PLANT ASSOCIATIONS OF WESTERN PENNSYLVANIA 101 



Floating Aquatics. The zone of submersed aquatics, which 

 constitutes the pioneer stage, is not usually sharply defined 

 from that of the floating-aquatics but overlaps it to a con- 

 siderable extent. 



To this latter group belong not only the few species which 

 float freely upon the surface such as Spirodela polyrhiza, and 

 Lemna minor, but also those which develop with roots or rhi- 

 zomes anchored at the bottom and send up leaves which spread 

 out flat upon the water, permitting the direct exchange of 

 gases through their upper surfaces. Many of this assemblage 

 bear two kinds of leaves, the second type being submerged and 

 more finely divided, or smaller than the floating ones. To this 

 zone, besides those already cited, belong the following members: 

 Nymphaea advena, Potamogeton natans, Potamogeton amplexi- 

 folius, Potamogeton angustifolius, and Polygonum amphibium. 



Always in association with these, there are found some of the 

 submersed species which persist in the shallower water. 



Aero- Aquatics. Following the stage which is dominated by 

 aquatics with floating leaves, occurs a region which is variable 

 in extent — the aero-aquatic zone, or marsh. This stage appears 

 in very shallow water, and the characteristic members develop 

 aerial vegetative leaves which rise from roots or rhizomes that 

 are commonly submerged. 



Many of the forms developmg here are quite xerophytic in 

 texture and general appearance. This is in part due to the 

 intensity of the light, both incipient and reflected; partly to the 

 exposure to desiccating winds, and partly to the lack of cor- 

 relation in root and shoot development. Among the species 

 commonly encountered here may be mentioned, Typha latifolia, 

 Scirpus americana, Scirpus validus, Polygonum Hydropiper, and 

 Slum cicutaefolium. 



The Open Swamp 



The marsh stage is seldom sharply defined but grades off into 

 the open swamp which more commonly occupies extensive areas. 

 In fact the larger number of western Pennsylvania swamps have 

 advanced beyond the aquatic stages, but retain the open swamp 



