Monday, 17 December 2012

Sun orchids

I haven't paid a lot of attention to orchids; I've been focused on other plant groups.  But I could get keen on them now, after having a look at some this week.

When I was a young lad, my Dad learned in a conversation at work that New Zealand had "a rare native orchid", and I imagined I'd never be lucky enough to see it.  Not long after, I lost my footing in what we called "the bush" (now the Wilf Mexted Reserve at Tawa), and when I'd slid to the bottom of the bank I looked at the vegetation in my hand that I'd grabbed to slow my fall.  There it was, the rare native orchid!  I'd pulled it out of the ground, maybe it was the last one.

I was too ashamed to tell anyone, until much later.  When I got to University, I found that my orchid was the common greenhood orchid, Pterostylis banksii, and that there wasn't just one species of native orchid, but lots.  Some of them are indeed rare, but many are common.  They're all interesting, in part for the exquisite shapes of their flowers and the complexity of some pollination arrangements.

Thelymitra longifolia on a bank in Karori, Wellington.
This is another of the common ones, a sun orchid, Thelymitra longifolia. Its flowers are usually white, but sometimes pink, and some people suspect there might be several species going under the one name. The bits and bobs attached to the central column are distinctive, and useful in telling if you have this or one of the other species of Thelymitra.

Thelymitra longifolia flower.
The outer parts of the flower comprise three sepals and three petals.  In most orchids the petal in the front of the flower is highly modified as the labellum (e.g., the slipper in slipper orchids), but in sun orchids all three petals are alike so the flower is quite symmetrical.  In the middle of an orchid flower, the central column combines the male and female organs.

Thelymitra longifolia, side view (left) and front view of column.
There's a single stamen and a sticky stigma, and a whole lot of other features (terminology follows Moore & Edgar 1970).  The column surrounds the stamen and stigma on three sides and extends into a hood at the top, furnished with two column arms, each with a tuft of white hairs.  Behind the stamen is a pointed post-anther lobe; you can faintly see the sides of it above, but its tip is hidden behind the hair tufts on the column arms.  The glistening knob between the anther and the stigma is the rostellum, and it's believed to be a modified lobe of the stigma.

Thelymitra longifolia is considered by some to be self-pollinating.  Charles Darwin quoted Mr Fitzgerald's Introduction to Australian Orchids that it's fertilized in the bud, although it does sometimes open enough for cross pollination to occur.  Part of the evidence for selfing is the pollen doesn't bind together into two cohesive masses, the pollinia, but the grains separate and scatter about the inside of the hood, many apparently landing on the stigma.  Also the flowers often stay closed; they only open briefly on the sunniest of days.  But if self-pollination's the whole story, then it's hard to understand why the column is so complex and how the pollen gets to the stigma without some disturbance.  In the first flower I photographed there was a thrips, a tiny pollen-feeding insect that some people believe are important pollinators.  He was head down-bum up in the anther, spilling pollen around, but none had reached the stigma.
Thelymitra venosa, here on Stewart Island, is brightly coloured and has a relatively enlarged labellum.  It's more likely to be insect pollinated.
The onion-leafed orchid, Microtis unifolia.
I photographed another orchid this week.  This one, Microtis unifolia, comes up wild in the plant pots at home.  They each have a single leaf, and a stalk of tiny green flowers, each just 3–4 millimetres across.
Microtis unifolia, note the labellum hanging down at the front of the flower.

References:

Darwin, C. 1862.  The various contrivances by which orchids are fertilised by insects.  Murray, London. (I have the 1904 edition.)

Moore, L.B.; Edgar, E. 1970.  Flora of New Zealand. Vol. 2.  Government Printer, Wellington.

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