150 THALLOPHYTA. [CH. 



definite families, and accepted as evidence possessing some 

 botanical value. 



A. DIATOMACBAB (BACILLARIACEAE). 



This family occupies a somewhat isolated position among the 

 algae, and is best considered as a distinct subdivision rather 

 than as a family of the Phaeophyceae or Brown algae, with 

 which it possesses as a common characteristic a brown-colouring 

 matter. 



Single-celled plants consisting of a simple protoplasmic body 

 containing a nucleus and brown colouring matter (diatomin) 

 associated with the chlorophyll. The cell- wall is in the form of 

 two halves, known as valves, which fit into one another like the 

 two portions of a pill-box. The cell-wall contains a large 

 amount of silica, and the siliceous cases of the diatoms are 

 commonly spoken of as the valves of the individual, or the 

 frustules. Diatoms exhibit a characteristic creeping movement, 

 and are reproduced by division, also by the development of 

 spores in various forms \ 



Th^ recent members of the family have an exceedingly wide 

 distribution, occurring both in freshwater and in the sea. 

 Owing to the lightness of the frustules, they are frequently 

 carried along in the air, and atmospheric dust falling on ships 

 at sea has been found to contain large numbers of diatoms'. 

 The siliceous valves are abundant in guano deposits, and they 

 have been found also in association with volcanic material. 

 Diatomaceous deposits are now being formed in the Yellowstone 

 Park district ; " they cover many square miles in the vicinity of 

 active or extinct hot spring vents of the park, and are often 

 three feet, four feet, and sometimes five to six feet thick'.'' 

 The gradual accumulation of the siliceous tests on the floor of a 

 fresh-water lake results in the formation of a sediment consisting 

 in part of pure silica. Such deposits, often spoken of as kieselguhr 



^ A monograpli on the Diatomaceae has recently been written by Sohiitt for 

 Engler and Prantl's systematic work. See also Murray, G. (97) and Pfitzer (71). 

 2 Darwin (90) p. 5. s Weed (87). 



