Just to make sure I understand the principle, transpiration bags get the water that is a byproduct of sugar production during photosynthesis.

So that's why the bag has to transmit light. The bag also has to be airtight and weighted down so that water will condense out (rather than venting as vapor) and collect at the bottom of the bag. The branch you pick should also have plenty of leaves and get plenty of sunlight.

Is all this correct? What do you recommend as a good bag type? Grocery store vegetable bags? They're very thin and transparent, but appear to be water-tight.

Is this deciduous only, or will evergreens work in winter?