Orchids unseen

there are no conservation efforts in place currently, and the outlook for the survival of R. gardneri , at least, is precarious in‐ deed. Further work should explore the population genetics, complex interdependencies, and the ecology and reproductive biology of the genus Rhizanthella. A combined approach of exploration surveys and ex situ conservation and re‐introduction to suitable habitats should be an urgent priority to help conserve the world's only known un‐ derground flowers.

F I G U R E 1 (a) The habitat of Rhizanthella gardneri in Western Australia; (b) The capitulum of R. gardneri; (c) Excavated mature inflorescences of R. gardneri; (d) The exposed bracts of R. gardneri. and may be more widespread than previously thought, given that there are extensive areas of suitable habitat across Southern and Western Australia (Bougoure, Brundrett, Brown, & Grierson, 2008).
The rarity and subterranean life history of Rhizanthella suggest that new taxa could await discovery. Exploration surveys and phylogenetic data from across populations are required to resolve better, this cryptic genus. Surveys are complicated by the need for excavation, which involves careful removal of the top centimeter of soil to reveal the tips of the bracts (Figure 1d; Dixon, 2003). Indeed, locating the orchids is established to be difficult and unpredictable: searches between 1980 and 1984, involving 3,000 person-hours by volunteers, identified orchids in fewer than 4% of likely habitats (Dixon, 2003).

| E VOLUTI ON AND LIFE HIS TORY
Mycoheterotrophy has evolved multiple times independently among flowering plants (Merckx, Bakkerb, Huysmansa, & Smets, 2009) and is particularly prominent among orchids (family Orchidaceae; Lam et al., 2018). Like other mycoheterotrophic orchids, Rhizanthella is parasitic, acquiring nutrients from a specific mycorrhizal fungus that forages for soil nutrients (i.e., Nitrogen) and at the same time, dependent on an autotrophic host for a continuous supply of carbon (Bougoure, Brundrett, & Grierson, 2010;Warcup, 1985). Thus, they are completely devoid of photosynthesis; indeed the plastid genome ("plastome") of Rhizanthella is established to be the smallest organelle genome described in land plants (Delannoy et al., 2011).
Underground orchid species appear to be ecologically distinct.
For example R. slateri occurs in relatively moist, shady eucalypt woodland of central-eastern Australia (Jones, 2006)

| REPRODUC TIVE B IOLOGY
Very little is known about the reproductive biology of underground orchids, and for species besides R. gardneri, the pollinators are virtually unknown. All species produce terminal capitula of small flowers enclosed by fleshy bracts. Rhizanthella gardneri is apparently clonal and persistent in a given location, and produces dormant daughter tubers (Dixon & Christenhusz, 2018;Hágsater & Dumont, 1996). Termites, fungus gnats, flies, and wasps have all been suggested to be pollinators of R. gardneri (George, 1980;Dixon, Pate, & Kuo, 1990;Dixon, 2003;Mursidawati, 2004;Swarts & Dixon, 2009). Interestingly, Rhizanthella is the only known angiosperm to be pollinated by termites (Dixon & Christenhusz, 2018 & Swing, 2007). The flowers of the parasitic plant Hydnora triceps also produces flowers beneath the soil surface from time to time, to which insects apparently are attracted through cracks in the ground (Musselman & Visser, 1989). Given that most species of Rhizanthella also bloom just below the soil surface, pollinators could access the The seeds of Rhizanthella are produced in fleshy fruits which take several months to mature; seed dispersal has never been observed, although native fossorial (underground-dwelling) marsupial mammals are suggested to be likely dispersal agents for R. gardneri (Dixon & Pate, 1984;Hágsater & Dumont, 1996;Mursidawati, 2004); these animals are now extinct in the region where the plant occurs (Swarts et al., 2009).

| CON CLUDING REMARK S
As orchids "unseen" Rhizanthella provides a fascinating narrative to engage people with plants. All species of Rhizanthella are scarce, have complex ecological dependencies and are of critical conservation concern. For example, just a handful of locations containing fewer than 20 individuals of R. gardneri exist, and the species is critically endangered (Delannoy et al., 2011;Swarts et al., 2009).
Habitat fragmentation, reduced fire occurrence and the loss of seed dispersal agents all point to grave conservation concern for R. gardneri. Indeed, up to 95% of the probable range of the species has been cleared, and remnant bushland thickets containing the orchid are isolated, and in one case, privately owned (Hágsater & Dumont, 1996). Similarly, the newly described species R. johnstonii is known from just a single location, and is projected to decline in abundance (Dixon & Christenhusz, 2018). Climate change and soil salinization, together indicate a high level of extinction risk for Rhizanthella (Swarts et al., 2009). Taken together, these exceptionally unusual plants present a highly unusual challenge for conservation biologists, and their growth characteristics make it difficult to assign an accurate conservation status to them (Hágsater & Dumont, 1996).
Botanic gardens have already played a major role in the conservation of R. gardneri to date through survey work and research into propagation and conservation management options (Swarts et al., 2009).
However there are no conservation efforts in place currently, and the outlook for the survival of R. gardneri, at least, is precarious indeed. Further work should explore the population genetics, complex interdependencies, and the ecology and reproductive biology of the genus Rhizanthella. A combined approach of exploration surveys and ex situ conservation and re-introduction to suitable habitats should be an urgent priority to help conserve the world's only known underground flowers.