Let’s start with the question “What is a fungus?” A fungus is an organism whose body consists of a mass of root-like threads, each only one cell thick. You can see this in the picture at the right, in the lower left-hand corner. This is what a fungus looks like under the microscope. Let’s say that the fungus is growing on wood chips or mulch, as the pictured mushroom often does, indoors, in the United States. The wood chips, the stuff that the fungus is eating, are called the substrate. The fungus grows through the substrate, digesting it as it goes.

When the fungus feels that it’s grown enough and needs to reproduce, it sends up a mushroom, which functions for the fungus the way a fruit does for a plant. If a mushroom is shaped the way you’re used to (like this one), the underside of the cap is covered with radiating blade-like structures called gills. On these gills, the mushroom produces millions of spores. These are essentially single-celled seeds; three of them are shown at the top of the picture, and six at the bottom. These spores are microscopic, so small that they blow away on the wind. Eventually (hopefully), they land on a suitable substrate, to grow into another fungus.

Fungal fruits come in all sizes and shapes, not just the medium-sized “mushroom” body-type that you know and love. And just as the fruits of plants include things that we don’t usually think of as fruits, like the winged fruits of maple and beech trees, fungal fruits include things that look nothing like “normal” mushrooms. In fact, we usually refer to fungal fruits as “fruiting bodies”, so as not to get them mixed up with those sweet things that grow on plants. But when a mushroom is up, we often say that the fungus is fruiting, just like one would for a plant.

Lepiota lutea, the "flowerpot mushroom", from Emile Boudier's Icones mycologicae (1901)

Lepiota lutea, the “flowerpot mushroom”, from Emile Boudier’s Icones mycologicae (1901)

For instance, conks, those shelf-like or hoof-shaped things that grow from the sides of trees, are also the fruiting bodies of a fungus. These have a number of important differences from mushrooms like the one pictured above.

First of all, conks don’t have gills on the underside of the fruiting body; they have a layer of tubes. These can be seen from the side when the conk is cut or broken open, as at the bottom and left-hand side of the picture. But if you just look at the underside of the conk, you will just see the mouths of the tubes, as shown to the upper left of center in the picture. In this view, the tube mouths just look like holes, or pores, and for this reason these fungi are often called “polypores”, for “many-pored” fungi. Sometimes the pores are a millimeter or two across, but often they are so small (as with this Fomes) that you need to look really closely (or with a hand lens) in order to see the pores; otherwise the underside just looks like a solid surface.

Another thing that’s different about polypores is that they usually last a long time. Ordinary mushrooms, like plant fruits, usually decay quickly. Polypores, on the other hand, often remain on the tree for several months, and some of them remain basically indefinitely, adding a new layer of growth each year, the way a tree adds rings. With this Fomes, you can see the layers of growth on the cap, and at the bottom where it’s been cut away, you can see that there are layers of growth both in the upper shell and in the tube layer. The white layer at the very edge of the fruiting body is the only one that is actively growing this season. Some large polypores have been found that have twenty or thirty layers!

The main body of the fungus – – the part that’s digesting the substrate – – is called a mycelium, and the threads that make it up are called hyphae (HIGH-fee). The fruiting body of the fungus is also made up of hyphae: the “fibers” in the stem of a mushroom are made up of hyphae running in parallel to make the stem strong; the cap of the mushroom is made of of hyphae so tightly interwoven that they seem to be one solid mass; and even a hard conk is made up of hyphae that have been specially strengthened with material from the host tree, to make them hard and woody.

Fomes fomentarius, a tinder polypore, from Emile Boudier's Icones mycologicae (1901)

Fomes fomentarius, a tinder polypore, from Emile Boudier’s Icones mycologicae (1901)

The Fomes picture also shows in more detail how these fruiting bodies bear their spores. Inside the tubes, large bubble-shaped cells called basidia form (one by itself is called a basidium). The basidia grow four points on top, like a crown (see the picture to the right). From each of these points (called sterigmata) grows a spore. In the center of the Fomes picture above, you can see a the tough, parallel hyphae that make up the conk, and bubbling out above them are the basidia, two of which have grown sterigmata and spores. At any given time during its fruiting season, there are always a few basidia that are growing spores, some that have already shot their spores off, and a some that haven’t begun the process yet. The layer of bubble-shaped cells where this all occurs is called the hymenium.

Typical basidia. Drawing by Joseph Henri Léveillé, from his 1837 article “Sur le hymenium des champignons”

You shouldn’t think that all polypores are hard, or that all polypores with color zones remain for years, piling up layers. The Turkeytail is a very common polypore; you have probably seen it in the woods already, without knowing what it was. It grows out in a single season, and is thin and leathery, rather than hard and woody. Its zones are formed in a single burst of growth (colored differently in each one) rather than the result of yearly layers piling up on one another.

Trametes versicolor, the “Turkey Tail.” Photo by John Denk

There is even a group of fungi that have “normal” mushroom-shaped fruiting bodies, and grow on the ground, but have pores underneath the cap, like a polypore. These mushrooms are called boletes. Some of the most prized edible mushrooms in the world, like the famed porcini, are boletes.

Suillus grevillei, one of the “Slippery Jacks”. Watercolor by Mrs. T.J. Hussey, from her Illustrations of British Mycology (1858)

In the picture above, you can see the tube mouths (pores) showing in the overturned mushroom on the left. Slicing through the cap of the mushroom (lower right) shows you the lengths of the tubes in profile.

Looking at a cross-section of a tube under the microscope, we would see (as with a polypore) basidia with their spores (one has already shot them off, and remains with just its “crown” protruding) sticking out into the interior of the tube, as shown in the drawing below.

Interior of a bolete tube. Drawing by Joseph Henri Léveillé, from his 1837 article “Sur le hymenium des champignons”

This by no means exhausts the forms of fungal fruiting bodies. There is, for example, a whole other phylum of fungi that form their spores inside sacs called ascii, instead of on basidia. These fungi, called Ascomycetes, are actually more numerous than the more familiar Basidiomycetes, and have more species. Morels, truffles, and most lichens are just a few of the Ascomycetes that you may have heard of. Also, basidia have other shapes besides the simple “bubble” one that we have shown here. One of the few sure things in working with fungi is that there is always more to learn.