280 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. 7 
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 
into a single heterogeneous zone of thickets. 
339. 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 other. Incompleteness is 
more frequently found in bilateral zonation, though this is a question of 
distance or extent, rather than one of symmetry. 
340. Vertical zonation is peculiar in that there is no primary ecotone 
present, on either side of which zones arrange themselves with reference to 
