Geastrum

Geastrum (orthographical variant Geaster) is a genus of puffball-like mushrooms in the family Geastraceae. Many species are commonly known as earthstars.

The name, which comes from geo meaning earth and meaning star, refers to the behavior of the outer peridium. At maturity, the outer layer of the fruiting body splits into segments which turn outward creating a star-like pattern on the ground. The inner peridium is called a spore sac. In some species, the outer peridium splits from a middle layer, causing the spore sac to arch off the ground. If the outer peridium opens when wet and closes when dry, it is described as hygroscopic.