STABLE AND MIGRATORY PLANT FORMATIONS 1 29 



petition of a mature succession, wiiile in semi-desert condi- 

 tions of drought or intense cold, the plants following these 

 tracks frequently thereby escape physical extremes. 



The migratory agents originate habitats with, on the one 

 hand, a surplus, or, on the other, a reduction of water, oxygen, 

 and salts, over that available by the neighbouring stable 

 vegetation. They may cause excessive transpiration, as in 

 areas of blown sand or hinder absorption, as in salt-marshes ; 

 they may palliate extreme conditions, as in deserts and arctic 

 regions, and generally tend to delay the type of stabilisation 

 under the climatic conditions prevailing. Mountains, seas, and 

 large rivers may form barriers to the spread of certain stable 

 types of vegetation, but the alpine, coast and river belts have 

 always preserved open routes of migration and places of 

 refuge for plants ousted from the neighbouring areas of 

 competition. They form sanctuaries for relic associations, 

 or outposts for pioneer invading species. 



These open ways and asylums may shelter plants that have 

 lagged behind or degenerated in competitive stabilisation, 

 preferring the easy existence of being continuously bathed in 

 water, or of annually happening on fresh open formations. 

 But the stations of the migratory formations in many cases are 

 peculiar to the agents of surface change, and are occupied by 

 endemic forms which have adopted lines of specialisation 

 meeting the physical conditions of a peculiar environment, 

 rather than those adapting them for competition over wide 

 areas of stable topography. They have become nailed to a 

 special physical environment, or have been forced to be 

 constantly accommodating themselves to unstable and chang- 

 ing surroundings. Thus we have lithophytes, aquatics, 

 sand-dune, swamp and marsh plants, halophytes and various 

 xerophytes, vagrants and annuals, excluded from the 

 neighbouring stable formations. 



Some survive or even flourish under rapid physical changes 

 in the habitat, and man\' have specialised in their mode of 

 growth, and in the nature of their fruits or seeds, particular!}- 

 adjusting them for dispersal by the sea or rivers, by the 

 wind, or by migratory animals. Many have doubtless 

 evolved under the migratory conditions in which we find 

 them, while others may be descendants of plants that had 



