44 H. A. GLEASON VEGETATIONAE HISTORY OF MIDDLE WEST 



change, either in physical conditions or in their control by plant life 

 or in parasites and enemies, sufficient to prevent the establishment of 

 new plants in the immediate vicinity of the parent, so that the species 

 disappears from the habitat with the death of existing individuals. 

 Succession results when migration is so complete and is shared by indi- 

 viduals of so many species that the nature of the vegetation is funda- 

 mentally changed. The close relation between succession and migration 

 have been noted elsewhere, 11 and the nature of the causal environmental 

 changes have been discussed in detail by Clements. 12 Instances are 

 of course well known in which species migrate into an association and 

 become an integral part of it without causing succession. Thus in 

 parts of New Jersey Daucus Carota has entered the hemlock-maple 

 forest, and in northern Michigan Rumex Acetosella has become widely 

 distributed through the aspen association. Such a result may be 

 termed taxonomic succession, as distinguished from the ecologic suc- 

 cession previously mentioned. 



Kate of Migration. — The simple production, movement, and estab- 

 lishment of reproductive bodies, constituting together the method of 

 migration, is not the only factor concerned in the process. The rate 

 of migration depends upon one or the other of two conditions: the 

 rate of environmental change and the migration-capacity of the plant. 

 The latter condition may in turn be resolved into two components ; the 

 mobility of the plant, depending on the structure of its migrating parts 

 and its ability to utilize the necessary environmental agencies, and the 

 length of its juvenile period, which must be completed before a new 

 crop of disseminules is produced to continue the march. Migration- 

 capacity may be formulated in general terms as mobility divided by 

 length of juvenile period. Hence those plants which combine excellent 

 structural devices for dispersal with a short vegetative cycle, such as 

 the dandelion (Tara&acum), migrate more rapidly than others with 

 less mobility, as lamb's-quarters (Chenopodium) , or with a long vege- 

 tative period, as the maple (Acer), while slowest of all are those plants 

 which lack both favoring features, as the oak (Quercus). The actual 

 rate of migration is fixed by the slowest of the two general conditions. 

 If environmental change is slow, migrating species may keep even 

 with it and continually scatter their seeds forward into territory where 

 they can not yet grow. If environmental change is rapid, the slower 

 species may not be able to keep even with it and consequently follow 



11 Gleason, H. A. The structure and development of the plant association. Bull. 

 Torrey Club 44: 463-481. 1917. 



12 Clements, Frederic E. Plant succession, an analysis of the development of 

 vegetation. Carnegie Inst. Washington, Publ. 242. 1916. 



