188 CLIMAX FORMATIONS OF NORTH AMERICA. 



In half-drained water-bodies, the submerged and floating associes are much 

 mixed, consisting of Potamogeton, Bairachium, Brasenia, Neluvibo, Myrio- 

 phyllum, Ceratoj^yUum, Elodea, Vallisneria, and Naias. The lake is rapidly 

 filled by these, and the amphibious plants advance upon it, led by Sdrpus 

 lacMstris, which is followed by Typha laiifolia, Phragmites, Zizania aqiuiUca, 

 Acorus calamus, Eriophorum, Sagittaria, and Sparganium. Sedges encroach 

 rapidly upon the reed-swamp, and are in turn replaced by grasses, which form 

 a prairie. Among the grasses occur Amorpha canescens, Petalostemon, Baptisia, 

 Eryngium, Phlox, Silphium, Lepachys, Solidago rigida. Aster, Imtris, etc. 

 The prairie may persist almost indefinitely as a subchmax, though there are 

 some signs that it will ultimately become forested. 



Davis (1900, 1901) reaches the following conclusions in regard to the rela- 

 tionship of the algse, especially Chara, to marl or lake lime deposits : 



1. Marl, even of the very white pulverulent type, is really made up of a 

 mixture of coarser and finer matter covered up and concealed by the finer 

 particles which act as the binding material. 



2. The coarser material is present in the proportion of from 50 to 95 per cent. 



3. This coarser material is easily recognizable with the unaided eye and 

 hand lens, as the incrustation produced on the algse, Schizothrix and Chara, 

 principally the latter, down to particles less than 0.01 inch in diameter. 



4. The finer matter is largely recognizable under the compound microscope 

 as crystalline in structure, and is derived from the algal incrustation by the 

 breaking up through decay of the plants of the thinner and more fragile parts, 

 or by disintegration of the younger parts not fully covered. 



5. Some of this finer matter is capable of remaining suspended in water a 

 sufficiently long time, after being shaken up with it, to make it unnecessary 

 to advance any other hypothesis to explain the turbidity of the waters of some 

 marlj lakes than that it is caused by mechanical stirring up of the marl by 

 waves or other agency. 



6. Shells and shell remains are not important factors in the production of 

 the marl-beds which are of largest extent. 



7. There is in marl a small amoimt of a water-soluble calcium salt, readily 

 soluble in distilled water, after complete evaporation. 



Lange (1901 : 621) has studied the colonization of an island formed in Lake 

 Phalen, Minnesota, as a consequence of the filling in of a trestle, which caused 

 the soft marl of the bottom to rise from 1 to 10 feet above low-water level: 



In June, the west portion consisted of bare mud, while the east showed 

 plants of Nymphaea, Typha, Alisma, and Sdrpus. By July, the latter were 

 in distress, and the area was covered with a large number of ruderals, such as 

 Trifolium, Brassica, Anthemis, Solanum, Polygonum, Phleum, etc. By Sep- 

 tember the east section had become a wilderiiess of weeds, containing many 

 yoimg wiUows, and with Zizania, Lycopus, Scutellaria, and Mentha in wet or 

 moist places. The west section was covered with an open growth of cotton- 

 wood, aspen, and willow. By midsummer of 1899 the west part was a thicket 

 of Salix amygdaUddes, 10 to 12 feet high, which concealed the cottonwoods 

 and aspens. On the east section, the Zizania community had yielded to 

 Leersia, and Nymphaea, Zizania, and Sdrpus had disappeared from the higher 

 areas. Polygonum incarnatum had become dominant in some portions, and 

 Salix in others. Of the 55 species found more than half were ruderal or sub- 

 ruderal. In 1900 the annuals began to yield to perennials, especially Solidago, 



