INVASION AND SUCCESSION 283 



changed, so that fairly complete and continuous invasion may 

 take place in it. Its continuance is largely due to the reaction 

 which each of its stages exerts upon the physical factors of the 

 habitat. The reactions thus produced are as follows: (1) influ- 

 ence upon weathering, (2) binding aeolian soils, (3) reduction of 

 run-off and erosion, (4) filling with silt and plant remains, (5) 

 enriching the soil, (6) exhausting the soil, (7) modification of 

 atmospheric factors. 



Plants influence weathering by hastening disintegration or 

 decomposition, or more rarely by protecting the rock surface 

 from the action of weather. This last effect is found in some 

 degree where lichens or mosses cover rocks completely, though 

 they serve at the same time to decompose the surface. Herbs, 

 and especially shrvibs and trees, hasten disintegration through 

 the growth of roots into cracks and clefts. When present in 

 masses, they increase the amount of water, and hence promote 

 the action of the latter. The shifting soils of dunes, blowouts, 

 etc., are inhabited chiefly by sand -binding plants, mostly per- 

 ennial grasses and sedges. These usually have masses of fibrous 

 roots, long, erect leaves, and vigorous rootstocks capable of push- 

 ing up rapidly through a covering of sand. They react by fixing 

 the sand with their roots, thus preventing it from being blown 

 away. They also catch the shifting particles among their stems 

 and leaves, which also serve to accumulate vegetable remains. 



In habitats subject to erosion, plants delay run-off and pre- 

 vent the formation of rills of sufficient size to erode the soil. Such 

 plants are usually perennial grasses or composites with well- 

 developed roots. On sand and gravel slopes the loose texture 

 of the soil leads to the production of sand-binders and of mats 

 and rosettes, which are especially effective in preventing the 

 slipping of the soil. In hydrophytic habitats the plants check 

 the movement of the water. In consequence they cause the 

 deposition of part or all of the sediment carried by it, and they 

 then retain and fix the particles deposited. The continuance of 

 this action, together with the accumulation of plant remains, gradu- 

 ally fills up the hal)itat. The enrichment of the soil by the decay 

 of the plants growing upon it is a reaction that is founckin some 

 c measure in all formations. The reverse process, the exhaustion 

 of the soil, is largely confined to cultivated fields. Only a few 

 doubtful cases of such action occur in nature. In lavered for- 



