Apart from a few highly specialized species and some general adaptive features, not a great deal more can be said about aquatic plants than can be said about plants in general. In freshwater environments however, plants serve two important functions. They help to oxygenate the water, and they provide refuge and habitat for much of the microfauna which would otherwise be quickly consumed by the smaller fish and other predators. They form a kind of buffer zone which prevents the population of food organisms for the larger pond dwellers from reaching zero -- they are havens of biodiversity.
More notes on aquatic plants will be added at a later date.

  Elodea (Canadian Pondweed).

As the name implies, Elodea is something of an exotic import in countries outside North America. It seems to offer little attraction to the rotifers and epiphytic growths which encrust other water plants such as Cladophora, and its leaves and stems are always fairly clean.

Its great attraction for the microscopist is the ease with which it affords a demonstration of cyclosis, or cytoplasmic streaming. The leaves are mostly two cell layers thick, and if a healthy green leaf is removed and mounted on a slide under a coverglass with just sufficient pond water to keep it flat, the movement of the plastids within each cell is easily seen using a forty power objective.
It is believed that similar streaming occurs within most plant cells, but few lend themselves to such convenient examination.
This is a highly recommended exercise, and some pictures of cyclosis in Elodea are shown below.

The high-power darkfield pictures below were taken using a Zeiss-Winkel x90 oil-immersion achromatic objective with inbuilt iris diaphragm, and a Beck focusing darkfield condenser, on Kodachrome 25 transparency film exposed with electronic flash. The generally green colour of the pictures is due to the fact that the illumination is arriving from below, passing through plastids in cells below the plane of focus.

	Picture captions anticlockwise from upper left. All pictures are darkfield. The magnification of the first is x400, the rest are at x1400. Most of the cytoplasmic streaming activity within a cell takes place around the inside of the cell wall, as evidenced here in most cells. The three cells of the top row have bridging structures however, as does the middle cell of the bottom row. Two cytoplasmic bridges across the body of the cell, crowded with slowly moving plastids. These plastids, or chloroplasts, are the only coloured bodies in the leaf, and are solely responsible for its colour. 4, 5 and 6 are all of different groupings of cells in a leaf of Elodea, showing the plastids in various positions in their slow drift about the interior of the cell.

 

	Since the underside of the Lemna leaf is always in contact with water, the stomata are all on the upper surface, in contact with air. This picture shows a single stoma, lit strongly from below. The wavy lines mark the junctions of the walls of adjacent cells at the leaf surface. Darkfield, x400.
	Lemna. Two stomata. Same specimen as above. Darkfield, x400.

 

	Cross-section of Nymphaea stem, showing sclereid. Darkfield, x200.
	Cross-section of Nymphaea stem, showing sclereids. Darkfield, x400.
	Cross-section of Nymphaea stem, showing sclereids. Darkfield, x400.