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Algae: Intro. Colonial Diatoms Filamentous Desmids Motile


Desmids are a distinctive group within the green algae (Chlorophycaea) and have always been amongst the favourite subjects of microscopists.

Each cell consists of two symmetrical cellulose-walled halves usually with a pronounced constriction in between -- two semi-cells joined by an isthmus. The nucleus is usually located in the isthmus. The walls of the semi-cells are frequently ornately sculptured, and this, together with the clear brilliant green of the chloroplasts make them very attractive creatures.
They occur commonly as planktonic forms in rivers and streams, and sometimes in large numbers in ponds.

  Closterium and Cosmarium.

Closterium with filamentous algae. Closterium is a common desmid in most freshwater habitats. It lacks the isthmus of the general description above but has at the extreme ends of each semi-cell a small spherical vacuole in which varying numbers of tiny particles can be seen in constant agitated motion.

These vacuoles have fascinated microscopists for the best part of two hundred years. Close examination reveals that certain of these particles will leave the vacuole and go on a meandering path through the ridges and crenellations of the large and elaborate chloroplast. Exactly what is happening is still unknown.

Closterium also shows motility in the form of a slow pivoting about one end or the other. This is known to be due to the expression of mucilage from pores in the outer cell wall.
Click for a diagram of Closterium.
Darkfield, x400.
Cosmarium. This single specimen of the desmid Cosmarium clearly shows the isthmus referred to in the general description above, and something of the ornate sculpturing in the form of closely spaced papillae on the cellulose outer wall.
Click for a diagram of Cosmarium.
Darkfield, x1500.


Netrium. This and the following specimens of Netrium were collected from the naturally acid waters of Thursley Common (UK). Like Closterium, they lack the isthmus, and possess an intricately scalloped and ridged chloroplast. The exterior cellulose wall is entirely smooth. The nucleus is located between the two chloroplasts, at the centre of the cell. Also like Closterium, at the highest powers of the microscope, tiny particles can be seen wandering between the outgrowths of the chloroplast.
Click for a diagram of Netrium.
Darkfield, x600.
Netrium showing chloroplast. Netrium as above, but with the focus of the darkfield condenser lowered so as to illuminate the chloroplast but not be reflected by the "equator" of the cell wall.
Darkfield, x600.
Netrium: Closeup of chloroplast. Netrium: same specimen as above at higher magnification. The space between the chloroplasts where the nucleus is located is clearly seen, and the bright round bodies are pyrenoids which store energy for the cell in the form of a starch-like saccharide polymer.
Darkfield, x1500.
Netrium showing crystals. A specimen of Netrium showing an internal crystalline growth. Can anyone explain what is happening here?
Darkfield, x700.
Netrium: Showing chloroplast and crystals. The same questions apply to this Netrium as to the specimen above.
Darkfield, x700.
Netrium amongst diatoms. Shows a single Netrium amongst a miscellany of diatoms from Thursley Common.
Darkfield. x500.

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