REACTIONS 125 



water content. Such conditions obtain for the most part 

 only in denuded habitats. 



(1) Succession by preventing weathering. 

 Reactions of this nature occur especially in alpine and boreal 

 regions, in the earlier stages of lichen-moss successions. 

 They are typical of igneous and metamorphic rocks in which 

 disintegration regularly precedes decomposition. The 

 influence of the vegetation i^ best seen in the lichen stages, 

 where the crustose forms make a compact layer, which dim- 

 inishes the effect of the atmospheric factors producing disin- 

 tegration. In alpine regions especially, this protection is so 

 perfect that the crustose lichens may almost be regarded as 

 the last stage of a succession. There are no recorded ob- 

 servations which bear upon this point, but it seems certain 

 that the pioneer rock lichens, Lecanora, Lecidea, Biatora, 

 Buellia and Acarospora, cover alpine rocks for decades, if not 

 for centuries. Ultimately, however, the slow decomposition 

 of the rock surface beneath the thallus has its effect. Tiny 

 furrows and pockets are formed, in which water accumulates 

 to carry on its ceaseless work, and the compact crustose 

 covering is finally ruptured, permitting the entrance of 

 foliose forms. The latter, like the mosses, doubtless pro- 

 tect rock surfaces, especially those of the softer rocks, in a 

 slight degree against the influence of weathering, but this 

 is more than offset by their activity in hastening decompo- 

 sition, and thus preparing a field for invasion. Rocks and 

 boulders {petria, petrodia, phellia) furnish the best examples 

 of this reaction : cliffs (cremnia) usually have a lichen cover- 

 ing on their faces, while the forces which produce disinte- 

 gration operate from above or below. 



(2) Succession by binding aeblian soils. 

 Dunes (thinia) are classic examples of the reaction of 

 pioneer vegetation upon habitats of wind-borne sand. The 

 initial formations in such places consist exclusively of sand- 

 binders, plants with masses of fibrous roots, and usually 

 also with strong rootstalks, long, erect leaves and a 

 vigorous apical growth. They are almost exclusively per- 

 ennial grasses and sedges, possessing the unique property 



