172 THE PLANT LIFE OF MAKYLAND 



Nymphaea zone. — In slightly shallower water (2 to 5 feet) there 

 frequently occurs an almost pure growth of the yellow water-lily. 

 Plate XIII, Fig. 1, illustrates a pond near Upper Marlborough, 

 Prince George's County, in which four "islands" of Nymphaea are 

 visible. The localization of these in limited areas must be due to 

 variations in depth of the water, for in shallow ponds Nymphaea 

 generally covers the whole area. The most striking feature of this 

 photograph is the group in the foreground, which may be styled an 

 atoll for it is plainly ring-shaj)ed, probably owing to the cause already 

 mentioned, — that is, greater depth of water in the middle of the 

 group. On Komney Creek, Harford County, Castalia odorata was 

 observed in the outer, deep zone together with Polygonum sp. and 

 Spirodela polyrhiza. 



Pontederia zone. — In still shallower water (1 to 2 feet) Pon- 

 tederia cordata is of very frequent occurrence as a marginal plant, 

 and it is successful as a pioneer along the edges of watercourses 

 which are silting up. On account of its system of rhizomes and 

 numerous roots it is able to hold its place in a moderate current. 

 Good examjdes may be seen along the Anacostia Biver, near Wash- 

 ington. Associated with this is frequently found Sagittaria lati- 

 folia or Sagittaria lancifoUa. 



Zizania zone. — This zone is usually much broader than any of the 

 preceding, and forms areas many acres in extent along such rivers 

 as the Patapsco, the Patuxent (as far down as White's Landing; 

 vide Scofield*) and the Gunpowder. It follows Pontederia on mud 

 flats, hence is usually found a little farther back from the margin 

 of a stream, though it is inundated constantly, to a depth varying 

 with the condition of the tide. On account of the height of this 

 grass it shades out most of its competitors, but on the margin of 

 the stream it is mixed with Pontederia, and in very shallow water 

 it is accompanied by Bidens laevis, Polygonum sagittatum and 

 Sagittaria. This zone is tenanted by numerous birds who use the 

 seed of Zizania as food. 



*Scofield, C. S. U. S. Department ot Agriculture, Bureau of Plant Industry. 

 Bull. 72. 



