PLANT ASSOCIATIONS OF WESTERN PENNSYLVANIA 109 



As previously stated, the most conspicuous feature is the shrub 

 stage and the strong hummock development associated with it. 

 The aquatic stages may be almost entirely absent; Lemna minor, 

 Spirodela polyrhiza, and certain algae, being the species present 

 when there is any representation. The reason for the tendency 

 to eliminate the aquatics is perhaps to be found in the acidity 

 of the water. The decomposition of the leaves sets free the 

 chemical materials formed and deposited there as byproducts 

 during their activity in summer. These compounds are largely 

 acid, and are detrimental to the development of aquatic species; 

 or if present in sufficient quantities may entirely inhibit such 

 development. 



Upon the hummocks the chief shrubby representatives are 

 Cephalanthus occidentalis , Ilex verticillata, Pyrus arbutifolia, 

 Cornus stolonifera, Viburnum cassinoides, and Vaccinium corym- 

 bosum. In addition to these they may be occupied by Osmunda 

 cinnamo7nea, Osmunda- regalis, Woodwardia virgi?iica, Onoclea 

 sensibilis, Aspidium spinulosum, Cypripedium acaule, and a 

 few others. Woodwardia areolata is a rare component in the 

 flora here, although never reported from western Pennsylvania. 



The following tree species appear upon the margins of such 

 ponds and succeed the shrubs, Nyssa sylvaiica, Quercus palus- 

 iris, and Quercus bicolor. During the replacement of the shrubs, 

 pure stands of either Quercus bicolor or Quercus palustris may 

 occur. Pure stands of this kind are of very frequent occurrence. 

 Later Acer ruhrum and Fraxinus nigra may make an appearance. 

 From this condition there is a direct passage to the mesophytic 

 forest types. 



THE LAKE-FOREST SERIES 



While the swamp series most commonly occurs in the natural 

 reclamation of western Pennsylvania lakes there is another 

 sequence giving rise to associations of a different nature, which 

 terminates more rapidly in the mesophytic forest of the region. 

 This order is found where there is more uniform and complete 

 drainage than in the cases of swamp successions. This elimin- 

 ates to a large extent the annual flooding of the region, due to 



