Barometer Earthstar

Astraeus hygrometricus
Monday, June 28, 2010

This earthstar is also called the “false earthstar,” due to the fact that it belongs to the family Astraeaceae, rather than the family of true earthstars, Geastrum. Although it outwardly resembles a true earthstar, and indeed was grouped there until 1889, it has several differences. Among other things, it has larger spores, no organized hymenium (The spore-producing “substrate” tissue), and it lacks a columella (the sterile section of the spore-producing structure). The new genus Astraeus was originally thought to contain only two species; later analysis revealed six distinct species spread all over the world.

As in most fungi, the identifiable part that we see is only the fruiting body. The real “body” of a fungi is its mycelium, the underground complex of fibrous, branching hyphae. Thus, picking a mushroom (or earthstar) does not harm the organism the way picking a small plant would. The fruiting body of this earthstar is usually around 5 cm across. It is a lighter brown, while the rays are darker.

The young fruiting body of a Barometer Earthstar is a round sac partially embedded in the soil. As it matures, the exoperidium (outer skin) breaks open into numerous rays in a star-shaped pattern.  As they split apart, they reveal the spherical spore sac, enclosed in the endoperidium (inner skin). The rays unfold, pushing the spore sac into the air where the spores can be spread. Sometimes, they push it up hard enough to break its connection to the mycelium.

Barometer EarthstarThe Barometer Earthstar is so named because of its peculiar ability to read the air humidity. The different parts of the rays absorb water at different rates, giving the earthstar this unusual power. At times when the air or the ground below it is moist, the rays unfold to their fullest extent, raising the earthstar high above the ground. When a raindrop strikes the spore sac, spores are released in a burst, and their newly gained elevation increases the chance of them being caught by the wind and scattered far and wide. However, when the ground and the air are dry, the rays will lower the spore sac back to the ground and curl up around it, probably to protect it from predators or the elements. An earthstar may take only five minutes to completely open up, an almost visible process. However, it can take days to close again, likely because of the moisture remaining in the rays. Earthstars can remain in states like that for several years, even separate from the mycelium. However, when all the spores have been distributed, the earthstar disintegrates, leaving the rays opens.

These fungi are ectomycorrhizal (part of the mycelium is connected to the root tip of a plant), and in this area generally grow in association with oak and pine. It is a symbiotic relationship:  The earthstars, in exchange for energy, help the tree obtain water and nutrients, as well as helping to defend against nematodes and soil pathogens.

These four earthstars were found in the woods next to Route 613 past the hotel. They have been exhibited in different stages of expansion.

Article by Hazel Galloway

Astraeus hygrometricus opening when exposed to water-- total elapsed time about 5 minutes
This animation courtesy of