Blog Archives

Bird Orchid Pterostylis barbata

24/9/2017

Greetings fellow Foxies,

I was excited to find a bird orchid at Newman Block south-east of Highbury, because I hadn’t seen one for 65 years. My interest in wildflowers began then when my mother and I used to look for orchids in remnant bush at Karrakatta Cemetery. Alas now all graves.
This orchid has always fired my imagination with its elegant shape, translucence and the strange wispy labellum that gives the species name. Barbata means beard.
with wispy labellums and elegant shapes, plumed greenhood orchids are the most evolved of the Greenhood orchids.
In this case the galea (hood) is folded inwards to form the entry and exit spots for gnats with essentially the same column tube and anther location.
Unlike most greenhoods, the bird orchid is pollinated by a tiny predatory fly dagger fly Emis species). Male flies present a gift to females (dinner date?). The image taken from this reference, shows a male presenting to an orchid.

0 Comments

Shell and Jug Orchids

23/9/2017

1 Comment

Greetings fellow Foxies,

After dissecting greenhood orchids to see how they trapped insects I moved on to shell and jug orchids that have a more open appearance. In the shell orchid image below one can see that the lower two sepals are upswept rather than being down to form a landing platform, the hood is more open, and the labellum is much skinnier. A few deft slices of my trusty scalpel revealed how this unlikely combination could force an insect to pollinate the flowers. Near the base of the interior the hood petals fold in and the labellum widens to seal the chamber containing the column tube.
In the cross-section below the labellum is closed, but if you picture it leaning forward, imagine a male gnat crawling down it, following the plant-made scent of a female (pheromone).
When the gnat get to the base, the labellum snaps forward, forcing it to climb the column (leaving behind pollen attached its body), through the tube where it gets another dose of pollen, and out of the flower.
Can you see a gnat that was trapped by a curly bit at the base of the labellum? It is the remains of a Fungus Gnat (rather appropriate that it has been consumed by a fungus), whose maggots consume mushrooms, but more of this below.

Pterostylis hamiltonii Red-veined Shell Orchid

Arrows show gnat's journey through the flower

The Jug Orchid, Pterostylis recurva has an even more open- looking structure than shell orchids, but it is just a modification of the shell orchid layout.
It has the same infolded petals and thin labellum with widened base and little horns at the top of the column tube to position the labellum when closed.

Pterostylis recurva Jug Orchid

Front and side section

Note the gnat trapped by a strange basal labellum structure. I originally thought that the evil looking hook was meant to poke the gnat in the bum if it tried to reverse down the column, but now suspect a more sinister purpose. The hook itself could stop the labellum from snapping too far back, but the fork underneath seems to be designed to trap insects, as does the curved projection in the shell orchid and the big donger in the Frog Greenhood.
An associate doing a PhD on greenhood orchid pollination agreed with me that evolution rarely produces intricate structures like this for no reason, but said that insect deaths may be collateral damage if the plant is successfully pollinated anyway. Is the hook just a fancy counterweight?
In the image below the column is smeared with pollen, so other gnats had been safely through this flower before this poor individual met its end.

However the mouldy gnat in the shell orchid reminded me that orchids require fungi to form a mycorrhiza. IF the mould spores could serve this purpose, this would be a neat way for these orchids to inoculate their seeds.
Who knows?

Gnat trapped in Jug Orchid

Gnat remains in Red-veined Shell Orchid

1 Comment

Plain but Ingenious Greenhood Orchid

21/9/2017

0 Comments

Horizontal galea Pterostylis concava

Greetings fellow Foxies,
Greenhood orchids (Pterostylis species) look plain but their flowers are little marvels of nature, which trap insect pollinators (mainly Mycetophilidae and Sciaridae families, commonly known a fungus-gnats and dark-winged fungus-gnats respectively). The hood called a "galea" consists of the dorsal sepal fused with two lateral petals. The galea curves forward to form a light-filled chamber containing the column. The column, which contains the sexual parts of the flower (stigma and pollinia) has wings which fold around to form a tube.
The lower sepals are partially joined make a landing platform on which, the lower highly modified petal, the labellum closes the trap when gnat passes over it.

There are two basic galea orientations. Local examples are

Horizontal galea with a short labellum:- dark banded greenhood orchid P. sanguinea, frog greenhood orchid P. sargentii, cupped greenhood orchid P. concava, and painted rufous greenhood orchid P. picta.
Semi- vertical galea with long labellum:- jug orchid P. recurva, red-veined shell greenhood orchid P. hamiltonii, hairy stemmed snail greenhood orchid P setulosa, bird orchid P. barbata.

Vertical galea P. hamiltonii

Vertical galea P. setulosa

Vertical galea P. recurva

Vertical galea P. barbata

Male gnats are attracted to flowers by a female pheromone scent, and walk over the labellum, which snaps shut and traps them. Light from the translucent galea above guides them through the column tube where they press against the stigma and pollinia to fertilise the flower then attach a pollen sac to the gnat's back.
The labellum is a miracle of evolution: a petal which has changed into a tiny trapdoor with an attachment on its base called a lure, and a hinge attachment. The lure is a counter-weight and pheromone emitter. I noticed that jug and shell orchid lures had pinned and killed unfortunate gnats that triggered them.

Pterostylis sargentii

Pterostylis sanguinea

Pterostylis picta

P. hamiltonii interior of flower

When a gnat crosses the labellum it snaps shut against the column. To escape, the gnat climbs up the light-filled hood, through the tube in the column and squeezes past the pollenia. Hairs at tube entry stop the gnat moving backwards. Arrows in images below show a gnat's journey through the flower.
If the labellum shuts and the flower is not pollinated, it reopens in ten to thirty minutes.

Pterostylis concava

Pterostylis recurva

More information
Foxypress Bird orchid
Overview of Pterostylis pollination in Victoria

0 Comments

Bird Orchid Pterostylis barbata

Shell and Jug Orchids

Plain but Ingenious Greenhood Orchid

Author

Categories

Archives