Porcini’s Prodigious Cousin

What am I looking at?

This is a cross section through the underside of the cap of a bitter bolete mushroom (Tylopilus felleus). It is composed of parallel tubes made from hyphae (1), thin branched filaments that condense to form the walls of the tubes. The lining of the tubes is called the hymenium; it contains specialized club-shaped cells called basidia (2), each of which has four finger-like projections called sterigmata that support the developing spores (3). The bulk of the hymenium is composed of sterile paraphyses (4), which are slightly smaller than the basidia. Interspersed between them are large, also sterile, support cells called cystidia (5). The yellowish-green spots are granules of glycogen – the primary metabolite in fungi (rather than starch, as in plants).

Click on the right arrow to see some additional views of a bitter bolete.

Biology in the background

This fungus is related to Boletus edulis, the tasty, well-known porcini mushroom. While the bitter bolete is technically edible, it is – as its name implies – extremely bitter and therefore rarely eaten. The species’ habitat is across East Asia, Europe, and Eastern North America as far south as Mexico and Central America.

Bitter bolete produces large fruiting bodies – what we commonly know as mushrooms. They can reach 40 centimeters across (about 16 inches) and can weigh up to 3 kilograms (almost 7 pounds), although most individual fruiting bodies are much smaller. But the vast bulk of the fungus is subterrestrial – concealed underground.

The underground, vegetative part of a fungus – the mycelium – can be quite extensive. In fact, mycelia are considered among the largest living organisms in the world. A single bolete mycelium, for example, can cover several square miles – even an entire forest! And mycelia are not only massive but can also be incredibly ancient. A renowned mycologist named Paul Stamets (a mycologist being someone who studies mushrooms) discovered the mycelium of a honey mushroom (Armillaria ostoyae) that covers over 2,200 acres on the West Coast of the United States, in Oregon; it’s estimated to be around 2,400 years old!

These pores/tubes are about 115 micrometers in diameter – almost twice the width of a hair.

Technique

These images were created using confocal microscopy.