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PLANT PHYSIOLOGY AND ECOLOGY 



ciers. The primary successions upon colluvial, alluvial, and seo- 

 lian soils are by far the most frequent and important. 



295. Succession in colluvial soils. Colluvial deposits are 

 found at the base of cliffs, ledges, and mountain sides. The for- 

 mation of talus, i.e., the mass of coarse irregular material at the 

 base, is due to the action of gravity upon the fragments of rock 

 split off by disintegration. Decomposition appears later as a 

 secondary factor. The fragments forming the talus are extremely 



Fig. 111. a mat formation, the first stage of an alpine colluvial succession. 



variable in size, but they agree in their angular shape. Talus may 

 originate from any kind of rock, the nature of the latter deter- 

 mining the size of its particles. Gravel slides differ from ordinary 

 talus in consisting of more uniform particles, worn round by 

 slipping down the slope in response to gravity and surface wash. 



The character of the successions in talus depends upon the 

 kind of rock in the latter. If the rock is igneous or metamorphic, 

 decomposition is slow and the resulting soil is easily dried out. 

 The corresponding successions consist of many stages, and the 

 formations are for a long time open and xerophytic. In talus 



