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Algae.
Diatoms. 		 Page
2 of 2
Algae: Intro. Colonial Diatoms Filamentous Desmids Motile


  Diatoms. (continued).

Diatoms: Page 1 Page 2

 
  Pinnularia and Tabellaria.

Pinnularia and algae. A brightfield picture of the large pennate diatom Pinnularia, seen in valve view. The absence of any visible plastids shows that the diatom is dead. The background is flocculent organic matter and various filamentous algae.
Click for a diagram of Pinnularia.
Brightfield, x400.
Tabellaria and algae. Tabellaria is a common freshwater diatom which forms zig-zag chain colonies by joining diatom groups corner to corner with pads of mucilage. In this picture, all diatoms are seen in girdle view.
Darkfield, x400.
Tabellaria and algae. Another chain of Tabellaria.
Darkfield, x300.
Diatoms from an acid marsh. A group of diatoms from a naturally acid marsh (Thursley Common, UK). The water is particularly clean because the acidity discourages the growth of bacteria, and favours the growth of a wide diversity of diatoms and rotifers. The four plastids of the central diatom are clearly seen. A careful examination of the Pinnularia (?) to the left shows the Petri dish-like structure of its frustule.
Darkfield, x1000.
Chloroplast of an unidentified diatom. This darkfield shot of the middle portion of an unidentified pennate diatom clearly reveals the pigment of the organism to be located in many granules clumped together into roughly symmetrical shapes as mentioned in the introductory notes.
Darkfield, x1500.

 
  Cleaned and Mounted Diatom Frustules.

The following group of pictures is of diatom frustules which have been prepared by treatment with strong acid to remove all organic traces in order to best display the intricate frustule markings.

Diatom: Cleaned frustule. Navicula is a common pennate diatom which shows motility. This motility is found only in certain of the pennates and is associated with a longitudinal slit in the valve called a raphe. Navicula has a raphe in both valves, but other diatoms showing motility have the raphe in only one valve. It is thought that a slime produced within the cell is exuded through the raphe enabling the diatom to glide along surfaces it contacts. The details of this process are still unclear.
No centric diatoms show motility.
Darkfield, x400.
Diatoms: Cleaned group. Part of a large group of cleaned diatom frustules arranged by the hand of some anonymous Victorian microscopist indulging one of the microscopical pastimes of the day. These arrangements were an excellent way of presenting a variety of diatoms to illustrate their diversity of shape.

The requirements were a microscope, a single hog's hair mounted in a brush-like handle, a very steady hand and loads of patience. If the diatoms were fossil forms from diatomaceous earth, they were first separated by treatment with concentrated hydrochloric and nitric acids, followed by washing, sedimentation and drying.
Next, a clean microscope slide (or coverglass) was coated with a thin film of diluted egg albumen and allowed to dry. Selected frustules were placed on the prepared slide one at a time under the microscope using the mounted hog's hair. When each was satisfactorily positioned, the slide was gently breathed upon to cement the diatom to the albumen.
(Hog's hairs were particularly favoured for this task on account of their rounded tips, and a suitable supply could be found by careful selection among the hairs of a good quality shaving brush).

The finished arrangement was then mounted under a coverglass either in air or a medium of high refractive index to enhance visibility of the frustules.
Darkfield, x60.
Diatom: Cleaned Frustule. A large centric diatom from a Victorian arranged slide showing some asymmetry on the pattern of the valve towards the very centre.
Rheinberg Illumination, x400.
Diatom: Cleaned Frustule. Another specimen from a Victorian diatom arrangement showing the fine detail in the pattern by the use of oblique lighting effects.
Oblique brightfield lighting, x400.
Fragments of diatom frustule. This picture is of a small fragment of the frustule of an unidentified diatom from a strewn preparation. It shows quite clearly that the "dots" on the frustule which can appear to be minute bumps at lower magnifications are indeed holes. Nineteenth century diatomists referred to this phenomenon as "postage stamp fracture".

The picture was taken using a Reichert oil-immersion fluorite objective, with a darkfield provided by a Beck focusing darkfield condenser. The high magnification figure is a result of cropping the original Kodachrome transparency.
Darkfield, x1500.