Mock up showing the different groups of Pterostylis orchids and their floral form (roughly to scale). Note the large differences in size, flower shape, presentation of labellum and number of flowers which correspond to genera proposed by some authors. From left (in brackets alternative genera): snail orchids (Diplodium), tiny greenhoods (Speculantha), shell orchids (Diplodium), greenhoods x2 (true Pterostylis), swan greenhoods (Hymenochilus), rufa greenhoods (Oligochaetochilus), bird orchids (Plumatochilos), tall leafy greenhoods (Bunochilus), banded greenhoods (Urochilus).
Greenhood orchids
Pterostylis, or greenhood orchids, are terrestrial herbs which sprout each year from an underground tuber. There are over 300 species primarily found southern and eastern Australia, along with some species in New Zealand, northern Australia and Papua New Guinea. Many greenhood orchids are woodland and forest species, and can be particularly common in wetter areas. The majority of greenhoods flower in late winter and early spring, although you can find a species flowering in each month of the year.
There is a remarkable diversity of floral form across the different groups of Pterostylis, and these distinct groups are commonly split into their own genera. One uniting (and unique!) feature is the presence of a motile (active) labellum which forms part of an intricate trapping mechanism. Insects that trigger the labellum are trapped inside the hood of the flower, and must escape via a passageway through the front of the flower, and in doing so they collect pollinia on their back. The orchid’s goal is not to kill the insect! Just to dab some pollen on its back so it can visit the next flower to deposit the pollen.
Distribution of Pterostylis (records sourced from the Atlas of Living Australia).
Sargent’s prediction
Writing in the Victorian Naturalist in 1934, Oswald Sargent predicted a bright future for Pterostylis pollination research, and presented some exploratory research on the subject. Sargent can probably lay claim to having been the first to observe the pollination process in Pterostylis vittata (1934), or at least being the first to have documented it, although others had noted dead flies inside the flowers (Cheeseman 1872, Coleman 1934).
“Though not lacking in modest beauty the “Greenhoods” are far from gorgeous. Yet, in some respects the genus Pterostylis is the most interesting of Australian orchid genera, and seems destined to occupy a very important place in the annals of research.”
Oswald Sargent, Perth-based pharmacist and keen amateur botanist, 1934.
Despite the promising outlook, Pterostylis was largely overlooked when Australian orchid pollination research took off in earnest the 1980s, 1990s, and 2000s, in favour of the sexier spider orchids, hammer orchids and bird orchids. A large number of Australian orchids have now been studied and shown to be pollinated by sexually deceived thynnine wasps which mistake the flower for a female. Several have even documented the precise chemistry required to attract and fool the male insect. Yet scientific research on greenhoods never really got going - perhaps Sargent’s quip about the greenhoods’ modest beauty held some truth.
The first scientific confirmation that Pterostylis orchids (in this case P. orbiculata) were sexually deceptive and pollinated by male fungus gnats appeared in 2014 (Phillips et al.). Remarkably, it had taken over 100 years to confirm what many anecdotal observations had already suggested (most notably observations from David Jones, Robert Bates, Rudie Kuiter and others).
Pterostylis sargentii, named after Oswald Sargent.
Phillips’ breakthrough
Ryan Phillips’ results (published in 2014) would not have come as a surprise to anyone familiar with Australian orchid pollination, as many anecdotal observations had indicated likely sexual deception of fungus gnats for pollination of Pterostylis. Nevertheless, Phillips confirmed that males from one species of fungus gnat (an undescribed species in the family Mycetophilidae) were the sole pollinators. He also set out criteria for how to confirm whether orchids (or other plants) are unequivocally sexually deceptive.
Like many first papers on a particular topic, Phillips’ paper raised as many questions as it answered. How many of the 300+ Pterostylis orchids are sexually deceptive? Is each species pollinated by a different fungus gnat? How do the orchids attract and sexually deceive the fungus gnats? Are different groups of Pterostylis also sexually deceptive? Are they also pollinated by fungus gnats? Are any Pterostylis species not sexually deceptive?
A male Mycomya sp. (Mycetophilidae) pollinator of Pterostylis orbiculata attempting to mate with the orchid labellum while carrying pollinia.
My PhD
When I started my PhD in 2017, I was of course curious about these questions, and want to be able to answer some of them. But more broadly, I could see that research on Pterostylis orchid pollination was lagging behind some of the other large and well-known orchid genera in Australia, and I felt that this research needed to be done.
And why is this research important? Because for many plants pollinators are required to transfer pollen from one flower to another and thereby ensure sexual reproduction and the continued survival of these plants. We know very little about fungus gnat pollination and about fungus gnats in general, but for Pterostylis orchids at least, they appear to be important pollinators. And if, as we suspect, many of these greenhood orchids are sexually deceptive, they will probably attract only one species of fungus gnat pollinator.
Plants with highly specialised pollination strategies may be at risk of extinction if their pollinators decline or go extinct. If we know nothing about the pollination of these orchids, we won’t know how to begin conserving them! I aim, through my PhD, to offer a broad overview of pollination in Pterostylis, which hopefully can form the basis for further, more targeted, research into how these incredible orchids evolved and how we can make sure they continue to survive into the future.
The fascinating and endangered Pterostylis despectans.
Reading:
Cheeseman, TF 1872, ‘On the fertilisation of the New Zealand species of Pterostylis’, Transactions and Proceedings of the New Zealand Institute, vol. 5, pp. 352–357.
Coleman, E 1934, ‘Pollination of Pterostylis acuminata R. Br. and Pterostylis falcata Rogers’, Victorian Naturalist, vol. 50, no. 11, pp. 248–252.
Phillips, RD, Scaccabarozzi, D, Retter, BA, Hayes, C, Brown, GR, Dixon, KW & Peakall, R 2014, ‘Caught in the act: pollination of sexually deceptive trap-flowers by fungus gnats in Pterostylis (Orchidaceae)’, Annals of Botany, vol. 113, no. 4, pp. 629–641.
Reiter, N, Freestone, M, Brown, GR & Peakall, R 2019, ‘Pollination by sexual deception of fungus gnats (Keroplatidae and Mycetophilidae) in two clades of Pterostylis (Orchidaceae)’, Botanical Journal of the Linnean Society, vol. 190, pp. 101–116.
Sargent, OH 1934, ‘Pollination in Pterostylis’, Victorian Naturalist, vol. 51, no. 3, pp. 82–84.