SUCCESSION 257 
-The effect of succeeding stages of vegetation upon a new or denuded habitat 
usually finds expression in a change of the habitat with respect to a particu- 
lar factor, and in a definite direction. Often, there is a primary reaction, 
and one or more secondary ones, which are corollaries of it. Rarely, there 
-are two or more coordinate reactions. The general ways in which vegeta- 
tion reacts upon the habitat are the following: (1) by preventing weather- 
‘ing, (2) by binding aeolian soils, (3) by reducing run-off and preventing 
erosion, (4) by filling with silt and plant remains, (5) by enriching the 
soil, (6) by exhausting the soi!, (7) by accumulating humus, (8) by modi- 
fying atmospheric factors. The direction of the movement of a succession 
is the immediate result of its reaction. From the fundamental nature of 
vegetation, it must be expressed in terms of water-content. The reaction is 
often so great that the habitat undergoes 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 ero- 
static, 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 pro- 
ducing disintegration. In alpine regions especially, this protection is so per- 
fect 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 cover- 
ing 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) turnish 
the best examples of this reaction; cliffs (cremnia) usually have a lichen 
