BACILLARIE^ 



(Diatoms) 



THE Bacillariese include a very large number of unicellular Algse 

 commonly known as Diatoms. They are mostly of minute size, and owing 

 to the beautiful sculpture of their cell-walls they very early attracted the 

 attention of microscopists. Investigations extending over the greater part 

 of a century, and in almost all countries of the world, have resulted in the 

 description of some 12,000 species; but it is only during recent times that 

 these plants have been studied from a biological standpoint. 



In such a large group of unicells it is not surprising to meet with a great 

 variety of external form, and the sculpture of the walls is no less varied ; 

 but at the same time all diatoms possess such salient features that the 

 observant student can scarcely fail to recognize a member of this group. 



Diatoms are equally abundant in both fresh and salt water, and numerous 

 fossil forms are known. As a rule, the species of marine diatoms are entirely 

 different from those which inhabit fresh water, but there are a number 

 of typically brackish-water species, found principally in estuaries of rivers 

 and in brackish marshes, which sometimes occur in marine, and more rarely 

 in freshwater, situations. 



One of the leading characteristics of the group is the siliceous nature 

 of the cell-wall, which consists of an organic matrix combined more or less 

 closely with silica. The durable nature of this cell-wall accounts for the fact 

 that so many forms have been preserved in a fossil state, there being far 

 more fossil representatives of the Baeillariea? than of any other group of 

 algae. The silica can be removed by the action of hydrofluoric acid leaving 

 the organic matrix behind ; or the organic matrix, which according to 

 Mangin ('08) consists of pectic compounds, can be removed by calcination 

 leaving behind the siliceous constituent. 



The individual diatom-cell, which in the language of the diatomologist 

 has long been known as a frustule, possesses a segmented cell-wall of an 

 almost unique character. It consists of four or more segments, the two 



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