ELECTRON AllCHOSCOI'Y 



Fig. 6. Shadowed carbon replica of the calcite 

 scales of a species of the family Coccolithophorida- 

 ceae, X 10,000. 



phyccae. The first of these to be described 

 here, CoccoHthophoridaceae, is of wide interest 

 since the unicellular organisms form minute 

 calcite scales. These scales form sedimentary 

 chalk deposits. They are deposited on the 

 ocean bed after the cells have died and the 

 protoplasm has disintegrated. There is a very 

 large number of species in the family and 

 classification is a difficult matter when the 

 light microscope is used because of the lim- 

 ited resolution available. However, the elec- 

 tron microscope provides the increased reso- 

 lution recjuired for a full taxonomic study 

 and as a result much useful information has 

 been obtained which is of particular value to 

 both botanists and palaeontologists. 



As in many other cases, electron micros- 

 copy of the scales (coccoliths) requires a 

 detailed terminology which is provided by 

 Halldal and Markali (17). These authors 

 give a comprehensive survey of a large num- 

 ber of species of the genus using direct 

 examination in the electron microscope. 

 Though much information can be gained by 

 studying coccoliths directly, the results are 

 much clearer if carbon replicas are prepared 



(18). Figure fi shows a carbon replica of the 

 calcite scales and the full surface topography 

 is clearly shown. 



The remaining two genera studied in de- 

 tail in ihe electron microscope are Synura 

 and MaUomonas. These are closely related 

 to the CoccoHthophoridaceae , bvit the latter 

 are marine organisms whereas the former 

 are fresh-water organisms. Synura and Mal- 

 lomonas are also unicellular and covered with 

 scales, but these arc generally much smaller 

 and are composed of silica. 



The genus Synura contains only a few 

 species. These have been studied in detail 

 in the electron microscope by Manton (19), 

 Fott (20), Petersen and Hansen (21) and 

 Harris and Bradley (22, 24). The latter two 

 authors concentrated on their taxonomy us- 

 ing the electron microscope, but Manton 

 studied internal morphology and employed 

 thin sections. 



The genus MaUomonas contains a much 

 larger number of species, many of which 

 have only been discovered recently. The elec- 

 tron microscope has been of great value as a 

 taxonomic aid, since the classification of the 

 genus depends almost entirely on the struc- 

 ture of the minute silica scales covering the 

 organisms. Harris and Bradley (23, 24), also 

 Harris (25), have studied the scales directly 

 and used carbon replicas to show up their 

 fine structure. Asmund used direct examina- 

 tion and has studied the occurrence of Mal- 

 lomonas species in Danish ponds (26). 



Although the cells of many MaUomonas 

 species disintegrate when dried, most of them 

 become sufficiently rigid after careful fixa- 

 tion to be studied complete in the electron 

 microscope. Figure 7 shows a direct electron 

 micrograph of a scale of a species of Synura 

 and Figure 8 shows a carbon replica of a 

 complete MaUomonas. 



A small genus, ChrysosphaereUa, about 

 which relatively little is known, has also 

 been studied in the electron microscope. It is 

 rather similar to Synura and only consists of 

 two or three species. The cells are also coated 

 with silica scales and have long spines at- 



84 



