The vast and varying array of fungi sprouting in forests, fields and gardens are the temporary reproductive organ (or fruit body) sent forth by its underground parent when climatic conditions are favourable.
Fungi do not belong to the plant kingdom as they do not depend on light as an energy source, therefore, do not photosynthesise.
The major part of a fungus is essentially invisible, consisting of microscopic threads (called hyphae), weaving their way through the substrate. The substrate could be soil, wood (living or dead), or other dead or living material. A mass of hyphae is called a mycelium. Occasionally these threads can be seen near the soil surface if a 'mushroom' is disturbed.
The main function of the fungus fruiting body in the many forms that we observe, is to produce spores for reproduction. These dust-like spores are held on the gills, spines or pores of the fertile surface of fungi, or in the 'sac' of puffballs and earthstars.
Depending on the structure of the fungus, spores are dispersed by air currents, rain drops or running water, or pressure produced by vegetation movement and passing animals. Some spores are ingested by animals and dispersed as they are passed through the gut. Amazingly, some fungi even project spore-filled 'packages' in a cannon-like fashion. And thus, new fungi colonies are established.
Earthstars start out as 'eggs' with a double layer of 'skin' (right). The outer layer splits to reveal the inner 'sac' (left)
Saprotrophic fungi break down components of dead organic material. These provide a vital role in recycling the Earth's accumulation of dead vegetation and animals, returning nutrients to the soil.
Parasitic fungi obtain nutrients from living orgainisms with no benefit to the host. This not only includes living trees and other plants, but also other fungi and animals such as underground larvae of moths.
Symbiotic fungi form an association with a living organism which is of benefit to both. Fungi whose hyphae form a relationship with roots of trees and other plants are referred to as mycorrhizal.
Notice the spore-containing sac of the aging earthstar on the left is beginning to disfigure due to pressure exerted on the paper-thin skin by wind and rain. On the right is a newly-emerged earthstar that has not split its outer layer.
I have found earthstars in a wide variety of habitats ranging from moist mountain rainforest, dry woodland, grassy fields and gardens, to the nutrient-poor sand plains of south west Western Australia. All the Geastrum specimens here were photographed in the Hunter Valley.
A Geastrum species that arches to push the spore sac up - in a park garden at Wollombi in the Hunter Valley
As yet, I have not attempted to identify any of my Geastrum finds, but according to Fungimap (a project run by the Royal Botanic Gardens Melbourne), there are only about 10 species in Australia, so it might be possible for me to narrow down likely identifications, with considerable persistence.
A mature Earthstar and 'egg' - Barrington Tops NP
You're likely to encounter Earthstars at any time of year, but predominately in autumn and early winter, and often following rain. I found several Geastrum in mulched public gardens as the ground started to dry out after the June 2007 Hunter Valley floods. Delightful little things, arn't they?