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Helpless, Passive Plants?

It is easy to regard plants as passive and helpless beings, just waiting to be eaten or cut. However, plants are tough survivors. They have successfully defended themselves against attack for hundreds of millions of years.

As the tree of life evolved, it expanded to include hundreds of millions of species of land animals. Most of  them are insects. Most insects are herbivores; they eat plants for a living. So do many mammals. Fungi do as well. Plants and herbivores have been in a
co-evolutionary adaptation race since each came to be. Plants find a new defense; herbivores of the same biological community discover a new way to penetrate that new defense. Plants do sustain damage:

A Sample of Plant Damage

beetle damage from night feeding
a fungus paints a picture on a yellow violet petal
maple leaf punctuated with enthusiastic grazing,
or virus attack
tiny grape leaf miner caterpillars eat inside leaves
ants farming aphids
larva of squash vine borer in damaged stem

During a 500 million year coevolution, plants have become  powerful chemists. They manufacture thousands of chemicals used for defense.

Plants communicate with multiple life-forms using signal-chemicals. Many signal-chemicals are volatile and spread through the air. Others are found inside leaves and stems. A host of signal-chemicals are produced in plant roots (see Rhizosphere).

Plants defend chemically in soil against  bacteria, fungi, protozoans, animals such as nematode worms and insects, and with other plants. Above ground, plants defend chemically against different bacteria, fungal spores, insect and mammal herbivores.

We larger mammals usually first encounter plants’ physical defenses. We get stuck by thorns, stung by nettles, rashed by poison ivy . Take a look at some of plants’ physical defenses that are always present.

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Continuous Direct Defense

Thorns, spines, and micro-needles:

Many herbs, such as thistles and blackberries, use  thorns to discourage grazers. Cactus are famous for their effective spines. Many woody plants use thorns against browsers. Spines and thorns reduce browsing by  large herbivores (deer, moose, antelope, goats, giraffes) by slowing the herbivores' feeding rate, and by wearing down their teeth.

A few herbs, notably nettles, cover their epidermis with microscopic needles that inject acid into animal skin at a touch.

Stinging hairs on a nettle leaf Photo credit Liz Hirst, Wellcome Images

The large stinging hairs are hollow tubes with walls of silica making them into tiny glass needles. The bulb at the base of each hair contains the stinging liquid that includes formic acid and histamine. The tips of the glassy hairs are very easily broken when brushed, leaving a sharp point, which easily pierces the skin to deliver the sting.

cat's claw acacia of the desert
gooseberry spines leave nothing to chance
cholla cactus spines are emphatic
prickly pear cactus

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Barriers: Cuticle, Bark, and Resins:

Woody plants have bark; herbs have waxy cuticles. Both are barriers that resist penetration by virus particles, bacteria, and the spores of fungi. Barks also provide some defense against insect jaws and stingers. Thick barks provide fire protection. The bark of redwood trees grows to one foot thick to resist fire.

Some plants cover their entire epidermis, leaves, stem, roots, with a contact poison to defend against herbivores. Poison ivy, poison oak, and poison sumac all coat themselves with the skin contact poison urushiol.

Most conifers produce resin, which fossilizes to become amber.  Famously, fossil amber contains insects trapped ages ago. Conifer resin is both antibacterial and antifungal. One study in 2008 found that wood ants bring large quantities of solidified conifer resin into their nests, which reduces the density of many bacteria and fungi and so protects the ants. Birch and other deciduous trees and shrubs also depend on resin defense.

Resin is also used by some conifers to protect their seeds, not by covering them, but by being nearby, possibly as a volatile fungicide. Such cones are thick with resin, as with this cone of the gray pine.

resin on the cone of a gray pine
resin drips down pine bark from an injury

Resin channels in birchbark lend the bark amazing resistance to fungi and bacterial decomposers. Often, a dead birch will lie on soil for decades, until the wood inside the bark is completely gone. Birch resin gives decomposers a very hard time.

 

 

 

 

 

Many barks also contain chemicals to deter attackers. Quinine  has been a lifesaving drug against malaria for hundreds of years; it was discovered by Quechua Indians of Peru in the bark of  the Cinchona tree.

