 Although A. hygrometricus bears a superficial resemblance to member of the " true earthstars " Geastrum , it may be readily differentiated from most by the hygroscopic nature of its rays . Hygroscopic earthstars include G. arenarium , G. corollinum , G. floriforme , G. recolligens , and G. kotlabae . Unlike Geastrum , the young fruit bodies of A. hygrometricus do not have a columella ( sterile tissue in the gleba , at the base of the spore sac ) . Geastrum tends to have its spore sac opening surrounded by a peristome or a disc , in contrast with the single lacerate slit of A. hygrometricus . There are also several microscopic differences : in A. hygrometricus , the basidia are not arranged in parallel columns , the spores are larger , and the threads of the capillitia are branched and continuous with the hyphae of the peridium . Despite these differences , older specimens can be difficult to distinguish from Geastrum in the field . One species of Geastrum , G. mammosum , does have thick and brittle rays that are moderately hygroscopic , and could be confused with A. hygrometricus ; however , its spores are smaller than A. hygrometricus , typically about 4 µm in diameter . 
