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, 
uc pectinatus L. 
es 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 
stems 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 
