114 SUCCESSION 



erosion by water, or by wind, though the initial stages of 

 revegetation derive their character more from the aggrega- 

 tion of the soil than from the nature of the erosive agent. 

 Eroded soils are as a rule xerophytic. In the case of eros- 

 ion by water, dysgeogenous soils are readily worn away in 

 consequence of their lack of cohesion, as in sanddraws, etc., 

 while eusreogenous soils are easily eroded only on slopes, 

 as in the case of ravines, hillsides, etc. In the former, 

 the extreme porosity and slight capillarity of the sand and 

 gravel result in a low water content : in the finer soils, the 

 water content is also low, on account of the excessive run- 

 off, due to compactness of the particles and to the slope. 

 The erosive action of winds upon soils bearing vegetation 

 is not very eeneral : it is found to some extent in more or less 

 established dunes, and exists in a marked degree in buttes, 

 mushroom rocks and blowouts. The first two are regularly 

 xerophytic^ the last as a rule, dissophytic. The early 

 stages of successions in eroded soils are composed of 

 xerophytes. In loose soils, these are forms capable of 

 binding the soil particles together, thus preventing wash, and 

 increasing the accumulation of fine particles, especially of 

 organic matter. In compact soils, the effect is much the 

 same : the pioneers not only decrease erosion, but at the 

 same time also increase the water content by retarding the 

 movement of the run-off. 



9. Succession in flooded soils. The universal 

 response of vegetation to floods is found in the amphibious 

 plant, which is aplastic form capable of adjustment to very 

 different water contents. Floods are confined largely to 

 river basins and coasts : in hilly and mountainous regions, 

 where the slope is great, any considerable accumulation of 

 flood waters is now impossible, although of frequent oc- 

 currence when land forms were more plastic. 



In all streams that have become graded, the fall is in- 

 suflScient to carry off the surplus water in the spring when 

 snows are ^melting rapidly, or at times of unusual precipit- 

 ation. These waters jaccumulate, and, overflowing the 

 banks, spread out over the lowlands, resulting in the form- 



