120 SUCCESSION 



year after year. In fields that lie fallow for several years, 

 or are permanently abandoned, the first ruderai plants are 

 displaced by new-comers, or certain of them become domin- 

 ant at the expense of others. In a few years, these are 

 crowded out by invaders from the adjacent formations, and 

 the field is ultimately reclaimed by the original vegetation, 

 unless this has entirely disappeared from the region. The 

 number of stages depends chiefly upon whether the final 

 formation is to be grassland or woodland. Other activities 

 of man, such as the construction of buildings, roads, rail- 

 ways, canals, etc., remove the native vegetation, and make 

 room for the rapid development of ruderai formations. In 

 and about cities, where the original formations have en- 

 tirely disappeared, the chance for succession is remote, and 

 the initial ruderai stages become more or less stabilised. 

 Elsewhere the usual successions are established, and the 

 ruderai formation finally gives way to the dominant type. 

 In mountain and desert regions, where ruderai plants are 

 rare or lacking, their place is taken by subruderal forms, 

 species of the native vegetation capable of rapid movement 

 in them. These, like ruderai plants, are gradually replaced 

 by other native species of less mobility, but of greater per- 

 sistence, resulting in a short succession operating often 

 within a single formation. Prom the nature of cultivated 

 plants, succession after cultivation generally operates with- 

 in the mesophytic series. 



(d) Succession by drainage. Successions of 

 this kind show much the same stages as are found in those 

 due to flooding. They proceed from aquatic or swamp for- 

 mations to mesophytic termini, either grassland or wood- 

 land. When drainage takes place rapidly and completely, 

 the pioneer stages are usually xerophytic : cases of this sort, 

 however, are infrequent. 



(e) Succession by irrigation. Irrigation pro- 

 duces short successions of peculiar stamp along the courses 

 of irrigating canals and ditches, and in the vicinity of 

 reservoirs. These are recent, as a rule, and are usually 

 found in the midst of cultivated lands, so that their complete 



