112 STRUCTURE AND UNITS OP VEGETATION. 



"The succession of these different zones, from the border of the water 

 towards the original border of sand, represents the succession of changes that 

 have taken place through time in each of the anterior zones, so that in propor- 

 tion as the reeds advance, new zones are forming behind the advancing reeds 

 on the same places which they thus abandon." 



It has not been recognized that it reveals a basic and a universal principle. 

 It is just as true of the climax formations of a continent, with zonal disposi- 

 tion in accordance with latitude and altitude, as it is of the zones of a lake or 

 river or those of hill or ridge. The latter are zones of actual succession, the 

 stages of existing seres; the former are zones of potential succession, and indi- 

 cate the further stage of development in the event of a change of climate. 

 Both are possible stages of the same great development, and are equally con- 

 trolled by the gradual change of conditions, though the change in one case is 

 climatic, in the other edaphic. 



--— -^^^ *■ ^___^ - y^adgeland 



^^\~l~^]^^^^^l Submerged 



Fig. 3. — Schematic representation of the development of the hydroser, showing 

 identity of zones and serai stages. Straight lines indicate zones, and dotted 

 ones their extension across the pond as the latter is shallowed. 



The intrinsic relation between zones and stages is best proved by the zona- 

 tion about water, on account of the relatively rapid decrease in water-content. 

 It is equally well shown by areas with rapid increase about a dry center, such 

 as islets in the lakes of arid regions, but these are relatively infrequent. Ponds 

 and streams with gently sloping margins often show at one time a complete 

 series of zones representing the successive stages of development to the climax 

 association. The relation of these zones in time is clearly demonstrated by 

 projecting them across the water center, as is seen in figure 3. Such a pro- 

 jection occurs by degrees during the course of development, until the center 

 is occupied in the proper sequence by every stage from the submerged to the 

 climax community. The proof of this is found in practically all peat deposits, 

 but especially in those where the development has been gradual and complete. 

 The actual extension of the various zones over the water-body or a portion of 

 it occurs when a pioneer or subpioneer community, such as a Sphagnetum, 

 develops as a floating mat which becomes anchored at the bottom or the side. 

 Such seres furnish the complete demonstration of the identity of zones and 

 stages, and also serve to emphasize the fact that every zone has a temporal 

 as well as a spatial relation, and hence is the result of development (plate 

 33, A, b). 



The filling by reaction of a pond or lake with a uniformly shallow bottom 

 and abrupt banks is of especial significance in correlating climatic zones with 

 edaphic ones. In such ponds, which are typical of the prairie region, the 

 spatial relation is over-emphasized, the temporal relation obscured. It not 

 infrequently happens that there is complete unconformity between the pond 

 commimity and the climax vegetation in which it occurs. In a word, the 

 usual zones are lacking, since there is no gradual shallowing of the water 

 toward the climax area. The consequence is that each stage, instead of form- 



