 Astraeus hygrometricus , commonly known as the hygroscopic earthstar , the barometer earthstar , or the false earthstar , is a species of fungus in the Diplocystaceae family . Young specimens resemble a puffball when young and <unk> . In maturity , the mushroom displays the characteristic earthstar shape that is a result of the outer layer of fruit body tissue splitting open in a star @-@ like manner . The false earthstar is an ectomycorrhizal species that grows in association with various trees , especially in sandy soils . A. hygrometricus has a cosmopolitan distribution , and is common in temperate and tropical regions . Its common names refer to the fact that it is hygroscopic ( water @-@ absorbing ) , and can open up its rays to expose the spore sac in response to increased humidity , and close them up again in drier conditions . The rays have an irregularly cracked surface , while the spore case is pale brown and smooth with an irregular slit or tear at the top . The gleba is white initially , but turns brown and powdery when the spores mature . The spores are reddish @-@ brown , roughly spherical with minute warts , measuring 7 @.@ 5 – 11 <unk> in diameter . 
