The mature fruitbody consists of a roughly spherical spore sac atop the centre of a star-like base and the fruitbodies are commonly called earthstars. The arms of the star are very wide near the centre of the star and each tapers outwards to a point. The wall of the spore sac is thin and flexible. In the immature fruitbody the spore sac is enclosed within a thick casing. At maturity that casing splits radially along several lines and the segments then fold out to create the star-like base.
In the genus Astraeus the arms are tough, almost woody when dry, and polygonally cracked (like ‘crazy paving’) on the upper surfaces. The arms are hygroscopic, folding in when dry and opening again when moist. Initially at the top of the mature spore sac there is rough apical hole or tear and this may enlarge.
Most species in this group belong to the genus Geastrum. In this genus the spore sac (from a few millimetres to 3 centimetres in diameter) has a well-defined apical hole of constant size. The spore sac holds the powdery spores within a bundle of entangled filaments (much like a wad of cotton-wool). The wall of the spore sac is thin and flexible with an apical hole through which the spores can puff out when the sac is compressed (e.g. by a raindrop, a falling twig, a finger). When the pressure eases, the compressed ‘cotton-wool’ expands, restoring the sac to its pre-puff shape and so allowing future puffs. Without the ‘cotton wool’, the spore sac would stay compressed and puff no more.
The surface of a Geastrum spore sac is smooth to velvety, the spore sac may be raised on a short stem or be stemless, the arms may be hygroscopic or not, the apical opening may be just a simple hole or a more complex structure, the undersides of the arms may be smooth and clear of soil grains/debris or be coated with a strongly glued layer of soil grains/debris. These are some of the macroscopic features that help identify species.
If you think you have an earthstar with a spore sac about a millimetre in diameter, check the slime mould Diderma subasteroides.
** Myrisotoma: This looks like an earthstar with a number of holes over the upper half of the spore sac (which is held up by a number of sort, wiry stems). In Australia it has been found only in the Sydney-Newcastle area and in Queensland at Oakey, north-west of Toowoomba. Given those widely separated locations it must occur elsewhere as well. Should you find it in the local region, it would be a very noteworthy sighting! Here is a photo of a Dutch specimen: http://www.anbg.gov.au/fungi/images-captions/myriostoma-coliforme-0213.html.
Lepp, H. (2008). Myriostoma coliforme in Queensland, Australasian Mycological Society Newsletter, No. 4, 7.
Rees, B. J., Taeker, F., & Coveny, R. G. (2005). Myriostoma coliforme in Australia, Australasian Mycologist, 24, 25-28.
Sousa, J.O., Baseia, I.G. & Martín, M.P. (2019). Strengthening Myriostoma (Geastraceae, Basidiomycota) diversity: Myriostoma australianum sp. nov., Mycoscience, 60, 25-30.
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