88 REACTIONS. 



characteristic "short grasses" (Shantz, 1911). Both "hardpan" and "ort- 

 stein " favor the persistence of the community which produced them. " Hard- 

 pan," however, brings about the disappearance of the preceding population, 

 while "ortstein" apparently does not appear imtil heath has long been in 

 possession, since it depends upon the production of heath-sand. Another 

 difference Ues in the fact that heath is at most a subclimax, while the "short 

 grass " association is the final climatic stage. 



(9) Reaction by preventing weathering or erosion. — ^A plant cover, whether 

 living or dead, everywhere produces an iinportaht reaction by protecting the 

 surface from erosion. It has a somewhat similar effect upon the weathering 

 of rock by' atmospheric agents, but this has much less significance, since the 

 plants themselves are producing weathering. In the case of erosion, the 

 reaction is much the same as that which occurs when plants stop drifting 

 sand or suspended silt. In open communities the stems and leaves reduce 

 the velocity of wind or water and make it difficult for them to pick up soil 

 particles; in closed associations the plants usually eliminate the effect of 

 wind and water entirely and the erosion is null. The influence of cover is 

 thus a progressive one, from the sparse population of the pioneer stage with 

 most of the surface exposed to erosive action, through more and more closed 

 communities to the climax. It is a stabilizing factor of the first importance 

 in that it prevents denudation and consequent initiation of a new area. At 

 the same time it assures continued occupation by the plants in possession, and 

 hence the continuance of the reactions which produce the normal sequence of 

 stages. The progressive increase of reaction tends to limit denudation and 

 the renewal of succession largely to the early stages, and makes it more diffi- 

 cult in the final ones. Its significance is of course clearly revealed when the 

 cover is partially or wholly destroyed (plate 23, a, b). 



WATER-CONTENT. 



Since water is the chief factor in succession, as in plant respc»nse, it is more 

 or less affected by practically all reactions. In addition, the increase or 

 decrease of water-content may be the direct outcome of the activity of the 

 plant itself. The effect, moreover, may be exerted on the chresard as well as 

 upon the total water-content. 



(10) Reaction by increasing water-content. — There seems to be no case in 

 which flowering plants increase water-content as a direct reaction. Their 

 influence in reducing loss by evaporation from the soil is really due to the effect 

 of shading. In the case of Sphagnum, however, the power of the plant to 

 absorb and retain large amounts of rain and dew is a direct reaction of primary 

 importance. Because of this property. Sphagnum is able to waterlog or flood 

 an area and to deflect the sere or initiate a new one. In the moss areas them- 



. selves the effect is essentially to produce a new area of excessive water-content, 

 which can be invaded only as the surface becomes drier. The ability of 

 Sphagnum to retain water, either when living or in the form of peat, is also a 

 controlling factor in the course of the development of the new sere. 



The accumulation of plant remains as humus is the universal process by 

 which the amount of water-content is increased. No plant community fails 

 i» produce humus in some degree; hence no soil escapes its action, though this 

 is often inconsiderable in the initial stage of xerophytic areas. Its influence 



