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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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