28o THE FORMATION 



the fact that conditions are unfavorable to the facies, outside of the nucleus 

 as well as beneath it. Floristically, each layer and its corresponding zone 

 are distinct, as the one consists of shade, the other of sun species. Lateral 

 zonation is radial when the habitat or physiographic feature is more or less 

 circular in form, and it is bilateral, when the latter is elongated or linear. 

 Vertical zonation is unilateral. 



338. Radial zonation is regularly characteristic of elevations and de- 

 pressions. From the form of the earth, it reaches its larger expression in 

 the girdles of vegetation corresponding 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 con- 

 stitutes a formation. One of these is regularly hydrophytic, the other is 

 usually mesophytic. 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 transition 

 zone of amphibious plants, which are, however, to be referred to the hydro- 

 phytic formation. Nearly all forest formations serve as a center about which 

 are arranged several somewhat complete zones. As a rule, these merge 

 :nto a single heterogeneous zone of thickets. 



339. Bilateral zonation differs from radial only in as much as it deals 

 with hnear elevations and depressions instead of circular ones. With this 

 diflference, 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 other. Incompleteness is 

 more frequently found in bilateral zonation, though this is a question of 

 distance or extent, rather than one of symmetry. 



SAO. Vertical zonation is peculiar in that there is no primary ecotone 

 present, on cither side of which zones arrange themselves with reference to 



