 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 . <unk> earthstars include G. <unk> , G. <unk> , G. <unk> , G. <unk> , and G. <unk> . Unlike Geastrum , the young fruit bodies of A. hygrometricus do not have a <unk> ( sterile tissue in the gleba , at the base of the spore sac ) . Geastrum tends to have its spore sac opening surrounded by a <unk> or a disc , in contrast with the single <unk> 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. <unk> , 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 . 
