The most fundamental task a mushroom needs to accomplish is to send the fungus’s spores out into the world, where hopefully they will land somewhere congenial and germinate into a new fungus. There are two main structural options for spore dispersal architecture: a broad surface called a hymenium from which the spores can drop (or be fired) into the air; or a glob of spores called a gleba.

When the mushroom’s spores are supposed to drop into the air and float away, they are usually produced on a surface called a hymenium. The type of hymenium most familiar to most of us is a set of gills. The mushroom produces spores on the faces of the gills, and then shoots them off into the air. This is known as ballistospory.

Basically, the fungus wants to fit as much hymenium surface area as possible into a given space. If you want to find the hymenium of a mushroom (and this is a fundamental part of identifying mushrooms), a good way to start is by finding where the mushroom seems to be trying to maximize its surface area.

Gills are one way to pack a lot of spore-producing hymenium into a small area. If you took a regular button mushroom from the store, just a few inches across, and opened up each gill so that its entire surface area was lying flat, you’d have an area the size of a trash can lid.

There are other ways that mushrooms maximize the spore-bearing area underneath their cap. As the gills go towards the edge of the cap, they spread apart. If the space between them goes unutilized, the fungus is growing cap space that isn’t being used for producing spores. Many mushrooms use this space for growing extra gills. These gills that grow in the nooks between others, are called partial gills, since they only extend a part of the way in towards the stem. Some mushrooms grow multiple tiers of these.

The above mushroom has gone to great lengths to fill in the space between its gills. You can even see where it has made little vein-like gills that cross and squiggle in between the radiating ones.

Gills are not the only form that a hymenium can take. Other mushrooms have a hymenium that forms pores, teeth, or is just very wrinkled. Architecturally, pores are gills with cross-walls. In some mushrooms, they really resemble gills with cross-walls, and in other cases they are tiny and round and distributed as if they are random pinpricks in a solid surface.

Most mushrooms with pores underneath fit into two groups: boletes and polypores. Boletes are basically mushrooms like you’re used to from the store, but with pores underneath instead of gills. They have the same texture as mushrooms from the store, and their pores are sometimes arranged radially, like gills. They grow on the ground, because they are symbiotic with trees.
Polypores are the shelving things that stick out of the side of trees. They are often thick and quite hard/woody, or thin and tough/leathery. Many lack a stem. There are a few polypores that grow on the ground, and a few of them even have a soft texture similar to boletes; however, these have a rooting base that goes down to the tree roots that they feed on, a feature that no bolete has.

Although polypore pores are usually quite round and small, in some cases the walls between the pores break down as the mushroom matures, merging round pores into longer ones, and eventually giving the underside a maze-like appearance.

In some polypores, the hymenium forms radiating plates shaped like gills. However, these mushrooms are still really tough or hard, which lets you know that they are polypores. Sometimes they are called gilled polypores. Although the hymenium of these mushrooms is gill-like, the “gills” usually do a lot of forking, again giving the underside a maze-like appearance.

Because the original genus name for these polypores was Daedalea (after Daedalus, the mythical Greek inventor who built the labyrinth) this sort of maze-like underside is sometimes called daedalioid.

The other major hymenium formation that has a name is the toothed or spinose hymenium. It is sort of the complement of the polypore hymenium – you can see that a toothed hymenium could fit right into a perfectly matched polypore hymenium, and from this we see that they have the same amount of added surface area.

Although a toothed hymenium is still a fairly standard type, we don’t have many terms for its features, as we do for gills and pores. Some fungi with toothed hymenium have a cap and stem and grow on the ground, like hedgehog mushrooms; others form stemless caps or masses bearing spines on wood, like the Hericium above.

Some forms of hymenium don’t even have names. Morels have a hymenium that consists of many spore-bearing pits that form a honeycomb on its surface.

False morels in the genera Verpa and Gyromitra have an irregularly wrinkled hymenium that is sometimes called “saddle-shaped” (and sometimes actually is) but is more usually pretty random.

Likewise, chanterelles and jelly fungi have a hymenium that is just wrinkled. The shapes of some jelly-fungi have been called “blobby” and others are compared to ears, but this really doesn’t narrow things down much.

Some fungi have a hymenium that is smooth. Some of these are crust fungi, which just produce a bare hymenium– no stem, no cap – lying directly on wood. These fungi don’t get the benefit of jacking up their spore-production per area, but they generally produce tough fruiting bodies that can produce spores over a long period of time, as opposed to the softer mushrooms that rot or are eaten by animals.

Some gilled mushrooms with just a few gills also pursue this strategy – they can dry up and then revive and resume spore production the next time it rains. Jelly fungi do this also.

There are also cup fungi, which form a rounded, concave hymenium. In larger cup fungi this hymenium is often wrinkled, but in small ones it is smooth. Some of the common cup fungi are quite small, brightly colored, and may have bristles around the rim.