156 ZONATION 



feature is more or less circular in form, and it is bilateral, 

 when the latter is elongated or linear. Vertical zonation 

 is unilateral. 



Eadial zonation is regularly characteristic of elevations 

 and depressions. Prom the form of the earth, it reaches 

 its largest expression in the girdles of vegetation corres- 

 ponding to the zones of temperature. The zones of 

 mountain peaks are likewise due largely to temperature, 

 though humidity is a very important factor also. Mountain 

 zones are normally quite perfect. The zonation of islands, 

 hills, etc., is due to water content. In the former, the 

 zones are usually quite regular and complete; in the latter, 

 they are often incomplete or obscured. Prairies and 

 steppes are not zoned as units, but are complexes of more 

 or less zonal hills and ridges. Ponds, lakes and seas 

 regularly exhibit complete zones, except in those shallow 

 ponds where the depth is so slight that what is ordinarily a 

 marginal zone is able to extend over the entire bottom. The 

 line between an elevation and a depression, i. e., the edge 

 of the water level, is the most sharply defined of all 

 ecotones. It separates two series of zones, each of which 

 constitutes a formation. One of these is regularly hydro- 

 phytic, the other is usually mpsophytic. The line between 

 the two can rarely be drawn at the water's edge, as this is 

 not a constant, owing to waves, tides, or periodical rise and 

 fall. There is in consequence a more or less variable trans- 

 ition zone of amphibious plants, which are, however, to be 

 referred to the hydrophytic formation. Nearly all forest 

 formations serve as a center about which are arranged 

 several somewhat complete zones. As a rule, these merge 

 into a single heterogeneous zone of thickets. 



Bilateral zonation differs from radial only in as much as it 

 deals with linear elevations and depressions instead of 

 circular ones. With this difference, the zones of ranges and 

 ridges correspond exactly to those of peaks and hills, while 

 the same relation is evident between the zones of streams, 

 and of lakes and ponds. The ecotones are identical except 

 as to form : they are linear in the one, and circular in the 



