Ecological study of brush lake. 155 



of the next zone, since it will grow readily in much shallower 

 water. The inner limit of the zone appears to be determined by 

 the depth of water alone, the plants being able to advance readily 

 so long as the petioles are able to elongate sufficiently to bring 

 the leaf blades above the surface. On the west side there are 

 several small gaps, probably because of the recent origin of the 

 present zone on this side and these gaps are filled with great 

 masses of Chara. Another prominent plant of this zone is 

 Potamogeton natans L,. , which in several places covers considera- 

 ble areas. 



The water-lilies occupying the shallow water are very efficient 

 agents in building up the shore of the lake. The stout rhizomes 

 are able to creep inward and bind the loose soil together, while 

 the petioles and leaves tend to produce a calm where debris may 

 accumulate quite rapidly. 



Among the floating and aerial leaves of the water-lilies and 

 other plants are usually great swarms of duckweeds, especially 

 Spirodela polyrhiza (L,. ) Schl., Lemna minor L,. , and Wolffia 

 columbiana Karst. Riccia fluitans L,. is also common. Riccio- 

 carpus natans (L.) Corda., although occurring in large quantities 

 in the swamps near by, was not collected in the lake. Batrachium 

 divaricatum (Schr.) Wimm. is abundant in some places at the 

 outer edge of the water-lilies, but Ranunculus delphinifolius 

 Torr, common in the swamps and ponds of the region, was not 

 present. 



Other plants collected in the water-lily zone were : 



Sparganium eurycarpum Engel. 



Potamogeton foliosus Raf. 

 " pectinatus L. 



" lucens L. 



Naias flexilis (Willd.) R. & S. 



Zannichellia palustris L. 



Ceratophyllum demersum L. 



Myriophyllum sp. 



3. Half-submerged Zone. This is essentially a Rumex- 

 Polygonum-Typha zone. It is characterized by plants which 

 have their roots, rhizomes, and the lower parts of their erect 

 steins under water, but the larger part of the erect stems is 

 exposed to aerial conditions. It is much more complex and vari- 

 able than the two preceding zones. It extends outwards from 



