Pumice stone:

Pumice is the stone formed from the lightest, most aerated component of volcanic lava. Imagine bubbly champagne froth, suddenly turned to stone, and you have a good comparison to pumice. Pumice stone has the capacity to wick up water in the tiny, inter-connected hollow pathways of it's structure. This movement of water - against gravity - is called capillary action. Because of this, pumice stone is used to wick water from one place to another.

The pumice wick:

Pumice stone is used to slowly draw water from a reservoir, move it via capillary action to dry soil, to re-hyrate plants. It can be used anywhere, from orchids in fine pottery to vegetable beds outside.