sometimes used loosely as a synonym 

 for invasion, but it is here employed in its proper sense of 

 removal or departure, i. e. movement, and is contrasted with 

 ecesis, the making of a home, the two ideas being combined 

 in invasion, which is a moving into and a taking possession 

 of. An analysis of migration reveals the presence of four 

 factors, mobility, agency, proximity and topography. Not 

 all of these are present in every instance of migration, as 

 for example in the simple elongation of a rootstalk, but in 

 the great majority of cases each plays its proper part. 



'From the Greek, o'lKfja-K, the act of coming to be at home, bene 

 adjustment to the habitat, oTkck. 



