Thursday, October 8, 2015

Slender Onion Orchid

Microtis parviflora 

This humble little fellow must be our least ornamental local orchid. The flowers have tiny “faces” and seem to be almost all ovary.

You have to be paying attention, to notice that they are orchids at all!

However, the minuscule flowers are really delightful, so if you find any, look at them closely.


Not that they are easy to find. (Yes, there are some in the photo below. If you click on it, it will enlarge. Can you find them?)

Some internet sources claim that onion orchids are plants of “bogs and damp places”, but this colony of plants looked perfectly happy today, on an exposed, dry slope on Mt Kynoch red soil. They are said to do better after fires. This particular grassy, cattle-grazed site hasn’t had a fire for years and the plants were thriving. It would be interesting to see how it might be improved with fire.


The plants grow from underground tubers. At first glance they seem be quite leafless, but you can see that each flower stem does have a leaf wrapped firmly around its stem.

Like most orchid tubers, they are probably edible, and their tendency to grow better after fires was one of the reasons Aborigines burned their land. Early white settlers couldn't see why the fires were lit, and believed so strongly that Aborigines weren't farmers that they were not inclined to find out whether these apparently (to them) pointless fires actually had a practical purpose.

These orchids are highly unusual because they are pollinated by  ants.
Ants are normally the enemies of plants, when it comes to pollination. They secrete an antibiotic substance which kills it. (The antibiotic is produced by their metapleural glands, for those who are interested in that kind of thing).
Just to add to the difficulty, some ants’ rough little skins are simply too hard on pollen and likely to kill it.
Plants that need animal help for pollination have evolved flower designs to attract their pollinators, so it is quite interesting to examine just what appeals to ants.
Onion orchids are unique among orchids, as their pollinaters are wingless worker ants - usually little tyrant ants, Iridomyrmex gracilis. There is no question of attracting flying male ants by looking and smelling like a female, as happens with the other ant-pollinated orchids. Colour is clearly not relevant, and it is very obvious that this plant’s pollinators are not being attracted by showy petals!

(Hope you like the photo. It looks like a studio shot, doesn't it? However I assure you that no onion orchid was harmed in the making of this blog. It was held steady with a clothes peg on a stake, and the background is an out of focus trouser leg.)
These little orchids produce a sweet fragrance to attract the ants, and they deal honestly with them, providing the sip of nectar that the perfume promises. Honesty is not something we expect from orchids. Most of them are cheats. They dress up to imitate nectar-producing flowers, even having "nectar guides" - those lines that lead to the centre of the flower - but they don't supply the goods. The trick works for most insects, as demonstrated by the fact that the orchid family is one of the largest plant families in the world. Perhaps ants are not so forgiving as other kinds of insects (or not so stupid as to keep going from one unrewarding flower to another).
Having attracted their ants, the next problem is to deal with their pollen-destroying capacities. These knacky little onion orchids have evolved flowers which organise the nectar seeking ants so they can only get their reward if they are correctly aligned to pick a dab of pollen on the fronts of their faces,  well away from their metapleural glands. The flowers may look small, but they’re bossy!
They also have pollen with short stalks to hold the pollen grains safely away from the ants’ destructive skin.
Onion orchids fit a pattern shown by other types of ant pollinated plants. They tend to have small flowers, each supplying only a little nectar. This means that larger insects are not interested, and that even little ants have to visit a number of flowers to collect enough for their purposes.
They also tend to have flowering stems with flowers that open serially. This means that ants can’t find enough nectar by foraging all the way up a single flower stem, so they have visit more than one plant, carrying pollen as they go.

These little flowers, which at first seem quite boring, have a lot to interest us!

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