28 ALFRED DACHNOWSKI 



basins have a zone of submerged vegetation with more or less 

 distinct alternation and layering. Ordinarily Ceratophyllum de- 

 mersum comprises the main part of the association in abundance 

 of individuals. Potamogeton species (P. pectinatus, P. lucens, 

 P. natans) are next in importance as principal and dominant 

 species. There is great variation in the number of species and 

 the order of their importance; no attempt is made here to give 

 a complete list. Upon decay the finer detritus reaches the deeper 

 and remoter parts of the lake bottom, filling it with a soft oozy 

 mud. Nearer the margin the debris is often greenish and dis- 

 tinctly laminated. 



2. Semi-aquatic or amphibious associations. The predominat- 

 ing species are the white and yellow water lilies (Castalia tuberosa, 

 Nymphaea advena) and the knotweeds (Polygonum muhlenbergii, P. 

 hydropiperoides, P. amphibium) . There is no sharp limit between 

 the several associations and those preceding and following them, 

 hence the individual zones contain species of secondary impor- 

 tance in great variety. 



3. Floating association. Some of the constant representa- 

 tives are the duckweeds {Lemna minor, L. trisulca, Spirodela 

 polyrhiza), liverworts, and mats of algae. They occur among the 

 members of the semi-aquatic associations. Clearly defined soci- 

 eties do not exist in this association as the plants are easily trans- 

 ported by wind and currents. The association corresponds in 

 point of vertical zonation to the ground layer in forests, but 

 light does not constitute the controlling factor. In sheltered 

 and enclosed pools, where the decay of the plant debris is incom- 

 plete, there is noticeable in the duckweeds a difference in root 

 production, in amount of frond, and in rate of reproduction. 

 This condition indicates a growing season that is shortened by 

 the limitation of the available water supply and by struggles with 

 the reactions exerted upon the habitat by each stage of the vege- 

 tation cycle. 



III. SHORE SUCCESSION 



The plants that follow as the filling-in of the water basin 

 continues, are either pioneers of the land vegetation or of the 



