116 



Bacill(U'ie& 



and arctic latitudes. Mountain ranges always yield a good diatom-flora. 

 A few diatoms are inhabitants of warm and hot springs, and have been 

 obtained in a living state at a temperature of certainly as high as 55 C. Some 

 species are cosmopolitan, occurring in all parts of the world, but there are on 

 the contrary many species and genera which occur only in certain seas and 

 climates. The same is true to a lesser extent of freshwater forms, and when 

 these forms are more fully and accurately investigated from a systematic 

 point of view, many marked geographical peculiarities will doubtless be 

 brought to light. 



The free and unattached diatoms often form a yellow-brown scum at the 

 surface of the water or on the sediment at the bottom of shallow pools. 

 They also occur in abundance among the leaves of aquatic phanerogams 

 or among the branches of larger algas. Many species are epiphytic, being 

 attached to the thallus of larger algae. In fresh waters the filaments of 

 various species of Cladophora, Rhizoclonium, and Vaucheria are frequently 

 thickly covered with Cocconeis Placentula or C. Pediculas (fig. 84), and often 



Fig. 84. A small portion of a filament of Rhizoclonium hieroglyphicum Kiitz. largely covered 

 with the epiphytic diatom Cocconeis Pediculus Ehrenb. x 375. Only the outlines of the 

 cells are shown. 



with Epithemia turgida. Several minute species of Achnanthes are also 

 epiphytes of this nature. In the sea the genera Isthmia, Grammatophora, 

 Rhabdonema, Licmophora, and others are epiphytes on the smaller seaweeds. 

 Numerous diatoms pass the whole of their existence free-floating in the 

 surface-waters of the sea, or of lakes and large rivers. These are the plankton- 

 diatoms which occur in prodigious quantity in the cold waters of the Arctic and 

 Antarctic Oceans, and to a lesser degree in the warmer oceans. The marine 

 plankton-diatoms are almost exclusively centric, and some of the principal 

 genera are Chtetoceras, Biddulphia, Thalassiosira, Coscinodiscus, Rhizosolenia, 

 and Dytilum. The freshwater plankton-diatoms, on the other hand, are 

 mostly pennate in character, consisting largely of species of Tabellaria, 

 Asterionella, Synedra, Fragilaria, Nitzschia, and Surirella. The only centric 

 freshwater genera of importance are Melosira, Cyclotella, and Rlnzosolenia. 

 The plankton-diatoms form the principal part of the food of countless fresh- 

 water and marine animals. They are the most important ' producers ' of 



