248 THE FORMATION 



off, due to compactness of the particles and to the slope. The erosive action 

 of winds upon soils bearing vegetation is not very general ; 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 xero- 

 phytic, 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 in- 

 creasing 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. 



301. Succession in flooded soils. The universal response of vegetation to 

 floods is found in the amphibious plant, which is a plastic 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 occurrence when land forms were more plastic. 



In all streams that have become graded, the fall is insufficient to carry off 

 the surplus w^ater in the spring when snows are melting rapidly, or at times 

 of unusual precipitation. These waters accumulate, and, overflowing the 

 banks, spread out over the lowlands, resulting in the formation of a well- 

 defined flood plain. This is a periodical occurrence with mature streams, 

 and it occurs more or less regularly with all that are not torrent-like in char- 

 acter. The effect of the overflow is to destroy or to place at a disadvantage 

 those plants of the flood plain that are not hydrophytes. At the same time, 

 a thin layer of fresh silt is deposited upon the valley floor of sand or allu- 

 vium. Flooding is most frequent and of longest duration near the banks of 

 the stream. It extends more or less uniformly over the flood plain, and dis- 

 appears gradually or abruptly as the latter rises into the bench above. Floods 

 destroy vegetation and make a place for secondary successions by drowning 

 out mesophytic species, by washing away the aquatic forms of ponds and 

 pools, and by the erosion of banks and sandbars. They affect the amphib- 

 ious vegetation of swamp and shore to a certain extent, but, unless the period 

 of flooding is long, they tend to emphasize such formations rather than to 

 destroy them. The still-water formations of many cutoff and oxbow lakes 

 owe their origin to a river which cuts across a meander in time of flood. 

 This result is more often attained by tlie alternate silting and erosion of a 

 meandering river by which it cuts across a bend in its channel. The usual 

 successions in flooded lands are short as a rule ; amphibious algac, liverworts, 

 and mosses soon give way to ruderal plants, and these in turn to the original 



