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Austroboletus lacunosus : Cooke illustration

Spore release and dispersal

In the TYPES OF FUNGI SECTION there were descriptions of various common types of fungal fruiting bodies and the opening page of WHAT IS A FUNGUS explained that the sole function of a fungal fruiting body was to produce and disperse spores.

The TWO MAJOR GROUPS SECTION explained how various fungi produced their spores - in asci or basidia and this section will explain how the spores are released and then dispersed further afield.

The spores may be released actively or passively. In the former the fungus, through its own actions, ejects the spores from the basidia or asci with considerable force. In the latter the fungus relies on some other agent to release the spores from the fruiting body. The agents are varied - wind, impact, water, insects. Note that after the initial release, some other agency may be responsible for further dispersal of the spores.

Basidiomycetes and ascomycetes each contain active and passive releasers.

Active spore release

Once again, we'll start with a common mushroom growing in the middle of a field. As explained in the TWO MAJOR GROUPS SECTION the gills of a mushroom are lined with spore-bearing basidia. When a spore is mature it is forcibly shot from the basidium, into the air space between the gills. This is explained in more detail in the MUSHROOM SPORE DISCHARGE SECTION. While the spore is ejected horizontally with considerable force (up to 25,000 times the force of gravity), air resistance stops the spore in a fraction of a millimetre. The air in the space between two gills is still, so after coming to a stop the only influence on the spore is gravity and the spore falls downward. Once the spore has fallen below the bottom edge of the mushroom cap, and is clear of the gills, it strikes air that is not still. Even on what looks like a windless day, there are always slight breezes. While a human may not feel them, they are usually strong enough to be felt by a spore that is only about a hundredth of a millimetre long. These micro-breezes may pick up the spore and carry it higher into the air and away from the parent mushroom. The spore may come to rest a metre or a kilometre or even further away from the mushroom. It may come to rest on a grass blade, be eaten by a kangaroo that then moves away a few more kilometres and deposits it in a dropping.

Basidiomycetes with similar mechanisms are the boletes click to enlarge, polypores click to enlarge, paint fungi click to enlarge, stereoid fungi click to enlarge, coral fungi click to enlarge and jelly fungi click to enlarge.

A basidiomycete with a very different active method of spore dispersal is the Cannonball Fungus (Sphaerobolus stellatus click to enlarge ). The "egg" inside the two millimetre diameter cup contains a mass of spores and is forcible flicked out of the cup. You can find out more about this in the CANNONBALL FUNGUS SECTION.

In a cup fungus click to enlarge the surface of the cup is lined with the spore-producing asci. As the spores in an ascus are maturing, fluid pressure builds up in the ascus. At maturity, the pressure is sufficient to force the spores out through the top of the ascus. In some species of cup fungi there is a little lid at the top of the ascus which is forced open to allow the spores out. In others the tip of the ascus ruptures more irregularly. The spores may be shot several centimetres up into the air and, as in the case of the mushroom, air currents carry the spores further afield.

From the structure of a cup fungus, you will realise that many asci can simultaneously shoot their spores. Often when you pick up fresh cup fungi the mechanical disturbance of picking up a specimen is enough to jolt thousands of mature asci into releasing their spores and, if you are attentive, you will see a small cloud of spores arising from the fungal surface.

As well as the simple cup fungi, the "compound" or "distorted" cup fungi such as Cyttaria click to enlarge, Morchella click to enlarge and Leotia click to enlarge release their spores in the same way.

Many, but by no means all, of the flask fungi click to enlarge release their spores actively. Flask fungi differ fundamentally from the cup fungi. In the latter the asci line the surface of an open cup or disk but in the flask fungi the asci are contained within a chamber that has only a narrow opening at the top. So there is no mass firing of asci. Instead, when an ascus is mature its tip extends to the opening, shoots out its spores and then collapses back into the chamber. Then another ascus can have its turn and so on. The flask fungi also get a mention in the passive release section below.

Passive spore release

A mature puffball is typically a flexibly-walled, apically open sack of spores. A raindrop or foot hitting the sack momentarily compresses the air inside thereby forcing a puff of spores through the apical hole and several centimetres into the air. You can see this for yourself if you flick a puffball with your finger. While some sort of impact triggers the initial release of the spores from the fruiting body, wind takes over as the agent of longer distance dispersal.

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Earthstars (Geastrum sp.)
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Scleroderma sp.

Earthstars release their spores in the same way as puffballs. Amongst the other puffball relatives the tough skin of Scleroderma splits to expose the spores to wind and water, allowing the spores to be washed or blown away. In both Calvatia click to enlarge and Pisolithus click to enlarge the fragile outer skin breaks away to, once again, allow wind and water to disperse the spores. Pisolithus has an interesting internal structure. In this photo click to enlarge of the cross section you'll see that the inside is full of what look like yellow to brown rice grains. Initially there are basidia in each of those grains (or peridioles). When the spores are mature, the basidia and peridiole walls disappear, leaving a powdery mass of spores. The peridioles at the top mature first, then the next layer down and so on through the fruiting body.

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A stinkhorn, Anthurus archerii

The spores of stinkhorns are contained in an external, foul-smelling, sticky slime that attracts flies which then transport the spores elsewhere. Some stinkhorns are coloured red, just like the dead meat of a carcase - so providing a visual cue to various carrion-loving insects.
Examples are Anthurus archerii, Aseroe rubra click to enlarge, Dictyophora multicolor click to enlarge.

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A birds-nest fungus (Cyathus novae-zealandiae)

The 'eggs' within the cup of a birds-nest fungus are aggregates of spores which are carried from the cup by rebounding raindrops. These eggs are called peridioles (the same term used above for the 'grains' inside a Pisolithus). This is an effective method and experiments have shown that the peridioles are generally carried about half a metre from the cup (the record distance is a little over two metres). In many cases animals eat the peridioles that land on plants, the thick skin on the eggs is softened in animal guts and the spore mass is passed out with animal droppings. If a peridiole remains uneaten, the thick skin will break down over time to allow the spores out. See BIRDS NEST & CANNONBALL FUNGI

The truffle-like fungi click to enlarge rely on animals to eat the truffle-like bodies, with the spores then passing out with the animals' faeces. The truffle-like fungi often produce aromas which attract animals, which then dig up the fruiting bodies. It's well-known that in Europe female pigs are used to hunt for gourmet truffles. That's because, when mature, the gourmet truffle emits an aroma like that of a sexually aroused boar. No wonder the female truffle-hunting pig gets excited.

While many flask fungi shoot their spores forcibly into the air, there are also many flask fungi which simply ooze their spores out of the perithecia. In some cases insects will pick up the oozed spores and carry them further away.