176 HYDROPHYTES sect, iv 



usually visible immediately beneath the actual surface of the sand. With 

 this plant-community is combined a remarkably peculiar community of 

 animals. This association has an enormously wide distribution along 

 shores of the North Sea, on places where high-sands and flats (aestuaria) 

 are miles in width. 



Closely allied to this subformation is a community of Cyanophyceae 

 and diatoms which is hkewise placed within reach of the tide-water, 

 as it occupies mud-flats and thrives on marshy soil that is flooded by 

 sea- water during the spring tides (aestuarium).^ 



We treat of these sand-algae here as essentially belonging to the 

 benthos of loose soil, but in our classification they have no less claim 

 to be considered amongst both the halophytic and psammophilous com- 

 munities. 



B. Saprophytic Microphyte-formation. 



The vegetation on dead organic accumulations at the bottom of 

 calm water consists of Oscillatorieae, Beggiatoeae, and Schizomycetes, 

 but sometimes also of additional algae whose true home hardly lies here. 

 The accumulations usually lie loose on decaying soil ; Beggiatoa lives 

 in chalky white flocculent masses (beggiatoeta) ; while Clathrocystis 

 roseo-persicina, as well as Bacterium sulphuratum, B. Okeni, and other 

 sulphur-bacteria coloured with bacterio-purpurin, are in red masses. 

 It is in calm inlets with shallow brackish water and accumulations of 

 Fucaceae and other algae, that they specially occur in large quantities 

 and form these associations which are rich in individuals and species.^ 

 The sulphur-bacteria here, as in hot springs, deposit sulphur within 

 their cells ^ ; for by the reciprocal action of the dead organic matter 

 and water there is produced sulphuretted hydrogen, which is absorbed 

 and oxidized by the bacteria, thus giving rise to water and sulphur. 

 According to Sickenberger red sulphur-bacteria play an essential part 

 in the production of soda in the Egyptian saHne lakes. 



Abyssal saprophytic associations. At great depths in seas and lakes 

 where the water is untroubled, the light is often feeble, and the tempera- 

 ture often low, there is frequently a collection of black mud which is 

 filled with decaying bodies, alive with animals (thread-worms), but 

 allows the growth of no highly organized autophytes. Probably there 

 occurs here a rich saprophytic vegetation composed of species of Beggiatoa 

 and other, perhaps mainly, anaerobic bacteria. But we know practically 

 nothing of this vegetation. As an example of a place where there is 

 probably a rich bacterial vegetation we may mention the Black Sea. 

 According to Andrussow,* in this sea, at a depth of 100-600 fathoms 

 and more, one encounters vast masses of mud containing sub-fossilized 

 remains of mollusca, which belong to brackish water and arose in the not 

 distant past when the Black Sea was a brackish lake, but which were 

 exterminated when the Mediterranean Sea forced its way in. The currents 

 are so constituted as to cause defective aeration of the deep water, which 

 is therefore poor in oxygen though rich in sulphuretted hydrogen. It 



' See Warming und Wesenberg-Lund, 1904; Warming, 1906. 



- Warming, 1875. ' As was first shown by Cohn. 



'' Andrussow, 1893. 



M 1 



