Doubling and dabbling in duckweed
A simple way to study the behaviour of the hypocotyls (stalks) of small seedlings
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Duckweed does not immediately present an exciting image to the young scientist. It is just green, has no flowers (that you can see), it is small and just sits there on the pond. But have you ever looked closely at your local duckweeds? There are at least 5 different species in Britain. The one you will know best is, no doubt, the lesser duckweed, Lemna minor, and this is certainly worth plenty of attention as an ideal plant for investigations. But before we sing its praises as an experimental organism lets introduce the rest of the family!

The duckweeds (Family Lemnacea) are a small and cosmopolitan group, found from the sub-polar regions to the tropics. They all prefer ditches, ponds, lakes and slow-flowing rivers. There are, surprisingly, only 22 species in the world and most are very widespread. One can imagine them jet-setting from one continent to another on the feet of birds. All have a few flattened, often rounded, disc-like leaves (they are really leaf-like stems or thalli) floating on the surface. New leaves bud off from these as the plant grows. Once there are half a dozen leaves the plants break apart. Often there is an unbranched rootlet hanging below the leaves. Some of these rootlets are photosynthetic. The table and diagrams describe some duckweeds you may find in your locality, identified by their individual leaf size, leaf shape and rootlet characteristics.

Lemna minor is undoubtedly the most often encountered, but the smaller Lemna minuta has recently been spreading rapidly in Britain. Spirodela polyrhiza is less common but much the largest and most elegant.

Experiments with duckweeds.

Investigating optimal growth conditions.

Studying growth and measuring its rate.

• A fraction of the duckweed can be collected every few days from the culture and dried on a filter paper to estimate dry weight productivity (use the top of your light-bank as a drier). You need a fine balance. Duckweed weight gained, per unit area, approaches yields comparable to cereals! Duckweeds are readily eaten by ducks. In the summer the ducks certainly cannot keep pace with duckweed production. What area of duck-pond does a duckweed-feeding duck need? Do goldfish eat duckweed?

   

  Species	Leaf size	Leaf character	Rootlets
  The great duckweed Spirodela polyrhiza	large 5 - 8 mm	shiny and circular, floating	many up to 30 mm long
  The lesser duckweed Lemna minor	small 1.5 - 4 mm	opaque and more or less elliptical, floating	single to 15 mm long
  The least duckweed Lemna minuta	very small 1 - 2.5 mm	elliptical, floating	single to 8 mm
  The ivy-leaf duckweed Lemna trisulca	elongated 5 to 15 mm	spear-shaped with opposite branches, submerged and translucent	single and hooked
  The gibbous duckweed Lemna gibba	very small 3 - 5 mm	ovate, convex below, fat and bouyant	single to 6 mm

Competition between species

Try growth rate races between different species.

Report back to SAPS on your success with doubling and dabbling! In the next Osmosis, we shall supply the duckweed doubling formula and deal with duckweeds and pollution... a wonderful subject for more experiments.

Stephen Tomkins

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Q. Most science journals seem to be aimed at teachers. Are there any which are written for students?

Q. There seems to be a lot of information and advice on growing plants available from lots of different sources. Has anyone gathered all this information into one convenient package?

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