REACTIONS 129 



the deposit of river- borne detritus, causing the water to 

 spread over the lowlands and form swamps. They often 

 throw back also the sediment that has been deposited in the 

 sea, the marsh vegetation acting as a filter in both cases. 

 Successions of the kind indicated above are regularly meso- 

 tropic. Where the soil is sandy, and the fllling-up process 

 sufficiently great, or where salts or humus occur in excess, 

 xerophytic formations result. In certain cases, these suc- 

 cessions appear to be permanently hydrostatic, changing 

 merely from floating or submerged to amphibious con- 

 ditions, but this is probably due to the slowness of the re- 

 action. As a rule, the accumulation of plant remains is 

 relatively slight, and plays an unimportant part in the re- 

 action. In peat bogs and other extensive swamps, the 

 amount of organic matter is excessive, and plays an import- 

 ant rdle in the building up of the swamp bed. Its greatest 

 reaction, however, results frum its partial decomposition 

 under water. This process sets free a number of so-called 

 humous acids, which inhibit the absorption uf water, and 

 cause the development of xerophytes in what are apparently 

 hydrophytic habitats. 



(5) Succession by enriching the soil. 

 This reaction occurs to some degree in the great majority 

 of all successions. The relatively insignificant lichens and 

 mosses produce this result upon the most barren rocks, 

 while the higher forms of later stages, grasses, herbs, 

 shrubs, and trees, exhibit it in marked progression. The 

 reaction consists chiefly in the incorporation of the decom- 

 posed remains of each generation and each stage in the soil. 

 A very important part is played by the mechanical and 

 chemical action of the roots in breaking up the soil 

 particles, and in changing them into soluble substances. 

 Mycorrhizae, bacterial nodules and especially soil bacteria 

 play a large part in increasing the nutrition content of the 

 soil, but the extent to which they are effective in succession 

 is completely unknown. The changes in the color, texture 

 and food value of the soil in passing from the initial to 

 ultimate stages of a normal succession are well-known, and 



