SOIL FORMATION. 



85 



loess. It is the outcome of the retardation of air-currents by the stems and 

 leaves of plants, especially pioneers in sand. The effect of the plant-body 

 is twofold; it is not only a direct obstacle to the passage of grains of sand, but 

 it also decreases the velocity of the wind and hastens the consequent dropping 

 of its load. The same action likewise tends to prevent the wind from picking 

 the sand up again and carrying it further. The underground parts of sand 

 plants exert a complementary reaction by binding the sand through the 

 action of roots and rhizones, and by developing shoots which keep pace with 

 the rise of the surface. Certain pioneers form rosettes or mats, which hold 

 the sand with such firmness that they cause the formation of hiunmocks with 

 a height of one to many feet above the bare areas. The behavior of sand- 

 binders has been a fruitful field of study, and there is probably no other group 

 of plants whose reactions are so well understood (plates 1a, 22 a). 



The primary reaction upon wind-blown sand is mechanical. The pioneer 

 grasses in particular stop and fix the sand and produce stable centers for inva- 

 sion. This permits the entrance of other species capable of growing in bare 

 sand, if it is not shifting actively. With the increase of individuals, however, 

 the amount of vegetable material in the soil becomes greater, increasing the 

 water-retention of the sand and the amount of nutrients. This is the primary 

 reaction in sand areas after the sand-binders have finished their work of 

 stabilization. The reaction which produced and colonized deposits of loess 

 must have been similar. The action of plants in bringing about the dropping 

 and temporary fixing of wind-blown dust must indeed have been almost identi- 

 cal. Because of their much smaller size the dust particles were much more 

 readily compacted by the action of rainfall. For the same reason they 

 retained more of the latter in the form of the holard, and loess areas were 

 probably xerophytic for a much shorter time. While the development of 

 the first stages was doubtless more rapid, each stage necessarily increased the 

 hiunus and hence the water-content, though to a less significant degree perhaps 

 than in sand. However, our knowledge of the initial stages on loess and of 

 their reaction is obtained mostly from analogy, since no deposits of loess 

 known to be forming at the present time have been studied critically (c/. 

 Shimek, 1908:57; Huntington, 1914^:575). 



(5) Reaction upon water-borne dMritus. — ^The effect of plant bodies upon 

 material carried by water is essentially similar to that noted for eolian sand. 

 Stems and leaves slow the current and cause the deposition of its load in whole 

 or in part (plate 6). They also make difficult the removal of material once 

 deposited, a task in which roots and root-stocks have a share likewise. This 

 reaction is often associated with the deposition of sand and silt by the retarda- 

 tion of currents as they empty into bodies of water, but the effect of plants is 

 usually predominant. The filling incident to this reaction has the conse- 

 quences already indicated for filling by the accumulation of plant remains. 

 In fact, both processes cooperate to decrease the depth of water wherever 

 plants occur in an area through which detritus is carried. The decreasing 

 depth controls the usual sequence from submerged to amphibious plants. 

 The latter continue the process, but the movement of the water is steadily 

 impeded as the level rises, until finally it overflows the area only at times of 

 flood. This sets a limit to the accumulation of detritus, and the further 

 development is controlled by decreasing water-content due to plant accumula- 



