Why stinging nettles understand us better than we do

Stinging nettles for most people summon up images of path verges and unwilling walks through fields somewhat less manicured than you were told. For me the pictures are of jagged glass needles, precision engineering, and silent chemistry labs churning out the products of millions of years of testing poisons.

I’m not denying that they’re damned annoying, just saying they’re a lot more interesting than they get credit for.

For a start, the fact that they’re a plant which stings people intuitively seems ridiculous. Nettles are one of those things in nature which seem more at home in the darker back-catalogues of Spike Milligan than in your garden.

Thinking down evolutionary lines, their existence doesn't become any less absurd. The screaming myopic toddler that is evolution can only slowly stretch and pull pre-existing things over the generations, so to evolve a function you need to start with something which can be co-opted to fit the new role. In other words you can’t evolve a sword without a knife to work from.

So given that wasps’ venom-syringe stings evolved from stabby appendages used for sticking eggs into things, but plants don’t have any injecty things to work from, what the hell did the ancestor of stinging nettles muster up into a weapon?

We can glean a hefty clue from looking at a nettle leaf under a microscope. I’m a firm believer that you should never illustrate something with a photo which could be as well illustrated by a 17th century engraving - so check out what is probably the first microscope image in the whole world of what nettles use to sting:

This image is taken from Robert Hooke’s micrographia. Published in 1665, it was the first time the public had ever seen now-familiar things like cells and insects up close, and became a bestseller. You can look at the whole intricate thing here. I recommend at least having a glance: it's part olde worlde English and part perception-bending scifi adventure into the microscopic world. And the pictures are great.

Above you can see the evil spikes jutting out of the surface of a leaf like the claws of an eagle with serious developmental issues. But amazingly, the view is actually fairly similar whichever plant you're looking at. Having tiny spikes is very normal for plants, they seem to keep insects off by threatening to trap their legs in the same way as thick undergrowth would deter a goat. Ever looked closely at the stem of a poppy growing in a field?

On most plants these tiny hairs are as inert as toothbrush bristles. But for some, evolution has crafted them into strange and useful machines. Cannabis plants, for example, have turned them into minute hollow chemical factories for manufacturing and storing mind-altering oils, probably to mess with any insects who think the leaves would make an interesting salad.

The mind-altering chemicals in these cannabis hairs are mostly thought to be produced to protect the plant against marauding insects: imagine being a hungry beetle and landing on dinner only to find beach ball sized bubbles of weed essential oil. It's unlikely you'd get round to eating much, to start with at least. The only thing is we're not actually sure if bugs get high, and unfortunately for stoners everywhere who would definitely find the answer hilarious, this important research is unlikely to be funded soon.

On stinging nettles, some impressive engineering has turned these hairs into weapons of war, capable of mildly irritating animals weighing 10,000 times as much as the humble plant and with the unfair advantage of movement. The real innovation was to cover the leaves with what are essentially closed up hypodermic needles:

These hairs, sitting on a nettle stem, are also hollow. But instead of drugs they're filled with chemicals like serotonin and histamine which power the nerves and inflammation responses in people, cows, and other mammals.

It's hard to overstress how weird this is.

Plants don't have the same biochemistry as mammals, they don't need to make these things for themselves. In fact they don't even have nerves or inflammation. What's more, they have no way of working out how nerves work or what chemicals are used inside - they don't 'know' anything. Yet through trial and error these masterminds have discovered which chemicals our nervous system uses to function, and by pumping us with inappropriate amounts of them they can fend us off.

With the millions of different molecules floating around in every cell, this isn't unlike being pushed around by someone who doesn't share any close language with you, and making random noises, gauging his response each time, until you stumble perfectly across the words for 'stop please, we don't want any trouble now, do we?'

The way these perfect irritants actually get into your body is also implausibly elegant:

The needles are made essentially of glass and sealed full of their irritant payloads, with a bulbous tip and an area just below it where the wall is thinner and weaker. Our microscope friend Robert Hooke described the green base of each needle as "a more pliable substance... almost like a little bagg of green Leather or a wilde Cucumber."

As you brush a leaf, the end of the needle snaps off at the weakened break-point, leaving a sharply angled and open end. When you push against this, it sticks into the surface of your skin and squeezes the "little bagg", squirting the hair's caustic contents around your waiting nerve endings.

Normally when you get little red welts it's because your body has made histamine in response to something you're allergic to. The ones which surely follow now are because the nettles have injected you directly with histamine they've made themselves. For when this happens, some anti-histamine cream will take them down better than any dock leaf will.

So there you have it. Stinging nettles have mastered human biochemistry to find exactly which chemicals we use to signal pain. And it's turned its own inert hairs into precisely engineered delivery mechanisms. Millions of years of craft and chemical ingenuity you can reluctantly admire the next time you go for a walk - just be careful looking too closely.