i62 HYDROPHYTES sect, iv 



temperate North America that we are well informed. Apparently great 

 similarity prevails everywhere. In the northern part of the temperate 

 zone and in the more southern alpine lakes diatoms dominate during 

 most of the year. In the flat countries of Central Europe it has been 

 observed that nearly everywhere there is a regular alternation of 

 diatom-plankton during the cold months, and Cyanophycea-plankton 

 during the summer. In not a few lakes an almost monotonous plankton 

 is formed during spring and summer by Flagellata, especially by Ceratium 

 hirundinella, and Dinobryon.^ The maxima of the respective associations 

 depend upon the temperature of the water, and other consequent changes 

 in it ; moreover, light and the amount of certain nutritive substances 

 in the water have their influence. ^ 



C. Saproplankton. Foul- water Plankton 



In this sub-formation is included vegetation consisting of Flagellata 

 such as Euglena viridis and E, sanguinea, of species like the colourless 

 Polytoma uvella, also of various Cyanophyceae and Schizomycetes. 

 Such vegetation generally occurs in small pools of stagnant water that 

 is rich in putrefying organic matter but very poor in oxygen, as is the 

 case with water (manure-water, puddles in roads) near human dwellings 

 which may be distinctively coloured. The water is usually very green 

 and sometimes stinking. The green organisms, Chlamydomonadae and 

 others, presumably assimilate carbonic dioxide, and obtain nitroge- 

 nous compounds as well as other nutriment from organic constituents 

 present in the water ; they are therefore probably hemisaprophytes. 

 Euglena sanguinea and others cause a red colour, and are most fre- 

 quently motile. Among the saprophilous organisms in such water are 

 many Infusoria. The farther putrefaction has proceeded the better do 

 chlorophyU-containing plants, such as Scenedesmus, Raphidium, and 

 diatoms, flourish therein.^ The ' self -purification ' of rivers, after the 

 water below large towns has been polluted, is due to the activity of 

 bacteria and other microphytes ; the process can be traced right to the 

 stage when the water is devoid of organic substance. Sometimes the 

 final product assumes the form of sulphide of iron, which is a constituent 

 of black mud. Schenck * investigated the Rhine between Bonn and 

 Cologne and came to the conclusion that Green Algae play no great part i 

 in this process, and that filamentous and rod-shaped Schizomycetes 

 absorb the organic substances.^ 



* Wesenberg-Lund, 1904-8. ^ Whipple, 1894, 1896. 

 ' See Kolkwitz und Marsson, 1902; Volk, 1903; Marsson, 1907-8. 



* Schenck, 1893. 



* An immense Uterature deaUng with plankton has sprung up within recent : 

 years : among the investigators may be mentioned Gran and Wille in Norway, 

 Cleve in Sweden, Ostenfeld, Ove Paulsen, and C. Wesenberg-Lund in Denmark, 

 Apstein, Hensen, Brand, Zacharias, Chun, Haeckel, G. Karsten, Kirchner, Lohmann, 

 and Schiitt in Germany, Kofoid in North America, Chodat, Bachmann, G. Huber 

 and Schroter in Switzerland. See also literature quoted by Oltmanns, 1905. 



