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I wouldn't claim any of the following as necessarily being  "good" photosythesis exhibits, but are certainly good attempts.

1) similar to the one Peter mentioned, a plant in a sealed box, light source, metered air flow through the unit( you need a lot of active plant, and a little air space), and a humidity sensor in the air outlet. (humidity sensors are a lot cheaper than the CO2 detectors, and measure transpiration.. Having both gives you a lot of information ( but can the public digest this much??). Turn on the light . Water vapour levels rise. Change the colour of the light ( coloured globes or filters) and see which parts of the spectrum are "active".

2)A support display ( of stomatal openning during photosynthesis) we had with the above unit was made from a big piece of bed foam ( expanded polyurethane 3 foot by 6)... Cut a slot about a 18 inches long, and bond an inner tube from a small car to the edges of this slot. Carve the surface to loof like plant leaf epithelium., paint with flexible silicon paints. Place a light detector in the leaf, and link this to a pneumatic valve. Point a flood light at the "leaf" and the tube would inflate and open the stomata. A small mist unit behind with a fan and the vapour is seen being transpired.

3) In the cells of many plants you have a process called cytoplasmic streaming ( the organelles - such as chloroplasts, mitochondria etc) flow around the edges of the cells. The rate of flow is linked to cell energy levels, and various with photosythetic activity.. ( not a direct part of photosyhthesis but observable, especially in color river algae, and some water plants with medium magnification.

4) A quirky demostration is to use a motile algae ( such as a dinoflagelate).. Put these on a slide under a microscope, and shine a spectrum of light onto the slide, the dinoflagelates will swim and accumulate in the areas which match the frequencies of light used for photosyhthesis... ( a few at the red end, but most up the blue end)...

Most of these are better at helping people with a basic idea take another step forward, than getting the idea across to a naïve visitor....AND all need a high degree of tech support, plants get stressed too..

Hope this is of some help.
Cheers
Stuart K
Manager New Concepts
Questacon.
Canberra


-----Original Message-----
From: Informal Science Education Network [mailto:[log in to unmask]] On Behalf Of Peter A. Anderson
Sent: Thursday, 3 November 2005 4:53 AM
To: [log in to unmask]
Subject: Photosynthesis

ISEN-ASTC-L is a service of the Association of Science-Technology Centers Incorporated, a worldwide network of science museums and related institutions.
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Olle, the Exploratorium has a good one;  a large fast-photosynthesizing leaf is sealed in a transparent chamber.  Air is passed over it and the output air goes througn a carbon dioxide analyzer (an automatic spectrophotometer).  Shortly after the visitor turns a strong light on the leaf, the carbon dioxide level falls as it is used up in photosynthesis.

Of course, there are also waterweeds that have bubbles of oxygen on them, from photosynthesis.

The actual process is a pretty complicated bit of natural organic chemistry, and difficult to put across.

Good luck

Peter Anderson

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