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Continuous Plant Poisons

Some plants keep permanent poisons in their leaves, and sometimes in the entire plant body. These discourage herbivores by tasting bad, by interfering with digestion, by interference with nervous systems, and, if the animal eats too much, with death. For example, garden flowers such as foxglove, larkspur, and monkshood are continuously toxic:

Foxglove gives us medical digitalis, used in heart treatment. Because it is poisonous in all parts, Foxglove has earned sinister folk-names: Witches’ Gloves; Dead Man’s Bells. Leaves of the upper stem are especially toxic; just a nibble can kill.
Tall larkspur, a Delphinium, a native wildflower, kills sheep and cattle on high rangelands. Garden Delphinium is also extremely toxic to mammals and insects. In old Transylvania,a delphinium was used to keep witches from the stables, perhaps because the wild delphinium flower there is black.
Monkshood, called Wolfsbane in werewolf movies, contains the nerve poison aconitum, formerly used in Western medicine, once widely used as an arrow poison in hunting cultures.
Poison Hemlock is the most notorious plant poison, for it was used to kill the Greek philosopher Socrates. Hemlock defends against grazers by being rich in coniine, a strong neurotoxin.
 
Deadly Nightshade defends itself with atropine, a drug still used in cardiac arrest, once used to dilate womens’ eyes for cosmetic effect. In that use it was called in Italy belladonna (beautiful woman). This use is recorded in the scientific name Atropa belladonna.
In Greek myth, Atropos was the one of three sister Fates who cut the thread of life.

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Defenses Triggered by Attack

Many plants defend when attacked, instread of continuously, Some plants do both. Plants know when they are being eaten. When a leaf, stem, or root is bitten or pierced, plants can produce signal-chemicals such as antibiotics, alkaloids, and other compounds with incredible speed, and send them throughout their tissues. These substances act directly against the herbivore.

The best known defense compounds are alkaloids; over 3,000 kinds are known. They include nicotine, caffeine, morphine, colchicine, cocaine, and strychnine. Another group of defense compounds are terpenoids. They include volatile oils such as citronella, menthol, camphor and pinene.

Signal-chemicals have multiple defense uses. Some volatile oils, such as citronella, discourage many insects. Some warn other plants of their species that herbivores are attacking, so that the others can jump-start production of defensive chemicals. Other signal-chemicals encourage specific insects that a plant calls to for help.

Plants Send for Help: Indirect Defense

Many plants produce volatile signal-chemicals that attract the natural predators of their attackers. These plant-to-insect calls are not general; they are aimed directly at one kind of insect. The plant is saying to the insect, "Your food is ready." Here is a typical scenario:

A corn earworm caterpillar chews on a corn plant. The caterpillar’s saliva reacts with corn tissue in the caterpillar’s mouth and produces the chemical volicitin. With the next bite, some volicitin is inserted into the plant. This signals the entire corn plant to produce a volatile (airborne) signal-chemical which accomplishes twin goals:

 
It attracts tiny wasps which home in on the signal and sting the caterpillar, then lay their eggs in it.
 
It tells nearby corn plants to activate their defenses.

 

 


Plants can even blend chemicals to release in response to feeding by different species of insects.

Pine Sawflies and Who Comes to Help

Pine trees "taste" when a sawfly lays its eggs inside their needles. The pine then produces a volatile signal-chemical cocktail that is aimed at attracting both parasitoid tachinid flies and parasitoid wasps.

a pine sawfly laying her eggs inside a pine needle
pine sawfly eggs in pine needles
pine sawfly caterpillars hatch in large numbers and can quickly defoliate the host tree.
A parasitoid tachinid fly
emerging from a sawfly cocoon
Photo credit Klaus Hellrigl
tiny parasitoid wasp laying
her eggs in a sawfly cocoon
Photo credit Piero Baronio
parasitoid wasp larvae inside a sawfly cocoon
Photo credit Annette Herz
sawfly parasitoid wasp, injecting her eggs into a sawfly larva's cocoon
this tiny egg parasitoid wasp smells gravid butterflies, hitches a ride on them,
then lays her eggs inside their eggs
A tiny parasitoid wasp called to
a plant by a signal-chemical
lays her eggs inside a moth's eggs.
A parasitoid tachinid fly homes in on
a caterpillar where she will lay her eggs
A parasitoid wasp prepares to
lay her eggs on a caterpillar
Photo courtesy Andrei Soukarov
Tachinid fly eggs on a cutworm.
Larvae will burrow in to feed
Photo courtesy U. Nebraska Entomology Dept.

