Column: How do mushrooms reproduce?
Earthstars open and close in wet and dry weather so as to disperse their spores more effectively. (Photo: Elizabeth Ellison/Courtesy of the artist)
Everything about a fruiting structure — dandelion seeds that become airborne, fruits and seeds that float, fruits that attract animals to devour them, whatever — is connected with the need to accomplish seed dispersal.
Some plants — like, say, touch-me-not — don’t require wide seed dispersal. When the touch-me-not pod is fully mature, it “explodes” and the seeds are propelled for a few feet as the rubbery strands they are connected to suddenly uncoil.
Magnolia trees, however, require much wider seed dispersal. Their seeds would never prosper under the shade of the parent tree. If you examine a magnolia cone in the fall, you’ll observe that the extruded seeds aren’t allowed to fall to the ground. Instead, they dangle on short rubbery strands called “funicular strands.” Their bright scarlet color helps migrating birds locate the seeds and thereby disperse them widely.
Fungi, of course, don’t utilize seeds to reproduce. They are non-vascular and reproduce via spores. But the above-ground portion that we think of as a mushroom is actually the equivalent of a fruiting structure, which are produced from underground strands called mycelium.
Spores are most often dispersed from slits or tubes underneath the cap. One of the methods utilized in mushroom identification is to make a “spoor print” by breaking off the cap and leaving it overnight (flat side down) on a white piece of paper. These prints can be quite beautiful in regard to color and conformation. And they are often diagnostic, allowing accurate identification down to species level.
Most everyone is familiar with the frequently encountered mushroom called puffballs, which don’t have gills or tubes; indeed, they don’t even have caps. Appearing in fields or other open areas, various puffball species can be smaller than golf balls or almost as large as volleyballs. Many are edible before they fully ripen, but most aren’t really tasty.
Most puffballs utilize a very primitive way of accomplishing spore dispersal. Once the puffball ripens, the outer membrane containing the spores becomes dry and cracks open. Wind, falling debris, raindrops, and other agents hitting the puffball cause the spores to pop out. It’s that simple. But it’s a crude method — all of the spores in a given puffball aren’t usually dispersed and thereby go to waste.
There is, however, a category of puffballs known as earthstars in the genus Astraeus. These star-like fruiting structures — named for “the starry one” in Greek mythology who was responsible for the ancient arts of astronomy and astrology — can be very attractive in regard to both color and conformation.
Most earthstars then disperse their spores like other puffballs. But one species sometimes called the barometer earthstar (Astraeus hygometricus) has devised an ingenious mechanism whereby full spore dispersal is obtained.
As the common name indicates, the outer covering of the barometer earthstar is sensitive to meteorological conditions. During wet weather the outer covering remains open and some spores are no doubt dispersed in the normal puffball manner.
But during dry weather the outer covering contracts and closes in on top of the inner core, thereby squeezing out any spores that remain. The opening and closing mechanism is triggered because the “star-like rays” absorb water at different rates, which causes them to close or open in dry or wet weather. It takes about five minutes to do either.
Many field guides indicate that earthstars appear in late summer or fall. For whatever reason, Elizabeth and I spot them more frequently this time of the year on into early spring, during rainy periods when the fruiting structures are fully open.
We have chanced upon them in sandy woodlands as low as 1,700 feet on our property and as high as 4,000 feet in the Highlands area. It would never occur to us to go out “hunting” for earthstars. They are something that you just happen to look down and see.
What we think of as a mushroom is actually the equivalent of a fruiting structure.