82 REACTIONS. 



prevented. This is the universal method by which biogenous soils are formed, 

 though it must be recognized that animals also usuaUy play a large or control- 

 ling part in the process. As a reaction proper it is brought about only by 

 plants which growin water or in wet places, but the formation of the soil maybe 

 hastened by the incorporation of transported material, including terrestrial 

 plants as well as animal remains and detritus. It is the characteristic reaction 

 of aquatic and amphibious communities, and occurs in salt water as well as in 

 fresh water. The peat substratimi which results is f oimd universally wherever 

 plants decompose in the presence of insufficient oxygen. As is well known, a 

 similar process has recurred throughout geological history, resulting in the for- 

 mation of coal at various times from the Paleozoic to the Tertiary. Along 

 with the biogenous formation of the soil occur certain secondary consequences, 

 such as the production of acids, of possible toxic substances, changes in soil 

 organisms, etc., which are considered elsewhere. 



The shallowing of water by pioneer aquatics first changes the conditions to 

 the detriment of submerged plants and the advantage of floating species, and 

 then to the respective disadvantage and advantage of floating and amphibious 

 plants. This is equally true when water-borne detritus plays a part, for it 

 merely hastens the outcome. The process is continued by the amphibious 

 reeds and sedges, which may yield finally to meadow grasses. In this stage, 

 surface-water usually disappears, and the acctunulation ceases entirely or 

 nearly so, because of the access of oxygen. In boreal and moimtain regions 

 Sphagnum usually enters in the amphdbious stage or near its close, and gives 

 a new lease to accumulation imder circumstances which may almost com- 

 pletely inhibit decomposition. After a time the moss layer becomes so thick 

 that other plants may enter because of the decreasing water-content of the 

 surface, which controls the further development. Sphagnum may also extend 

 as a floating mat over pools and ponds, and eventually fiU them with peat 

 (plate 21a). 



A host of investigators have studied the formation of soil by peat-producing 

 plants, as is indicated in a later section. Various kinds of peat have been dis- 

 tinguished on the basis of the component species and the degree of decompo- 

 sition and compression. These have Kttle bearing on the reaction here con- 

 sidered, since the mere accumulation is the chief fact. The direct reaction 

 which influences the sequence of stages is, however, the change in water 

 depth and content incident to the increase of thickness of the peat. In the 

 submerged and floating stages the directive factor is the decreasing depth 

 which permits the entrance of species with floating leaves. Such plants cut 

 off the Ught from the submerged pioneers and probably change other condi- 

 tions unfavorably also. A further reduction of depth allows the ecesis of 

 amphibious reeds, and these first dominate and then displace the floating 

 plants, partly, it seems, in consequence of hght reduction. From this point 

 the essential change is a decrease of water-content, largely by continued filling 

 but partly because of the relative increase in transpiration. This is the ruling 

 reaction throughout the rest of the development, imless the latter is deflected 

 by the appearance of Sphagnum, or until it reaches the shrub or forest stage. 



The formation of soil by the deposition of diatom shells is relatively insig- 

 nificant, though frequently found on a small scale. It probably played a 

 larger part in the geological past, if one may judge from the existence of exten- 



