Tuesday, 9 December 2014

Pollen shot a danger to insect life

I've always enjoyed firing off the trigger mechanism of the Trigger Plant (Stylidium) flower and it was one the subject of one of my first posts on Talking Plants, back in 2008. The fused male and female parts of the flower are cocked, ready to flick back into the middle of the flower when an insect (or my twig) makes contact with the sensitive area at the base of the trigger (a form of thigmotaxis, if you recall). This all happens within 20 milliseconds.

There are contenders for the plant speed record. The Bunchberry Dogwood (Cornus canadensis) has what are called 'exploding flowers', with pollen fired off at a reputed speed of 6 metres per second, just under half a millisecond after opening. Impatiens pallida has seeds that reach similar speeds and the Sandbox Tree, Hura crepitans, is faster, with seeds reputedly reaching speeds of up to 70 metres per second.

But let's back track to Cornus canadensis. The Missouri Botanical Garden Angiosperm Phylogeny Group website states that 'the anthers of Cornus canadensis have explosive dehiscence; the maximum acceleration rate of the pollen grains has been estimated at 24,000 metres per second squared (remember that acceleration is a measure of the change in velocity - that is, metre per second, per second...).

Cornus canadensis

To put that in context, this acceleration is 800 times that experienced by astronauts on liftoff from Earth. The pollen is travelling about 4 metres per second (14 kilometres per hour) when it leaves the flower. Botanist Ted Mosquin wrote a charming article on 'the explosive pollination mechanism in Cornus cadensis' in 1985 and summarised it online in 1998, in Botanical Electronic News.

Mosquin explains that the Bunchberry Dogwood is well armed for speed. It has a 'sensitive antenna-like structure' sticking out of one of the petals in the unopened flower, 'petals on a tensile spring' and male parts (stamens) with what are called 'elbow springs'. It's these stamen elbows that fling the pollen upwards and out of the flower.

In most plants, including the Trigger Plant, the firing mechanims involves one (or a fused part) of the flower working alone or perhaps constrained a little by petals. Ted Mosquin cites species of Pilea (the Artillery Plant), Urtica, Kalmia, Medicago, Sarothamnus, Lopezia, Hyptis, Hucuna, Ilex, Odontonema and Ravenala, noting that there are also some with 'less rapid floral movements related to pollination'.

The flower of the Bunchberry Dogwood is one of those flowers that isn't. What we often call the flower is a collection of a dozen or so inconsicuous flowers, within four big showy bracts. Most of the floral parts (and the bracts) are white, greenish-yellow or generally light in colour. The only exception are the female bits, which are dark purple and therefore quite prominent.

According to Ted Mosquin, the first thing you notice about the Bunchberry flower is that it is either in bud (as all are in the above, with the little antenna pointing skywards) or fully open. If you look closely you also notice that in open flowers the pollen has all been released (from the anthers). This is because when the antenna is prodded by a visiting insect or botanist the flower not only opens but there is an immediate explosive release of pollen. The petals peal back and the anthers spring out - flexing their elbows no doubt - spraying their entire pollen load into the air. The anthers are actually fully open in the bud, so that when the flower is triggered the pollen is ready to fly.

The flower has no need of nectar and this in effect wind pollination, set in motion by an insect, rather than what we would normally call 'insect pollination'. It would hard for an insect to eat the pollen, which might be part of why such a mechanism evolved. This triggering is so sensitive that any insect, even a small midge, can set it off. In fact Mosquin muses that the popping of the flower may well 'pose some theat to the life and limb of the smaller and more fragile of the woodland insects'.

It seems this explosion, or pop, happens extremely quickly and is hard to observe with the naked eye - one moment the flower is closed, the next fully open. It all happens in less than 0.4 milliseconds, 'less time than it takes for a bullet to travel the length of a rifle barrel'! Mosquin's implied concerns would seem warranted, so look out for insects riddled with pollen holes.

Images: unusually I've had to source all my pictures from other sources having never seen this plant myself. The picture at the top from the Adirondack Almanack, the second from White Flower Farm and the last from Portland Nursery.

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