132 THE SCOTTISH BOTANICAL REVIEW 



aquatic, helophytic, lithophytic, psychrophytic, psammo- 

 phytic, oxylophytic, and rabble formations, etc. (13), but the 

 desirability of further analysing them from the standpoint of 

 physiography has doubtless been apparent to some ecologists 

 for a considerable period of time. Aquatic formations always 

 vary with the physiography. Rock formations may support 

 certain common species over wide areas of different physio- 

 graphy, but the majority show signs of being distinct in type 

 and association with the physiographic factors leading to the 

 rock exposure. Shingle beaches and alpine screes must 

 obviously be separated, as habitats for plant life, and the 

 meadows, marshes, and swamps of the higher and lower parts 

 of the drainage system come under very different physio- 

 graphic conditions of existence, and vary widely in their 

 plant associations. 



Certain types of plant association only follow in the wake 

 of the migratory agents of surface change ; thus we have 

 various spring, flush, marsh, aquatic, alluvial — meadow and 

 swamp associations, confined within the sphere of action of 

 the drainage system of the streams, rivers, lakes, and seas, the 

 world over. Halophile associations are confined to the mari- 

 time coastal belt or to places marking the present or late foci 

 of centripetal, inland drainage. The latter may form zones 

 around inland seas or lakes, or appear in the form of playas, 

 dry lakes, and salt marshes. Alkali soils have a somewhat 

 wider distribution than recent foci of centripetal drainage, 

 especially in the desert areas subject to much erosion, as the 

 salts are redistributed by the winds, and old salt deposits 

 are frequently re-exposed at the surface. Hilgard has shown 

 how the salts tend to accumulate at or near the surface of the 

 soil in sandy, saline deserts, by the effects of evaporation 

 bringing a considerable amount of the soil water to the 

 surface. 



All spring associations have certain common factors in the 

 water supply coming directly from beneath the surface of the 

 ground and in being continuously changed, but various 

 springs differ much in the volume and temperature of the 

 water, the dissolved salts and gases, and other ways. These 

 factors may completely suppress the growth of vegetation, 

 only allow the presence of a peculiar flora, or cause an 



