How does a toadstool get its shape?
(from New Scientist, March 7, 1992)
MUSHROOMS and toadstools (agarics), and bracket fungi are the most
well-known fungal fruit bodies. These come in a wide range of shapes and sizes, all the better to disperse the spores they bear.
In stalked forms the spores are ejected from the cap, which is born aloft to ensure that spores can be dispersed by passing air currents. Stalks may be solid or hollow, but are invariably cylindrical. Although a girder form would provide more stability, it would create greater aerodynamic disturbance, blowing spores back onto the stalk. Stalks grow away from the pull of gravity to ensure that gills hang vertically, so that the spores can be easily ejected. Large agarics have relatively stout stalks, and small ones relatively slender ones, because the load increases as the cube of the linear measurements. So as a toadstool doubles in size, the cap volume (and hence its weight) increases eight times, whereas the cross section of the stalk merely increases four-fold. To compensate, stalks become relatively thicker as the fruit body grows larger.
If you turn a toadstool upside down, you'll notice that most have gills radiating out from the stalk, often with intercalated gills at the edge. But long-lived bracket fungi and Boletes such as the Penny Bun (used in packet soups) have a sponge-like matrix of narrow tubes or pores from which spores are ejected. This polypore structure makes more efficient use of the area below the cap than a gill system would, with less wasted space. However, polypores, unlike gills, cannot finely readjust to a vertical position if a toadstool is slightly dislodged.
A fungus's source of food also determines the size of its toadstools. Large forms tend to be mycorrhizal, or wood decomposers, with access to a rich source of nutrients. Smaller toadstools may be litter decomposers, growing only on a single leaf or twig, and have conical tops and thin gills to make the most of resources.
Agarics may be many different colours, but no one is sure why. Pigments may protect against ultraviolet radiation, or warn predators of toxicity. Yet brightly coloured Amanita species, deadly to humanity, are quite happily eaten by rabbits and deer.