78 REACTION: THE INFLUENCE OF COMMUNITY ON HABITAT 



is produced by the underground parts, primarily the roots, while that 

 upon snow is due to stem and branches, and to leaves also in the 

 case of conifers. The process known as "slumping" involves, in addi- 

 tion, the saturation of the soil mass so that it flows, producing lava- 

 like streams of mud under extreme conditions. Slumps occur with 

 increasing frequency as highway cuts become deeper with more abrupt 

 sides, and their prevention by means of plant reactions has become a 

 matter of much practical importance from the standpoint of both 

 safety and economy of maintenance. 



The species best adapted to retaining the soil on slopes are mats 

 or rosettes with tap roots, or long branching ones which anchor the 

 plant firmly, their greatest extension often being uphill, and the cluster 

 of stems or horizontally appressed leaves prevents the slipping of the 

 surface materials. Each plant or colony exerts a stabilizing effect for 

 some distance below its own area, partly by intercepting small slides 

 above it. The primary reaction is a mechanical one, leading to in- 

 creasing aggregation and finally to invasion. Where rain or snow is 

 a factor, as in slumping, the grass or bush forms possess additional 

 advantages, particularly in utilizing artificial succession to hold the 

 slopes. 



Soil Structure 



The reactions that build soils regularly continue to act to bring 

 about modification in them to a larger or smaller degree. The struc- 

 ture of a soil may be changed mechanically by superficial accumula- 

 tion or by the inclusion of plant and animal remains, by the penetra- 

 tion of roots, and by the disturbances wrought by animals. With 

 these go a number of chemical changes, often of fundamental impor- 

 tance. In addition, plants react upon the soil profile in such a manner 

 as to protect it against the action of modifying forces, such as weather- 

 ing and erosion by water and wind. The soil is thus a complex of 

 reactions, in which the role and significance of each process can be 

 definitely ascertained only by thorough-going analysis and measure- 

 ment. This applies particularly to the respective parts taken by 

 plants and animals, though it is obvious that the major effect of ani- 

 mals will be exerted through various kinds of disturbance. 



Reaction by Adding Organic Matter. The most important changes 

 in the structure and texture of the soil are caused by the addition of 

 organic matter or humus. This is derived chiefly from the decay of 

 plant parts, though animal remains usually play some part in it, and 

 its working-over and incorporation are due primarily to animals, 

 everywhere the smaller soil organisms and in dry region the rodents 



