300 



PLANT PHYSIOLOGY AND ECOLOGY 



of the water-level, is a sharply defined ecotone. It separates 

 two series of zones, each of which constitutes a formation. One 

 of these is regularly hydrophytic, the other usually mesophytic. 

 The line between the two can rarely be drawn at the water's edge, 

 as this is not constant, owing to waves, tides, or periodical rise 

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

 tion zone of amphibious plants, wliich belong properly to the 

 hydrophytic formation. j\lany forest formations serve as a center 

 about which are arranged several incomplete zones. 



Fig. 123. RacUal zonation of Sparganium anguatifolium in one of the 



Seven Lakes. 



329. Bilateral zonation differs from radial only because it is 

 produced by linear elevations and depressions rather than cir- 

 cular 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 in form; they 

 are linear in one, circular in the other. Incompleteness is more 

 frequent in bilateral zonation, though this is due largely to the 

 length of l^ilateral zones. 



