402 



ECOLOGY. 



water. The second zone then is in the water shown in the fore- 

 ground of the picture. The plants here are also submerged, or 

 only a small portion reaches the surface of the water, and so the 



Fig. 495. 

 Sagittaria heterophylla. Often forms a zone just outside of the Sagittaria variabilis. 



zone does not show. In this zone occurs the curious Vallesneria 

 spiralis, with its corkscrew flower stem, and various potamoge- 

 tons. 



715. In the third zone, or the first one which shows in the 

 picture, are great masses of the arrow-leaf (sagittaria) so variable 



