a profound change in the course of the succession, changing from hydrophytic to mesophytic or xerophytic, or the reverse. This is characteristic of newly formed or exposed soils. Such successions are _xerotropic_, _mesotropic_, or _hydrotropic_, according to the ultimate condition of the habitat. When the reaction is less marked, the type of habitat does not change materially, and the successions are _xerostatic_, _mesostatic_, or _hydrostatic_, depending upon the water-content. Such conditions obtain for the most part only in denuded habitats. =316. 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 is best seen in the lichen stages, where the crustose forms make a compact layer, which diminishes the effect of the atmospheric factors producing disintegration. 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 observations 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 protect 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 decomposition, 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 covering on their faces, while the forces which produce disintegration operate from above or below. [Illustration: Fig. 66. Thicket formation (_Quercus-Holodiscus-driodium_), stage V of the talus succession.]