An invisible flower
In May 1928, a farmer named Jack Trott was ploughing land near Corrigin, Western Australia, when he noticed a strange, succulent plant completely buried in the soil. It was Rhizanthella gardneri, an orchid that lives its entire life underground. The discovery caused such a stir that a wax model of the plant was later toured around the British Isles.
This plant has no leaves and produces no chlorophyll, the pigment that allows most plants to create energy from sunlight. Instead of photosynthesis, the orchid is a mycoheterotroph, stealing all its energy from a fungus. Its horizontal stem, a rhizome, grows 6 to 12 centimeters below the soil surface. From late May to early July, this subterranean plant flowers, producing a fleshy, tulip-shaped head called a capitulum. This structure pushes up to just below the leaf litter, sometimes creating a small crack in the soil. Inside are up to 100 small, inward-facing flowers, each about 4 to 5 millimeters wide and colored in shades of cream and deep reddish-purple.
A three-way relationship
Rhizanthella gardneri does not simply parasitize any fungus. It has a highly specific, three-way relationship with a particular fungus and a particular shrub. The orchid is completely dependent on a mycorrhizal fungus, Thanatephorus gardneri, for its survival. This fungus, in turn, has a symbiotic relationship with the roots of the broom honey myrtle (Melaleuca uncinata and related species).
The fungus forms a bridge, drawing carbohydrates produced by the Melaleuca shrub's photosynthesis and transferring them to the orchid. The orchid hacks this network, stealing the energy it needs to grow and flower in total darkness. This makes the orchid an epiparasite, a parasite on a parasite. This extreme specialization is why R. gardneri is only ever found growing in thickets of these specific shrubs.
The plant's reproductive strategy is adapted to its underground life. The flowers emit a scent that attracts soil-dwelling insects like termites and fungus gnats, which are thought to be the primary pollinators. After pollination, each flower develops into a fleshy, berry-like fruit containing up to 150 seeds. Unlike most orchids which produce dust-like seeds dispersed by wind, R. gardneri's seeds are likely dispersed by small mammals that eat the fruit. The plant can also reproduce asexually by budding off new plants from its rhizome.
Due to extensive land clearing for agriculture, rising soil salinity, and drought, the orchid is classified as critically endangered. Only a handful of populations are known to exist, with possibly fewer than 250 individual plants remaining. Conservation efforts are underway, with several populations protected within nature reserves.