35 2 George E. Nichols, 



their vegetation. But, on the whole, aquatic plants are apt to 

 be relatively more abundant here by reason of the lesser depth of 

 the water, its comparative quietness, etc. Largely because of the 

 absence of any appreciable amount of wave action, the shores 

 of small ponds tend to be more muddy than those of the larger 

 bodies of water. The vegetation of sandy shores is similar to 

 what has been described above, and the same is true in general 

 of muddy shores. Here, however, there is often a rank growth 

 of cat-tails (Typha latifolia) and bulrushes (Scirpus occidentalis, 

 S. cyperinns, S. atrocinctus, etc.), through the activity of which 

 the pond tends to become filled in and converted into a swamp. 



Sink-hole ponds frequently exhibit the phenomenon of marl- 

 formation (see Nichols '15, pp. 194-196). In such ponds there 

 is usually a luxuriant growth of Chara, one of the most impor- 

 tant marl- forming plants, and of various algae. Among the 

 prominent aquatic seed plants here may be Potamogeton pecti- 

 natus and P. pusilhis. Leaves and stems of all submersed forms 

 are usually incrusted with a thin, whitish, flaky deposit of marl. 



The association-types of periodic ponds. — Periodic ponds are 

 not sharply delimited from permanent ponds on the one hand 

 or from periodic swamps on the other. Very shallow depres- 

 sions, which during the growing season contain water for only 

 a brief period, are commonly occupied by a rank growth of 

 such species as Scirpus cyperiniis and 6'. atrocinctus, Juncus 

 effusus and /. hrevicaudatus, and Iris versicolor. In the case of 

 ponds which disappear completely only for a short period dur- 

 ing the summer, there may be a striking concentric zonation of 

 plant associations. In one instance, for example, the wetter cen- 

 tral area is largely occupied by the moss, Amblystegium riparium. 

 Proceeding from here toward high water level there are 

 encountered (i) a zone of more or less amphibious species such 

 as Sparganium americanum, Juncus effusus, Ranunculus Flam- 

 mula reptans, Hypericum canadense, Lysimachia terrestris, and 

 Sium cicutae folium; (2) a zone of Iris versicolor; (3) a zone 

 of Alnus incana. Elsewhere Eleocharis palustris and the species 

 cited earlier in this paragraph may be prominent as marginal 

 plants, while in some cases the liverwort, Marchantia poly- 

 morpha, develops profusely on the muddy shores of periodic 

 ponds. 