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Nectar to Keep Helpers Nearby

Plants can be subtle defenders. Many plant attacks are seasonal, when the rewards to the herbivore are largest. Many kinds of plants keep potential helpers in the neighborhood out-of-season, before they are needed, by offering them nectar before and after flowering. Commonly, the nectar is offered in special little nipple-like organs called nectaries, but some plants offer nectar directly on their leaves. This out-of-season nectar insures the plant of protection by ants and parasitoid wasps.

Ants will vigorously attack any leaf-eater that shows up on their nectar source. In season, when a nectar-offering plant sends for winged help, the wasps will already be in the neiborhood.

A Gallery of Extra-Floral Nectar

a guardian ant about to feed
on an Acacia tree nectary
nectary offering sweets beneath cactus spines
an ant feeds from a Cholla cactus nectary
Cherry tree nectaries on a leaf stem.
Aspen nectaries are similar.
Photo courtesy Pat Doak, U.Alaska Fairbanks
elderberry nectaries on the young stem
passionflower nectaries can occur
on every part of the vine

the nectary on this wild ginger stem in Costa Rica
looks like a yellow stripe.
Photo courtesy Dan L. Perlman, EcoLibrary.org

Pitcher plants recruit ants with large nectaries.
an ant drinks nectar exuded directly
from a leaf, without a nectary
Photo courtesy Dale Ward, link here

an ant struggles with sticky nectar exuded from a sunflower leaf, without a nectary
Photo courtesy Dale Ward, link here

ants attack a katydid feeding on cholla cactus
Photo courtesy Barry Sullender, link here
ant attacking swallowtail caterpillar
on a leaf it is defending
photo credit Shannon Murphy

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Egg Mimicry Defense

One of the most startling plant defenses is mimicry. Some plants mimic the presence of insect eggs on their leaves, dissuading insect species from laying their eggs there. Because female butterflies are less likely to lay their eggs on plants that already have butterfly eggs, some passion flower vines grow physical structures on their leaves that mimic the yellow eggs of Heliconius butterflies.

Yellow faux "butterfly eggs" grown by a Passion Flower vine to dissuade egg laying
by Heliconius butterflies
Dead cells surround a virus that is isolated through programmed cell death. The dead cells will appear on the leaf as a tiny brown leaf spot.

Virus Defense: Programmed Cell Death

Many plants, when invaded by viruses, fungi,and other microbes, kill their own cells that surround the infected cell. Programmed cell death is also known as hypersensitivity response. The attacker is cut off from the nutrients it needs to survive, so it cannot spread the infection. Programmed cell death is used by the immune system of many animals as well. See graphic above right.

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Explore Further in Plant Pages

Plants: What's New? Including Symbiosis with Fungi and Bacteria
Plant Diversity: Major Groups (Images)
Plant Defense including Symbiosis with Wasps
Plant Pollination including Symbiosis with
Insects, Mammals and Birds
Seed Dispersal Including 'Compelled Symbiosis' with Birds, Insects and Mammals

Explore Further in Biosphere

 
Biosphere: Introduction
 
Biosphere as Place: Introduction
 
Biosphere as Ocean: Life Zones
 
Biosphere as Ocean Floor: Benthic Biomes One
 
Biosphere as Ocean Floor: Benthic Biomes Two
 
Biosphere on Land: Terrestrial Biomes
 
Biosphere on Land: Anthropogenic Biomes
 
Biosphere as Process: Introduction
 
Biosphere Process: Floating Continents, Tectonic Plates
 
Biosphere Process: Photosynthesis
 
Biosphere Process: Life Helps Make Earth's Crust
 
Biosphere Process:
Rock Cycle--Marriage of Water and Rock
 
Biosphere Process: Marriage of Wind and Water
   
Biosphere Process: Gas Exchange
 
Biosphere as An Expression of Spirit
 
The Ecological Function of Art
 
The Earth Goddess
 
The Tree of Life
 
The Green Man
 
Earth Art
 
Biosphere as Community
 
Biosphere Microcosm: Bacteria and Archaea
The Procaryote Domain
 
Biosphere Microcosm: Germs
 
Biosphere Community: The Eucaryote Domain
 
Biosphere Community: Protists 1: Algae
 
  Biosphere Community: Protists 2: Protozoa
 
Biosphere Community: Plants: What's New?
 
Biosphere Community: Plant Diversity--Major Groups
 
Biosphere Community: Plant Defense
 
Biosphere Community: Plant Pollination
   
Biosphere Community: Plant Seed Dispersal
 
Biosphere Community: Kingdom Animals
 
Biosphere Community: Kingdom Fungi
 
Biosphere Community: Six Great Extinctions
 
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