A gleba is sometimes called a “solid mass of spores.” This is not quite true: sometimes, even in maturity, there are tough thread-like cells called capilitium that run through the gleba, and in stinkhorn gleba the spores are embedded in slime. Before maturity, there are actually many non-spore cells running through the gleba, to bring the developing spores water and food. But even in this state, the area where the spores are developing is called the gleba.

The big difference is actually that spores grown on a hymenium stay attached to the hymenial surface until they are mature and are fired off into the air. Gleba spores at maturity are not attached to anything, but are still lying around the mushroom. In some species, a packet of glebal spores is fired off by the mushroom (perhaps this should be called ballistogleby!) but usually the spores of a gleba are launched by the energy from outside sources, or distributed through other means.

One flagship example are the giant puffballs, various species in the genus Calvatia. The gleba grows inside a thick outer skin called a peridium. In the early stages of growth, the gleba is mostly support cells and is white, with a texture like cheese or tofu. In this stage, the mushroom is a standard edible.

Once the spores are more or less able to fend for themselves, the support cells liquefy themselves and drain away as a stinky yellow liquid. In this stage, the mushroom is rather disgusting, and even a small showing of yellow in a firm white gleba means that the mushroom will be too bitter to eat.

Once the support cells are gone, the spores are left as powder, and the peridium cracks open to let the spores be blown away by the wind. Mechanical agitation such as kicks from small children may also serve to shake the spores loose and distribute them. Although the supporting cells have drained away, even in maturity tough strands of capilitium traverse the remaining spores.

Some smaller puffballs that used to be in the genus Lycoperdon have a single opening in the top center of the peridium. The consensus on these is that their spore dispersal is that the impact of rain drops drives the spores out of the hole. The spores are thus dispersed at a time when the moist conditions are best for them to germinate. As with the Lycoperdon puffballs, these earth stars also have a single opening at the top center of the peridium, for spores to escape through when driven by raindrops.

Still other small puffballs in the genus Bovista don’t release their spores in situ at all. They have a very light peridium and a very fragile attachment to the ground, so they typically come free of the ground and are driven by wind and whatever else might move them until weathering and repeated collisions break the peridium and the spores escape.

The puffball model is the closest to a “solid mass of spores” that we have among the gleba-bearing fungi. The peridium plays an important role in this, as it holds the mass of spores together until they are ready to be released, and it shelters and protects the developing spore mass from adverse weather and casual consumption. Its role is thus similar to that of the universal veil of some gilled mushrooms and stinkhorns. In fact, there’s no real reason for the difference in terminology, except that a universal veil tends to be a membrane, while a peridium is usually thicker and tougher, and may be quite hard.

A mushroom may also have multiple layers of peridium – an inner one that serves mainly to keep the powdery spore mass together, and an outer one whose function is more protective. The outer peridium may be ornamented with spines, and in the case of earth stars (fungi in the genera Astraeus and Geastrum) the hard, woody peridium splits into rays that give it a star-like appearance. In some earthstars, these rays fold back over the gleba when the air is dry. This prevents spores from escaping when conditions are unfavorable. The rays then open up again when the air is moist and escaping spores would have better conditions for survival.

A mushroom’s gleba need not all be in one mass. Bird’s nest fungi have their gleba packaged in multiple capsules that are shaped like tiny lentils. Raindrops splash them out of the cup-shaped outer peridium to start a new fungus growing elsewhere.

Stinkhorns are another group of fungi that have a gleba. Stinkhorns have a fairly unique reproductive strategy in that they embed their spores in a slime that smells like rotting stuff. The smell attracts flies and other insects that feed on carrion, and these critters eat the slime (it’s actually very nutritious – lots of sugars and trace minerals) and poop out the spores somewhere else, and it is also thought that they get some of the slime stuck to them and this too ends up getting deposited elsewhere. In both cases, hopefully it’s somewhere that the spores can germinate and start growing another fungus.
There is also a whole phylum of fungi – the Zygomycetes – that produce spores in a gleba atop a long stalk. The Zygomycete that most of us are most likely to see is Spinellus fusiger, which produces a host of tiny stalk-borne gleba. Like the gleba of puffballs, these break open upon maturity and the spores are scattered by wind and rain.

Zygomycetes are also common molds on fruit. Look closely at the fuzzy gray spots on your strawberries and peaches and you may see tiny threads topped by a dark ball of gleba.