86 REACTIONS. 



tions, to transpiration, etc. Frequently the deposit of silt and subsequent 

 heaping-up of plant materials go on more rapidly in some spots than in others, 

 producing hummocks on which the future com-se of development is traced in 

 miniature. 



(6) Reaction upon slipping sand and gravel. — ^A characteristic feature of 

 the Rocky Moimtains is the steep talus-slope known as a gravel-slide. The 

 angle of the slope is usually so great that some slipping is going on constantly, 

 while the movement downward is materially increased after a heavy rain. 

 The fixation of such a slope is a problem similar to that which occiu's in dunes 

 and blow-outs. The coarse sand or gravel must be stopped and held in opposi- 

 tion to the downward pull due to gravity. The movement is slower and is 

 somewhat deeper-seated. Consequently, the species best adapted to gravel- 

 slides are mats or rosettes with tap-roots or long, branching roots. The latter 

 anchor the plant firmly and the cluster of stems or horizontally appressed 

 leaves prevents the slipping of the surface area. Each plant or each colony 

 exerts the stabilizing effect for some distance below its own area, owing to the 

 fact that it intercepts smaU slides that start above it. The primary reaction 

 is a mechanical one, and a large number of species invade as soon as the sur- 

 face is stable. These increase the humus production and water-content, and 

 the subsequent reaction resembles that of all dry sand or gravel areas (plate 3 a). 



SOIL-STRUCTURE. 



The structure of the soil may be changed mechanically by plants through 

 the admixture of plant remains, the penetration of roots, or the compacting 

 incident to the presence of plants. Associated with these are chemical changes 

 often of the most fundamental importance. In addition, plants react upon 

 the soil in such a away as to protect it against the action of modifying forces, 

 such as weathering and erosion by water or wind. None of these are simple 

 reactions, but the mechanical effect of each may constitute a primary reaction. 

 The opportimity for greater clearness and analysis seems likewise to warrant 

 the consideration of their influence upon soU-structxire alone. 



(7) Reaction by adding humus. — ^The change in the texture of the soil due 

 to the admixture of humus is caused by animals as weU as by plants. In 

 grassland and woodland soil, animals indeed play the chief part in the distribu- 

 tion of humus in the soil. The effect of the humus is much the same, however, 

 quite apart from the fact that soil organisms work over only material which is 

 destined to become humus at all events. All plant commimities produce 

 hiunus in some degree by the death of entire plants, annually or from time to 

 time, and by the annual fall of leaves and the aerial parts of perennial herbs. 

 The amoimt produced depends upon the density and size of the population 

 and upon the rate and completeness of decomposition. It is small in the 

 pioneer stages of a sere, especially in xerophytic situations, and increases with 

 each succeeding stage. It reaches a maximum in mesophytic grassland and 

 woodland, but falls off again with the decrease of population in a completely 

 closed community (plate 22 b). 



The physical effect of humus is to make light soil more retentive of water 

 and heavy soils more porous. Hall (1908: 47) states this as foUows: "Humus 

 acts as a weak cement and holds together the particles of soil; thus it serves 

 both to bind a coarse-grained sandy soil, and, by forming aggregates of the 



