Ross: Classification of Diatoms 409 



Summary 



Our present knowledge of the ultrastructure of diatoms covers only a very 

 small proportion of the total number of species, and some of the published infor- 

 mation is inadecjuate or misleading. The variations in types of ultrastructure 

 found do not, in a number of cases, correspond with the current classification, 

 which is based almost entirely upon characters of the valve as seen under the 

 light microscope. On the other hand, the observations made with the light 

 microscope have not been contradicted by work with the electron microscope. 

 For these reasons it has been contended that ultrastructure does not provide 

 information that can be used in diatom taxonomy. This view is criticized. 



If the characters of the ultrastructure are to be used in diatom taxonomy, 

 information about most species in a group is needed. As many species are 

 often sparsely represented in gatherings, individual specimens need to be 

 selected and mounted for examination in the electron microscope. A technique 

 is described. 



In a study just completed, electron microscopy has confirmed that a small 

 group should be removed from Stauroneis and placed in a separate genus. A 

 continuation of this work now in progress points to the need for further division 

 of Stauroneis, and there are indications that ultrastructure may provide infor- 

 mation that will assist in a revision of the present unsatisfactory generic classi- 

 fication of the Naviculaceae. In such a revision the characters of the frustule 

 structure as seen under the light microscope, of the ultrastructure, and of the 

 cell contents should all be given equal weight. It is, therefore, necessary to 

 obtain information about the ultrastructure and cell contents of a large propor- 

 tion of the species in the family: a prerequisite for this is the development of 

 a technique for preparing specimens in such a way that both their cell contents 

 and the structure of their frustules can be studied. 



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