224 THE FORMATION 



concomitant structural changes. The former explains such various habitat 

 forms of the same species as are found in Galium borcale, Gentiana acuta, 

 etc., and the latter the morphological constancy of plants like Chamacnerium, 

 which grow in very diverse habitats. 



The vegetation form of the invading species is often of the greatest im- 

 portance in determining whether it will become established. The vegetation 

 form represents those modifications which, produced in the original home 

 by competition, i. e., the struggle for existence, are primarily of value in 

 securing and maintaining a foothold. These comprise all structures by 

 means of which the plant occupies a definite space in the air, through which 

 the necessary light and heat reach it, and in the soil, from which it draws 

 its food supply. These structures are all organs of duration or of perenna- 

 tion, such as root, rootstalk, bulb, tuber, woody stem, etc., which find their 

 greatest development among trees and shrubs, and their least among annual 

 herbs. But while the invaders are aided in securing possession by the proper 

 vegetation form, the occupation of the plant already in possession is in- 

 creased by the same means, and the outcome is then largely determined by 

 other factors. To avoid repetition, the bearing of occupation upon invasion 

 will be considered under succession. 



BARRIERS 



272. Concept. DcCandolle^ seems to have been the first to use the term 

 barrier and to distinguish the various kinds, though Hedenberg^ clearly saw 

 that stations of one kind were insurmountable obstacles to plants belonging 

 IG a very different type. De Candolle pointed out that the natural barriers 

 to continuous invasion ("transport de proche en proche") are: (i) seas, 

 which decrease invasion almost in inverse proportion to their extent; (2) 

 deserts; (3) mountain ranges, which are less absolute on account of passes, 

 valleys, etc. ; (4) vegetation, marshes being barriers to dry land plants, 

 forests to those that fear the shade, etc. Grisebach'', in discussing the eflFect 

 of barriers upon the constitution of vegetation, laid down the fundamental 

 rule that: "The supreme law which serves as the bdsis of the permanent 

 establishment of natural floras is to be recognized in the barriers which have 

 hindered or completely prevented invasion." 



Any feature of the topography, whether physical or biological, that re- 

 stricts or prevents invasion, is a barrier. Such features are usually perma- 

 nent and produce permanent barriers, though the latter may often be 

 temporary, existing for a few years only, or even for a single season. In 



*Essai Elemcntaire dc Geographic Botaniquc, 4.'>. 1820. 

 'Stallones Plantarum Amocn. Acad., 4:64. 1754. 

 Die Vegetation dcr Erdc, 4. 1872. 



