REACTIONS ON LAND 75 



counts made to determine relative numbers of individuals (Taylor, 

 1930, b). Even small forms such as insects are not without impor- 

 tance in this connection, since caterpillars are known to add several 

 times their own weight of pellets consisting of partially digested plant 

 material (Jacot, 1936, a, b) . 



Reaction by Accumulating Shells and Concretions. The shells of 

 animals, whether chitinous or calcareous, regularly play a minor part 

 in this process on land, though quantitative studies may assign them 

 a greater importance than recognizable at present. The sole group of 

 plants with a similar reaction is the diatoms, though their effect is 

 relatively insignificant today in comparison with the geological past. 

 The production of diatomaceous soil on a small scale may be observed 

 along the margin of many pools and small streams, but marshes of this 

 type are rare. The most extensive and best known are found in 

 Yellowstone Park, where the hot waters have apparently promoted 

 the growth of diatoms, and the consequent accumulation has produced 

 a hydrosere, ending in characteristic meadows. 



Apart from shells, the concretions due to direct physiological activ- 

 ity are limited to aquatic plants found in shallow waters. The result- 

 ing substrata may be calcareous, represented by marl, tufa, and 

 travertine, or siliceous, as in sinter and geyserite. By far the most 

 widespread of these deposits is marl; it is produced chiefly by Chara 

 and occurs more or less regularly as a layer of variable thickness in 

 fresh-water marshes. It is frequently mixed with terrigenous mate- 

 rial and contains small amounts of organic matter from the decaying 

 shoots of the stonewort. Somewhat similar incrustations are pro- 

 duced by a few mosses, while the massive forms known as travertine 

 are the work of microscopic algae, as oolite may be likewise. Sinter 

 and geyserite are also secreted by blue-green and yellow-green algae, 

 but are restricted to the cones and basins of hot springs and geysers, 

 and are consequently of slight importance. 



Reaction through Weathering. The conversion of rock into soil, 

 in the usual sense, is the combined effect of mechanical forces and 

 plant and animal activities. The former take the more conspicuous if 

 not the more important part, but the production of fine particles is 

 chiefly the work of the organisms. In the case of woody plants, the 

 growth of roots in thickness is an important factor in mechanical 

 weathering, but the paramount reaction is exerted by root secretions, 

 primarily carbon dioxide in solution. Animals apparently have only a 

 small and indirect part in weathering, and then only after cracks have 

 appeared which they can occupy. 



The corrosion of rock by plant roots is the most significant process 



