Hi Soticgrad,

As you know, the idea is to heat the moisture in the leafs to evaporate it and then to condense the vapor to a liquid on the plastic. Since we are talking about solar as the source of heat, we are talking about radiated heat. Radiated heat, unlike conduction (movement of heat from solid mass to solid mass) or convection (movement of heat from molecule to molecule of gas in the air) is not warm until it strikes a solid dark object. Upon striking the dark object, radiation converts to heat. (simplified explanation)

In a solar still or transpiration bag we want the leaf to be heated in order to vaporise the moisture but we want the plastic bag to be as cool as possable so the moisture will condense on the cool surface to become a liquid again. A clear bag will allow the sunlight (radiation) to pass through the bag without converting to heat where we dont want the heat and to continue on to the leaf where we do want the heat. If the bag is dark, it will heat up and become the hottest part of the system and condensation cannot take place on the hottest part of the system.

The more clear the bag the better the still will work. You want the light to go through the bag, not heat the bag. the more opaque the bag the less light will go through but will heat it.

So a still has TWO different parts. A boiler, to heat the water and a condenser, to condense the water to a liquid.
The leaf becomes the boiler and the plastic bag becomes the condenser. If you heat the condenser, it will NOT condense.

Neal