The part of a passive watering system the nutrient passes up to be absorbed by the medium and roots, a passive hydroponic system uses a wick suspended in the nutrient solution.
Merriam-Webster Online Dictionary
wick (noun) a bundle of fibers or a loosely twisted, braided, or woven cord, tape, or tube usually of soft spun cotton threads that by capillary attraction draws up to be burned a steady supply of the oil in lamps or the melted tallow or wax in candles
transitive verb
to absorb or drain (as a fluid or moisture) like a wick - a fabric that wicks away perspiration
filament, thread, cord, string, twine