DEVELOPMENT AND STRUCTURE. 113 



ing a zone as normally, occupies the whole area for a longer or shorter period 

 in the usual sequence of succession. It shows no organic connection with the 

 climax association until the development is completed, and in itself furnishes 

 no direct evidence of succession. In fact, when the area occupied by such a 

 community is large, it simulates a climax association. Apart from its resem- 

 blance to other communities whose development is known, its real nature 

 can be ascertained only by actually following the sequence of stages or by 

 probing the deposits of plant remains. As a matter of fact, water seres are 

 too well understood to cause difBculty in this connection, and the illustration 

 is of importance only because it clarifies the developmental relation of the 

 great climatic zones. The latter also seem to have no successional connec- 

 tion, but this is only a seeming, as has already been indicated under potential 

 climaxes. An effective swing of climate at once places each climax area in 

 successional articulation with an adjoining one, and reveals its essential natxu'e 

 as a developmental zone. It has been shown above that the climax changes 

 of the glacial and the postglacial periods not only transformed climax zones 

 into successional stages, and the reverse, but also that it left a record of such 

 zonal stages in the layers of peat-bogs. 



The zonation of hiUs and ridges in the prairie formation is typical of the 

 relation between structure and development. Owing to the more or less 

 uniform nature of grassland, distinct zones are rarely evident, but a careful 

 scrutiny shows that a majority of the societies are in zonal relation. Exposed 

 rocky crests bear lichen colonies, about which are xerophytic communities 

 of Lomatium, Comandra, Meriolix, and others. Middle slopes are occupied 

 by more mesophytic species, such as Astragalus, Erigeron, Psoralea, etc., and 

 the bases and ravines by meadow species, especially grasses and sedges. In 

 some of the ravines small marshes or ponds develop and add one or more 

 zones to the series, while in others, thickets of Salix, Rhus, or Symphoricar'pus 

 appear, making possible the invasion of woodland herbs, and the occasional 

 entrance of Populus or Fraxinus. Imperfect as the zonation of the prairie is, 

 it furnishes an indubitable record of the development of the association from 

 xerophytic ridge communities on the one hand and from ravine communities 

 of meadow and marsh on the other. In addition, the ravine thickets suggest 

 the fate of the prairies when confronted by an increase of rainfall, or when 

 artificial barriers to the spread of woodland are withdrawn. 



The zonation of fringing forests is perhaps best seen in prairie and plains 

 regions, owing to the fact that the decrease of water-content from the edge 

 of the stream to the dry grassland takes place rapidly. In addition, there is 

 a similar rapid decrease of humus and increase of light intensity. In many 

 places actual zones are lacking or fragmentary, owing to local conditions; in 

 others, the complete series may find expression. In the Otowanie Woods 

 near Lincoln, Salix, Populus, and Ulmus either indicate or constitute narrow 

 zones from the water to the oak-hickory climax. In the direction of the grass- 

 land, Fraxinus, Rhus, and Symphoricarpus constitute as many zones in more 

 level areas, while on steep slopes only a narrow band of thicket may occur, or 

 the scrub oak (Quercus macrocarpa) may gradually dwindle into a shrub but 

 a foot or two high. 



Relations of climax zones. — Like aU zones, climatic ones are due to a gradual 

 change in the amount of one or more controlling factors. They differ from 



