P is for Plant Defences

As the great glam metal band Poison sang in 1988, “Every Rose Has Its Thorn“.

Like so many glam metal bands to grace the world’s stages before them, none of Poison’s members were botanists.  If they were, they might have known that roses actually have prickles, not thorns.

It’s an easy enough mistake to make.  But because I am a pedant at heart, I want Poison to know that, technically, thorns are modified branches, spines are modified leaves, and prickles are modified skin.  That means roses have prickles.  I therefore petition that the lyrics of the chorus of the song be changed from:

Every rose has its thorn
Just like every night has its dawn
Just like every cowboy sings his sad, sad song
Every rose has its thorn

to the more scientifically accurate:

Every rose has its prickle
Just like brine turns veggies to pickles
Just like every cowboy is really quite fickle
Every rose has its prickle

But how did plants come to develop all of these different ways to impale gardeners’ fingers in the first place?

To answer that question, let’s imagine a world without thorns, spines, or prickles.  No toxins, sap, poisons, or deterrents of any kind.  In a world like that, as long as there were herbivores, plants wouldn’t last long.  They’d get eaten up pretty quick and getting eaten generally isn’t good for your reproductive health (with a few noticeable exceptions *cough* black widow spider *cough*).  So there’s a lot of evolutionary pressure on plants to develop ways to avoid becoming lunch.

Predation from salad-eaters isn’t the only pressure on plants, though.  They also need to compete with other plants around them by growing to capture more sunlight and they need to devote resources to reproducing.  This is called the growth-differentiation balance.  Plants, with limited resources, must choose between straightforward growth and developing specialized defences.  If this theory is true, we would expect plants that didn’t have to worry about getting eaten would be less thorny.  In October 2014, an international team of researchers showed that in an herbivore-free zone, like the favourite hangouts of leopards on an African savanna, non-thorny plants thrive, whereas the thorny plants do best in the favourite hangouts of salad-eating impalas.

Salad-eaters (aka herbivores) don’t just take an evolutionary backseat to this escalation of plant defences.  Ever since the first animals started emerging from the sea and started choosing salad, plants have been trying to send them back and animals have kept coming.  It’s an evolutionary arms race.

And if the Cold War taught us anything, its that arms races lead to some pretty ridiculous specializations.  Here are a few of my favourites on the plant side of things:

Sorcerer Corn

It looks like regular corn.  And that’s because it is.

Regular corn seedlings, when exposed to a chemical in the saliva of beet armyworms, will release a chemical that summons a cloud (or, less dramatically, attracts) parasitoid wasps which will lay eggs inside of the armyworms.  These eggs will hatch after two days and eat their way through the armyworm from inside out.

Corn isn’t the only plant that releases signals like this.  In fact, you know the smell of freshly-cut grass?  That turns out to be the plant equivalent of screaming out to any relatives in the area to “GET READY! THERE’S SOMETHING THAT WILL HURT YOU NEARBY!”

Flinching Flowers

We normally think of plants as stationary things, unable to move.  This is usually true, but there are some plants which have the ability to quickly shut their flowers or droop on contact.  The most famous example of this is the Venus Flytrap, but that is more of an offensive flinch.

Mimosa pudica, or the sensitive plant, also has this flinching (thigmonastic) ability.  When touched, this species will close its flowers and fold away its leaves, thus decreasing its surface area and making it harder to see and eat.

Whether its by developing thorns, spines, prickles, the ability to fold up, or the ability to call helpful predators, plants have not been idle in the fight against salad-eaters.  Every rose has its prickles (not thorns!) because of this ancient struggle and while they may be annoying, I guess we should be happy roses haven’t evolved to attract parasitoid bears that will leave cubs inside of our stomachs to gnaw their way out over the course of a week.

Yet.