Friday, 7 March 2014

Miniature marvels

Orchids not only produce the tiniest seeds in the world, but they do in the millions. Check out a great article on the Kew Gardens website about why this is so.

Did you know a  typical orchid seed is merely the size of a speck of dust?

A single capsule of the tropical American orchid Cycnoches chlorochilon produces almost four million seeds, and one gram of seeds of the southeast Asian species Aerides odorata contains 3.4 million seeds.

Those of the New Caledonian species Anoectochilus imitans are said to be the smallest of all, measuring just 0.05 mm in length. At a ‘gigantic’ 6 mm, the seeds of the lopsided star orchid (Epidendrum secundum) are allegedly the longest of any orchid.

Because orchid seeds lack a food reserve in the form of an endosperm or a large embryo, most of them, especially terrestrial ones, are generally unable to germinate on their own. They first have to engage in a mycorrhizal relationship with a fungus that helps to feed the emerging seedling. Some orchids are able to join up with many different species of fungi whilst others only accept a very specific fungus to enter their lives (or rather roots). Few orchids don’t need any fungus at all for their germination, such as certain species of Disa from South Africa, a remarkable exception among terrestrial orchids.

The article explains their dependence on fungal partners. With their small size, low weight and balloon-testa, orchid seeds are perfectly adapted to wind-dispersal. However, their strategy is not to travel long distances. Scattering large numbers of seeds with the wind merely heightens the chances that at least some end up in a place where they are lucky enough to meet their specific fungal partner without which they cannot germinate.

This does not mean, however, that their seeds can’t cover long distances. Orchids managed to reach isolated islands far away from the mainland. As famously documented, they were among the first pioneers to resettle on the islets of Krakatoa after the catastrophic volcanic eruption of 27 August 1883.

If you like speckly vanilla ice cream, unless you make it yourself, the specks could be fake.

Image: Seeds of orchids, plate 2 of 3 - by J.G. Beer (1863) published on Beitrage zur morphologie und biologie der familie der orchideen. Vienna, Austria: Druck und Verlag von Carl Gerold's Sohn. 1863

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