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Imagine That

Imagine That


Have you ever discovered that something you thought quite ordinary (or even ugly), was actually a priceless antique? I remember harbouring such sentiments when I was a teenager. Since then, of course, I have learned better how to identify valuable items.

In a different context however, recently I discovered that a common wildflower of the boreal forest floor, a plant which we see everywhere in woodlands in spring, is actually an exceptionally remarkable biological specimen. I can’t wait for next spring to come so that I can look at bunchberry more closely.

The bunchberry plant is native to the North American boreal forest everywhere from Greenland to Newfoundland to Alaska. Each plant has a short stem, 7.5-15 cm (3-6 inches) tall, topped by a whorl of 4-7 shiny evergreen leaves. The flowers occur in a cluster above the point where the leaves are attached. However the blossoms are not exactly conspicuous. Greenish or white in colour, each flower is about 2 mm long. No wonder few people notice such flowers.

There is however something that people do notice. In the same way that tiny yellow Poinsettia flowers are surrounded by showy red bracts, so also the bunchberry flowers are surrounded by four showy white bracts. The bunchberry “flower” that most people identify, is actually a flower cluster with bracts. Later in the season the inflorescence develops bright red berries which are just as showy as the “flower” stage. The plant obviously takes its name from this cluster of fruit.

The plants are striking in their dense stands, but few people pause too long to examine them since there are usually other, less common blossoms to find and identify. Nevertheless, if we were to sit with a magnifying glass to observe these plants while the flowers are still in bud, we might discover that the flowers open in a spectacular fashion. Measurements on a miniature scale reveal that the bunchberry flower opens so quickly that it out-competes some organisms which are famous for their speed. Everybody knows about the snap of the venus flytrap (accomplished in 100 milliseconds or thousandths of a second or ms). Well bunchberry flowers open faster than the venus flytrap closes. Bunchberry flowers even manage to snap open faster than the famous stealth attacks of the mantis shrimp (complete in 2.7 ms).

One might suppose that such a fancy design feature provides the bunchberry flower with a special benefit and indeed it does. These flowers need to receive pollen from another plant in order to set seed. There are various ways to achieve this such as dispersal by wind or insects, but this plant uses another method for enhancing pollination as well. The buds open explosively. In the process, pollen is catapulted to comparatively impressive heights. At the exalted height of an inch (2.5 cm)!! above the blossoms, wind can better disperse the pollen to nearby flowers.

The process goes like this. First the petals, which are fused at their tips, pull apart and move out of the way. This process takes a mere 0.2 milliseconds. As they open outwards, the petals achieve a maximum speed of 6.7 metres per second with the impressive acceleration rate of 22,000 metres per second per second!

Once the petals are out of the way, bent filaments which have been holding the stamens in position against a central column (the style of the stigma), now begin to unfold. Once again we discover that living organisms employ designs that man has also developed, but the latter without the finesse exhibited in nature.

In the middle ages for example, armies used catapults to deliver rocks or fire to enemy castles. Specially effective catapults were called trebuchets (from an Old French word meaning “to overthrow”). These maximized the throwing distance by attaching the payload to a flexible strap. This device propelled the payload upward faster than an ordinary catapult could manage. Well guess what! Bunchberry stamens are attached to their filament by means of a thin flexible strap. The whole system constitutes a trebuchet.

Once the petals are out of the way, the bent filaments pull back in an arc. The stamen is then accelerated upward to a maximum vertical speed. Immediately, with a jerk, a cloud of pollen is released. During the first 0.3 milliseconds, the stamens accelerate at up to 24,000 metres per second per second. The duration of the process is so short however that the stamens reach only a maximum speed of 3.1 metres per second. The whole event does not last even a second, only about 0.5 milliseconds. Nevertheless, the result is that the pollen grains are launched to a height of 2.5 cm (more than 10 times the height of the blossom).

To put this in context, imagine that you could throw a ball upward to ten times your height in less than 0.5 thousandths of a second. Imagine the lucrative sports contracts you could command!

However, getting back to bunchberry, in quiet air over the forest floor, the pollen may reach a distance of 22 cm (100 times the flower’s diameter). With a breeze, flowers a metre away may receive a dusting of pollen.

It is evident that if these flowers were several cm in diameter, the impressive talents of this plant would be much more famous. When events happen in miniature however, we often miss the action. The source of the energy for the speed and force of the pollen catapult is stored mechanical energy in the form of water pressure in specific cells.

Biologists are only now beginning to investigate what allows a cell to release water pressure that quickly. Something impressive is happening, something that we do not yet understand. It is evident that even organisms which seem quite ordinary, may nevertheless exhibit remarkable talents. It is all part of the richness and variety of the creation. (see Nature 435 May 12/05 p. 164)

October 2005

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