100 J- E. CRIBBS 



vented from a rapid advance to a more mature stage by the 

 seepage of water into them from the surrounding morainic 

 clays, or because they are fed by springs. Upon the unglaciated 

 area east of the Allegheny River the topography is rugged, the 

 gradient, steep, and the drainage system more efficient. Hence 

 the development of swamps there is quite uncommon. Glacial 

 lakes which have not advanced far in the process of reclamation 

 frequently show four distinct stages of development, namely — 

 the Aquatics, the Open Swamp, the Shrub Stage, and the Swamp 

 Forest, the last exhibiting Deciduous and Evergreen Types. 

 While these more or less distinct regions are arranged horizont- 

 ally and in a concentric fashion, they really represent successive 

 phases in the vertical sequence of the plant series. 



The Aquatics 



The aquatic species are the first to infringe upon both lakes 

 and streams. Because of their habit of growth they may be 

 subdivided into three groups, namely — Submerged Aquatics, 

 Floating Aquatics, and Aero- Aquatics. These are regularly 

 distributed in zonation, in the order given; and constitute an 

 assemblage of plants well adapted, because of their abundant 

 developeent, to fill up shallow water to a stage which admits of 

 the entrance and development of aerial species. 



Submerged Aquatics. The submerged aquatics advance far 

 out into the lake. They are attached species with vegetative 

 structures entirely beneath the water surface, but with flowers 

 which develop at the surface. They are mostly annuals, and 

 because of this fact, by their decay in the fall, add yearly to 

 the humus, which by gradually collecting on the bottom raises 

 it's level to a position near the water's surface; thus affording a 

 condition which is conducive to the entrance of the succeeding 

 stages. Among the species encountered here Elodea canadensis 

 is the most characteristic both because of it's early entrance 

 and prolific development. The following species are also promin- 

 ent members in this stage: Najas flexilis, Najas guadalupensis . 

 Myriophyllum heterophyllum, Ceratophyllum demersum, Nitella, 

 and Ranunculus aquatilis. 



