224 THE FORMATION 
concomitant structural changes. The former explains such various habitat 
forms of the same species as are found in Galium boreale, Gentiana acuta, 
etc., and the latter the morphological constancy of plants like Chamaenerium, 
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 ccnsidered under succession. 
BARRIERS 
272. Concept. DeCandolle? 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 
ic a very different type. De Candoile pointed out that the natural barriers 
to continuous invasion (“transport de proche en proche”) are: (1) seas, 
which decrease invasion almest 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 effect 
of barriers upon the constitution of vegetation, laid down the fundamental 
rule that: “The sttpreme law which serves as the basis 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 Elémentaire de Géographie Botanique, 45. 1820. 
?Stationes Plantarum Amoen. Acad., 4:64. 1754. 
3Die. Vegetation der Erde, 4. 1872. 
