9 Plant Defense Mechanisms Britannica.com

7/26/2019 9 Plant Defense Mechanisms Britannica.com

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Botanical Barbarity: 9 Plant Defense MechanismsWritten by: Richard Pallardy 2

Theres no brain in a cabbage. Thats axiomatic. But the

lack of a central nervous system doesnt prevent them,

or other plants, from protecting themselves. Some

species boast armature such as thorns, or spines that

ward off slavering herbivores. Some are loaded with

poisons to sicken or kill them. And while tomatoes may

not be able to scream, as L. Ron Hubbard once claimed,

some plants do emit compounds that warn their

compatriots of approaching threats: the botanical

equivalent of a smoke signal. Rooted to the fundament

though they may be, theyre not just vegetative victims content to idly watch their hard-won greenery

masticated into oblivion and ejected from the back end of some lazy cud-chewer.

Thorns are basically pointy branches or stems.

Their defensive function is somewhat self-

evident: theyre stabby. And contrary to what (the

band) Poison would have you believe, not a

single rose has one. See the next item for

explanation. (As to the sad, sad songwriting

abilities of cowboys, I cant say either way.)

Roses actually have prickles, not thorns. Unlike

thorns, prickles are actually pointed

protuberances from a plants epidermis. Think of

them as razor-sharp freckles. While they shield

the plants that bear them from some

depredations, certain species of planthopper,

small enough to squeeze between them and

slurp up sap, actually mimic their spiky

appearance to avoid predators.

9 Thorn

8 Prickle
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Stabbiness comes in handy when youre a plant

leaves (and the related stipules) have evolved to

draw blood, too. Many plants have spines, but

they are perhaps most memorably marshalled by

the cacti, who sport them in abundance. Spines

not only defend the juicy stems of cacti against

ravenous succulent-eaters, but also shade them

from the relentless desert sun. (Cacti that grow

in non-desert environs wear a lighter covering of

spines.)

If youve ever had the misfortune of brushing against a

stinging nettle, you know the pain-inducing power of the tiny

trichome. Nettles and other plants grow a bristling fur of

these pointed structures to shield themselves from browsing.

Had the proverbial very hungry caterpillar made the mistake

of wandering onto a trichome-bearing plant, he wouldnt have

been hungry for much longer. He would have been impaled, or

eviscerated. Some plantslike the nettlehave associated

glands that inject poison into the trichome-inflicted wounds.

Some tropical nettles can cause permanent nerve damage

or death.

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7 Spine

6 Trichome

5 Idioblast


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Not all plants bear their defenses on the surface.

If thorns, spines, prickles, and trichomes are the

spear brigade, idioblasts are the landmines.

Specialized cells that contain a variety of

defensive compounds, from razor-sharp crystals

to pain-inducing chemicals, idioblasts detonate

when the first line of defense has been

breached. The dieffenbachia, a common

houseplant, contains idioblasts that fire barbedcalcium oxalate crystals into the mouths of

predators and then release an enzyme

analogous to reptilian venom. This can cause paralysisand thus loss of speechhence the

common name dumb cane.

Some plants have opted to hire mercenaries.

Several species of South American and Africanacacia tree both house and feed aggressive

ants. The stinging little soldiers make their

barracks inside swollen thorns and feed off of

food bodies produced by the plant especially for

them. The ants savagely defend their giving

trees against all comers, be they animal,

vegetable, or fungus. They even snip off the

foliage of any other plants that have the nerve to

encroach upon their acacias personal space. In experiments where the ant colonies were

removed, the trees died.

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The sensitive plant (Mimosa pudica) closes its

leaves when they are touched, making them

appear dead and therefore unappetizing. These

plants are often sold as curiosities and featured

4 Commensalism

3 Crypsis
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at botanical gardens. Watching plants react in

real-time serves as a visceral and memorable

demonstration of the fact that plants are, in fact,

animate. Erasmus Darwingrandfather of

Charlessaid in The Botanic Garden: [plants]

possess ideasofmany of the properties of the

external world, and of their own existence. Darwin may go a step too far here, but his

observations indicate that the notion of plants as being in some way more than just lumps of

passive herbage is far from novel.

Plants that are attacked by browsers or insect

pests or subjected to stressful conditions such

as drought or microbial infection may warn other

plants of the impending crises by releasing

volatile organic compounds (VOCs), which

precipitate physiological reactions in nearbyplants. They may increase concentrations of

toxic compounds to ward off the enemy, or they

may release compounds of their own that attract

the enemys predators. Some recent experiments

have shown that plants also communicate

through chemicals released by their roots and

even via networks of fungal symbionts.

Everyone knows that some plants are poisonous.

But what constitutes a poison to one organism

may very well be an inert compound to another.

Birds, for example, are unfazed by urushiol, the

toxic oil produced by poison ivy, and in fact seem

to love the berries produced by the plants.

Monarch butterfly caterpillars chomp on

milkweed and sequester the glycosides

produced by the plant in their own tissues,

making them poisonous to predators. Of course,

humans have turned all manner of plant poisons

to their own devious ends, from the

chrysanthemum-derived pyrethrins used as

insecticides to the castor bean-derived ricin,

which Walter White of Breaking Badattempted to

use to eliminate his enemies on several

occasions (and which was successfully used in

the 1978 assassination of a Bulgarian writer).

2 Chemical signaling

1 Poison

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