REACTIONS OX LAND 77 



their share in this process. This reaction is often associated with the 

 deposition of sand and silt by the retardation of currents as they enter 

 pond or lake, but the effect of plants is regularly predominant in such 

 deltalike areas. The resultant filling has much of the consequence 

 already indicated for the accumulation of the plant remains, and the 

 two processes usually cooperate to build up the level. However, the 

 movement of water is progressively hindered as the level rises, until 

 the area is overflowed only at times of flood. This sets a limit to the 

 deposition of sediments, and the further reaction is chiefly one of 

 decreasing water content due to the fall of plant parts, to trans- 

 piration, etc. 



The role of jjlants in impeding runoff and preventing erosion is 

 even more striking and important, though the action itself resembles 

 in some respects that of a shallow stream. In fact, it is the formation 

 of rills and gullies that renders erosion so effective. A good cover of 

 vegetation operates, in the first place, to prevent the direct impact of 

 raindrops on the soil, but much more important is its action in holding 

 back the fallen rain until it can be absorbed. It further restrains 

 both rills and sheet floods, reducing the momentum as well as the 

 surface affected and consequently minimizing both load and erosive 

 power. In the case of wind, the decrease in velocity through the 

 action of cover is the major factor in reducing erosion. Frequently, 

 especially in arid regions, water and wind act together, producing a 

 landscape of hummocks, which are the result of alternating erosion 

 and accretion. 



The organization of the Soil Conservation Service and the nation- 

 wide installation of projects in soil conservation and flood prevention 

 have served to draw attention to the plant cover as the paramount fac- 

 tor in protection and control. As a consequence, the reactions of cover, 

 both natural and cultural, have come to be regarded as of the utmost 

 significance to economic and social progress, not merely in terms of 

 agriculture, grazing and forestry, but also to urban populations. No 

 field of conservation research equals this in importance or has been 

 more neglected until recently, and the next decade is bound to see an 

 enormous expansion of knowledge in it, together with almost unlimited 

 application to human affairs. 



Reaction upon Slipping and Sliding. The reaction of a plant cover 

 on the soil of slopes may be exerted upon the mass as well as upon 

 the surface. This is particularly true of loosely aggregated materials, 

 as in talus, steep slopes of gravel, sand, snow, and so forth, and it 

 applies likewise to the faces of cuts and fills in the grades of highways 

 and roadways. In the case of sand, volcanic ash, or gravel, the effect 



