SECONDARY SUCCESSIONS 115 



ation of a well-defined flood plain. This is a periodical 

 occurrence with mature streams, and it occurs more or less 

 regularly with all that are not torrent-lilre in character. 

 The effect of the overflow is to destroy or to place at a dis- 

 advantage those plants of the flood plain that are not hydro- 

 phytes. At the same time, a thin layer of fresh silt is 

 deposited upon the valley floor of sand or alluvium. Flood- 

 ing is most frequent and of longest duration near the banks 

 of the stream. It extends more or less uniformly over the 

 flood plain, and disappears gradually or abruptly as the 

 latter rises into the bench above. Floods destroy vegetation 

 and make a place for secondary successions by drowning out 

 mescphytic species, by washing away the aquatic forms of 

 ponds and pools, and by the erosion of banks and sandbars. 

 They affect the amphibious vegetation of swamp and shore 

 to a certain extent, but, unless the period of flooding is long, 

 they tend to emphasize such formations rather than to de- 

 stroy them. The Stillwater formations of many cutoff and 

 oxbow lakes owe their origion to a river which cuts across 

 a meander in time of flood. This result is more often 

 attained by the alternate silting and erosion of a meander- 

 ing river by which it cuts across a bend in its channel. The 

 usual successions in flooded lands are short as a rule : am- 

 phibious algae, liverworts and mosses soon give way to 

 ruderal plants, and these in turn to the original mesophytes 

 of meadows, or dissophytes of sandbars. In the case of 

 ponds and pools, the process of washing-out or silting-up 

 merely removes or destroys the vegetation, without 

 effectively modifying the habitat, and the secondary suc- 

 cessions that follow are extremely short. 

 10. Succession by subsidence. Subsidence is a 

 factor of the most profound importance in changing vege- 

 tation. It operates over vast areas through immense 

 periods of time. For these reasons, the changes are so slow 

 as to be almost imperceptible, and the resulting successions 

 can be studied only in the geological record. Extensive 

 subsidence is confined today to coastal plains, as in Green- 

 land, the south Atlantic coast, and the region of the Missis- 



