Friday, 4 October 2013

That Natural Wow! Factor

The walk and jump of Equisetum spores

One of the things I get from reading about science is that little frisson of pleasure in finding out something new and being amazed at the sheer inventive ingenuity of nature and especially of the process of evolution.

You can quite understand why designers of things like modern aircraft wings use a genetic algorithm. The unintelligent, trial and error approach, free from preconceptions and fixed ideas about what something should look like or how it should work, often comes up with the the completely unexpected; the sort of solution to a problem an engineer sitting at a draughtboard would take several lifetimes to come up with.

To appreciate this fully it helps to know a little about a group of plants known as horsetails. Briefly, horsetails, sometimes called horsetail ferns, though they are not true ferns, are considered to be intermediate between the ferns and the flowering plants. Characteristically, they have a stem which looks in cross-section like a bundle of simple stems and have feature looking a little like the 'vascular bundles' of flowering plants. Like the ferns they reproduce through spores rather than the seeds of the flowering plants.

Like all plants, one of the challenges they face is in dispersing their spores or seeds. The last thing a spore or seed needs is to find itself germinating right next to its mother, who will have grabbed all the resources. The study of how flowering plants alone have overcome this challenge is a major branch of botany in itself. Apart from wind dispersal, the precise mechanism used by the horsetails to disperse their spores was unknown, until now. And it's in the method of dispersal of these spores where our little surprise is to be found.

They walk and jump.

Horsetail spores. Above - contracted; below - expanded.
Equisetum plants (horsetails) reproduce by producing tiny spherical spores that are typically 50 µm in diameter. The spores have four elaters, which are flexible ribbon-like appendages that are initially wrapped around the main spore body and that deploy upon drying or fold back in humid air. If elaters are believed to help dispersal, the exact mechanism for spore motion remains unclear in the literature. In this manuscript, we present observations of the ‘walks’ and ‘jumps’ of Equisetum spores, which are novel types of spore locomotion mechanisms compared to the ones of other spores. Walks are driven by humidity cycles, each cycle inducing a small step in a random direction. The dispersal range from the walk is limited, but the walk provides key steps to either exit the sporangium or to reorient and refold. Jumps occur when the spores suddenly thrust themselves after being tightly folded. They result in a very efficient dispersal: even spores jumping from the ground can catch the wind again, whereas non-jumping spores stay on the ground. The understanding of these movements, which are solely driven by humidity variations, conveys biomimetic inspiration for a new class of self-propelled objects.

The secret to how these ribbon-like elaters work is in their two layers. One of these layers absorbs moisture out of the atmosphere more quickly than the other does, so they straighten out, or curl up, according to the prevailing humidity. This causes them to move around randomly. Sometimes they become stuck to the surface of the spore or tangled up, so a tension builds up which is suddenly released and the elater spread out causing the spore to jump. Given that the movement is random and there are more ways of moving away from the parent plant than towards it, the result of this random walking and jumping is dispersal.

And that's it.

The sort of unexpected yet functional mechanism which evolution can come up with and which probably hasn't been improved upon since the Paleozoic era when horsetails evolved, some 250 to 540 million years ago.

Further reading:
Pamela J. Hines, Dessicated Dispersal; Science 4 October 2013: Vol. 342 no. 6154 p. 17
DOI: 10.1126/science.342.6154.17-b

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