I think that there were a lot of such experiments made in the 1960s. Some
involved lie detectors. The way I remember it is, that when they tested the
siblings in the same room the sibling reacted, but when it was removed from
direct possible chemical (pheromonoe) contact it did not. This was found out
when the 'myth' was debunked by scientists. I remember that one plant to try out
is Mimosa Pudica. No special detectors are needed (it reacts visibly).

Peter


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