Mycomorphology Part 1:
Why Do Mushrooms Look Like Mushrooms?

© Peter Werner
Original publication: Mycena News, December 2002

While fleshy fungi come in a variety of shapes, we usually associate higher basidiomycetes with variations on a characteristic parasol-like morphology, with an elevated pileus that bears various kinds of hymenophoral structures (gills, tubes, spines, etc.) underneath. But just why is a mushroom shaped like a mushroom, or to put it more technically, what is the adaptive significance of the characteristic "mushroom" morphology?

If one understands that a mushroom is the spore-bearing reproductive structure of the larger fungal organism, the short answer to this question is obvious - a mushroom is adapted to drop spores into the flow of air underneath, ensuring dispersal away from the parent. However, to fully appreciate the degree that this morphology represents an optimal adaptation for basidiospore dispersal, we must first look at the mechanism by which spores are shot from the basidia and escape from the hymenophores of a mushroom.

The mechanism of spore release has been the subject of much study. What is clear is that spore release begins with the formation of a liquid droplet, known as Buller's drop, at the base of the spore. Buller's drop is formed when the spore releases a dense hydrophilic solution of mannitol and hexose sugars. This solution is so strongly hydrophilic that it literally draws water out of the air, causing the Buller's drop to grow.

Buller's drop & spore dispersal
Formation of the Buller's drop and spore disperal.
(from Carlile & Watkinson, (1994) The Fungi based on Webster et al. (1989), Trans. Brit. Myc. Soc. 91: 199-203)

About a minute after Buller's drop forms, the droplet rapidly spreads over the surface of the spore, an event that is immediately followed by spore release. Many different hypotheses about this phenomenon have been advanced and rejected over the last century, but recent consensus seems to be that the droplet plays two roles. First, the initial formation of Buller's drop displaces the center of gravity in the spore, rocking it along its connection with the sterigma (the basidial tip on which a forming spore rests), causing this connection to loosen.

More importantly, the spread of the droplet over the surface of the spore involves the breaking of surface tension. This breaking of surface tension apparently creates a net force that pushes the spore downward against the sterigma. The sterigma is an elastic but highly pressurized body, and the pressure of the downward force of the spore results in an equal and opposite force that is strong enough to launch the spore away from the basidium. This mechanism has been called "the surface tension catapult". The spore is launched far enough so that before reaching free-fall it will be well clear of the hymenium (the basidia and cystidia, collectively) from which it originated, but not so far that hits the opposite hymenophoral wall.

Two critical microenvironmental parameters must therefore be maintained in the air spaces between the hymenophores. The first is that the air must be sufficiently humid for the hydrophilic compounds to draw moisture out of the air and form a Buller's drop. At the same time, liquid water must be excluded from this space, or the hydrophilic compounds will simply be washed away before they get a chance to form a Buller's drop. The second parameter is that the air in this space must remain absolutely still, both to prevent humidity from dissipating and to ensure that the free-fall of the spores between the walls of the hymenophores is not disrupted. The air beneath the pileus must also be relatively still so as to allow the spore to fall free of the mushroom and into the airflow without being blown back up to the hymenophores.

The role of the characteristic mushroom shape in maintaining the first of these parameters is quite straightforward when one notes the resemblance of a mushroom to a sunshade or umbrella. Shading the hymenophores from direct sunlight helps prevent humidity from evaporating away, while preventing rainwater from reaching the hymenium prevents hydrophilic compounds from being washed away.

The role of the mushroom's shape in maintaining the second of these parameters, stillness of air in and around the hymenophores, is not quite so self-evident, but was illustrated quite dramatically by a recent experiment which involved the placement of mushroom caps in a low-speed wind tunnel. Plumes of smoke were blown through the wind tunnel and over a mushroom pileus fitted onto a wooden "stipe". Airflow was shown to decrease in velocity at all points close to the pileus, but with a particularly significant decrease in velocity beneath the pileus. (The airflow several millimeters above the pileus increased in velocity, which showed the pileus to be aerodynamically not unlike an airplane wing.)

Hence, a mushroom's characteristic shape makes it particularly well suited for ensuring the conditions necessary for basidiospore release and dispersal. It also illustrates why fleshy ascomycetes have such a different morphology. Asci disperse their spores through a much more powerfully explosive mechanism, firing their spores much further than do basidia. Because of this, the hymenophore of an ascomycete cannot be closely spaced, like the gills or pores of a basidiomycete. The asci must also face upward, or the ascospores would be shot straight into the ground. This is why fleshy ascomycetes typically have hymenophores that are variations on a cup or saddle shape, with asci found on top of the fruiting body, rather than the underside.

I will also note that in order for a mushroom to properly ensure the efficient release of basidiospores, the surface of the hymenophore must be exactly perpendicular to the ground. Mushrooms have a highly intricate set of adaptations for ensuring this. The story of how mushrooms keep themselves upright (and how they even sense "up" and "down") will be the subject of my next column.

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