156 HYDROPHYTES sect, iv 



Trichodesmium, including T. erythraeum (which colours the water red, 

 and occurs in the Red Sea, also in other seas, but especially near the 

 coast); Nodularia spumigena (which causes a greenish-grey colour, and 

 is common and even extremely abundant in the Baltic Sea) ; HeUotrichum 

 (in tropical parts of the Atlantic Ocean) ; in fresh water there are species of 

 Anabaena and Polycystis, Aphanizomenon fios aquae, Oscillatoria rubescens 

 (especially in mountain-lakes), Coelosphaerium kuetzingianum, Gloeo- 

 trichia echinulata, and others that give to water a sap-green or bluish- 

 green tint, and a peculiar aroma. The majority of them are genuine 

 plankton-organisms which are found floating below the surface, but 

 when the water is quite undisturbed swim in numbers on its surface, 

 just like cream on milk. Klebahn and Strodtmann^ have minutely 

 investigated certain small irregular bodies in their cells, and express the 

 opinion that these are air-containing spaces within the protoplasm 

 and enable the Algae to ascend ; on the contrary, H. Molisch and R. 

 Fischer 2 maintain that the corpuscles are not air-vacuoles, but are com- 

 posed of a peculiar substance of undetermined nature. Fischer and 

 Brand 3 come to the conclusion that they have nothing whatever to do 

 with the power of flotation. Ripe spores contain no air-vacuoles, and 

 they sink. 



2, Schizomycetes may be mentioned after the Cyanophyceae. They 

 are found in the ocean even far from land down to depths of 800-1 loo 

 metres, and in considerable numbers at depths of 200-400 metres. One 

 and the same species shows great variability in form and size. The 

 Schizomycetes concerned are motile, and most of them are of the spiral 

 type, while some are luminous. In lakes the number of bacteria present 

 shows the widest differences, as it varies between a few and many thou- 

 sands of individuals to each cubic centimetre. The pelagic region of most 

 lakes has the fewest.* In Zurich Lake there were at a depth of 80 metres 

 28-30 per cubic centimetre, in Lake Constance at a depth of 60-65 metres 

 31-146. The number present is smallest at the surface, and larger in 

 the somewhat deeper strata. In the opinion of some investigators the 

 bacteria at the surface are killed by light, but according to others the 

 larger number of bacteria in the deeper strata of water is due to the 

 greater abundance of decaying organic matter (dead plankton). 



Among Schizomycetes nitrifying and denitrifying organisms are of 

 special significance in regard to metabolism in water, as they oxidize 

 ammonia to nitric acid, or reduce any excess of the latter to nitrogen. 

 Brandt ^ has put forward the hypothesis ^ that the greater activity of 

 denitrifying bacteria in warm seas is the cause of plankton here being 

 poorer than in colder seas. 



3. Diatomaceae occasion brownish or greenish ^ tints in water, 

 especially in arctic seas, where they occur in huge masses composed of 

 countless individuals, particularly belonging to the genera Thalassiosira, 

 Chaetoceras, Rhizosolenia, Coscinodiscus, and Thalassiothrix. In fresh 

 water there occur Melosira (especially in lowland lakes), CycloteUa 



^ Klebahn, 1895 ; Strodtmann, 1895. "" R. Fischer, 1904, 1905. 



" Brand, 1905. * Forel, 1878, 1901 ; Schroter, 1897. 



* K. Brandt, 1904. * See also H. H. Gran, 1905, and Reinke, 1904. 



' In regard to greenish water in the North- Atlantic Ocean, see K. J. \\ Steen- 

 strup, 1877. ' See Gran, 1905. 



