Underneath, however, boletes have tubes instead of gills and in this they resemble bracket fungi. Boletes differ from bracket fungi in that they are soft, fleshy and rot quickly. They are sometimes called sponge fungi. Bracket fungi, on the other hand, are leathery or woody and persist for long periods.
Tubes can be very shallow or more than a centimetre deep in some species.
The mouths of the tubes appear as pores on the underside of the fruitbody. Pores vary in size and shape from species to species. In some species pores are circular, in others angular or maze-like. In some the pores are radially arranged and in others irregular. Pores can be more than a centimetre across or so tiny they are hard to see without a magnifying glass.