42 II. A. GLEASON VEGETATIONAL HISTOEY OF MIDDLE WEST 



of it deals with the migration of individual species, and Clements' 

 exposition of the whole subject is without doubt the best general treat- 

 ment extant. 



Invasion, Migration, Ecesis. — Following Goeze, he considers the 

 general movement of species or associations under the term invasion, 

 and recognizes that this consists of two distinct processes. One of 

 these, to which he restricts the term migration, involves merely the 

 movement of the plant by means of its reproductive bodies or vegeta- 

 tive processes. The mechanics of such movement, the methods of seed 

 dispersal or vegetative propagation, while exceedingly diverse, are 

 already well understood and require no discussion here. The second 

 process, which he terms ecesis, includes the actual establishment of the 

 plant in a new location. Ecesis depends entirely upon the environ- 

 mental conditions of the new location in their relation to the physio- 

 logical requirements of the seedlings. The details of these relations 

 are for most plants entirely and for all plants mostly unknown, and 

 are 1o be determined by the methods of experimental ecology. Migra- 

 tion, in Clements' use of the term, is completely unsuccessful in the 

 failure of ecesis, and ecesis, to be effective and complete the proce=s 

 of invasion, must be carried to the maturity of the plant and the pro- 

 duction of a second generation of disseminules. The distinction be- 

 tween the two processes is fundamental, and Clements' clear analysis 

 has done much to systematize the knowledge of the subject. 



The term invasion, which Clements applies to the whole phenomenon 

 of movement, is in many cases scarcely appropriate. His own defini- 

 tion and the general meaning of the word both imply a movement of 

 plants into an area not before occupied by them, that is, an extension 

 of their range. As a matter of fact, the migration of plants is almost 

 or quite as frequently out of an area previously occupied as into a 

 new one, and then involves a retreat or restriction of their range, a 

 result to which the term invasion can not be properly applied. The 

 term migration, in the sense of Clements, is distinctly a process of one 

 generation. Ecesis involves the development of a second generation 

 of individuals, but scarcely applies to the whole population of a species. 

 Yet these processes, participated in by a whole population and con- 

 tinued regularly through successive generations, may lead to broad 

 results far more inclusive than those chiefly considered by Clements. 

 For the purposes of this article, the migration of plants is understood 

 as any general movement by which the range of a species is changed. 



Efficiency of Plant Dispersal. — The efficiency of plant dispersal 

 is well understood. Each normal individual produces one or many 



