Another possibility -- the weight of the roots on top of the plastic causes the plastic film to be depressed, however slightly. More condensation will tend to collect underneath the depressed areas. I've noticed this on plastic that has folds or pleats, even in the absence of any external weight.
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From: "firstname.lastname@example.org" <email@example.com>
To: Discussion of aroids <firstname.lastname@example.org>
Sent: Tuesday, June 10, 2008 5:13:32 AM
Subject: [Aroid-l] Root Physics and Disarticulated Anubias Roots
Jan's pictures were most interesting.
It did look as though the roots had been cooler than the surrounding root-free
plastic, causing condensation along the root length. I suggest, however,
that this is caused by the difference in mass between the roots plus plastic
versus the plastic sheeting alone. That is, the thin sheet of plastic gets
warmer faster because it has less mass. This creates, in effect, a cooler
temperature under the roots, leading to preferential condensation. Another
effect that may play here is that the underside of the root is in shade,
also keeping it cooler or slowing its warming in the sunlight.
On the subject of root disarticulation,
I have seen this with Anubias species, which have a good grip on the substrate,
perhaps due to the same mechanisms that Peter is describing. But I have
never seen a fragment of disarticulated root regenerate into a new plant.
I have, from time to time, inadvertently created fairly long pieces of
isolated Anubias root. They seemingly survive for months, but eventually
give it up without organizing a new growth point for leaves or green stems.
Has anyone on the list had them regenerate?