We may sometimes think of plants as weak, defenseless victims at the bottom of the food chain. They can't fight back with force, and they can't run away or hide when a hungry herbivore happens by. However, if you look more closely, green growing things aren't so wimpy.
Plants exist to reproduce themselves. Annuals must produce lots of seeds and spread them as far as possible in just one growing season. Perennials expend energy creating a healthy root system so they can produce seeds or new plants in the future. Plants need defense systems to do this. Any adaptation that allows a plant to live until it propagates either by setting seeds or by creating new plants through its root system is a successful one.
From a plant's perspective, herbivores—things that eat plants—are the enemy. A 1,000-pound bull moose or a microscopic fungus can be equally deadly. The plant's foes include mammals, reptiles, birds, insects, fungi, and even other plants. In response, the plants try to make their world a dangerous place for herbivores, full of appetite suppressants, digestion disruptors, spikes, armor, and toxins.
Insect Invasion. Insects do the most damage to the plant world. Insects account for half to three-quarters of all species on earth, and half of all insects are plant eaters. Small plants like clover may have to contend with 100 to 200 species of insect. Trees may have a thousand species or more feeding on them. Insects prey on every part of the plant—flowers, pollen, leaves, roots, and sap. To survive, plants must find some way to defend themselves from these hordes of voracious herbivores.
Some plants do this by producing toxins. Native white sage (Artemisia ludoviciana) releases jasmonate hormones when it is wounded or tasted by insects. These chemicals inhibit leaf digestion in the insect's gut and can sicken or kill the insect. Jasmonate hormones in sage plants can also travel through the air, priming other sage plants to begin producing the hormones before they are even attacked.
Can you imagine mild-mannered common milkweed (Asclepias syriaca), a key food source for the beautiful monarch butterfly, as an aggressor? The milky sap of the milkweed contains a chemical called a cardiac glycoside that interferes with the function of cell membranes in most worms and many other animals that chew its leaves—but not monarch caterpillars.
When the caterpillars morph into butterflies, they still contain enough of the glycoside to be toxic and are therefore protected from bird predation.
Getting Physical. Physical defenses include thorns (modified branches), spines (modified leaves), prickles, tough leaf surfaces, sap that traps insects, and tough stems.
The nonnative bull thistle (Cirsium vulgare) has one of the most straightforward physical defense mechanisms. All thistles look aggressive, but bull thistles seem downright medieval with huge spines on their 5- to 7-foot stalks and slightly smaller, but more numerous, spines on their 3- to 12-inch-long leaves. Even their ½-inch to 2-inch flowers, with beautiful magenta petals, are surrounded by prickly sepals. Perhaps one reason these thistles are so invasive is that almost the entire plant is unpalatable to grazing and browsing animals. Even goats, which will eat just about anything, nibble only the flowers, leaving the spiny sepals behind. If the blooms are removed before they set seeds, the thistle just grows more flower stalks.
Because this plant so successfully fends off mammal species, each bull thistle produces hundreds of flowers that each release hundreds of seeds that remain viable in the soil of disturbed areas—hay fields, pastures, roadside ditches, and disturbed prairies—for more than 10 years. That successful strategy is one reason that bull thistles are considered invasives in Minnesota and are thriving all over the state.
The field thistle (Cirsium discolor) is a native plant that is much less widely distributed than nonnative thistles. It is found mostly in the eastern part of the state. The field thistle can grow as tall as seven feet with 1½-inch to 2-inch flowers, but the stems are hairy rather than spiny and the leaves have only small spines on their edges with a larger spine on the leaf tip. Like other native thistles, field thistle should not be eradicated: It is good for pollinators and is not invasive.
Native field horsetails (Equisetum arvense) are survivors of a plant family going back 300 million years. While some ancient horsetails were as tall as trees, modern horsetails grow only one to two feet tall. They are most often found in damp soils. They are known as biosilicifiers because they take up silica, the same chemical as the quartz in sand, and deposit it in their cell walls and other plant structures. Some scientists believe that this silica protects the horsetails from being eaten by most animals and insects. The silica is also what gives the horsetail family its common name, scouring rush. Horsetails were once used for polishing pewter and wood, and many campers have used them for scouring cooking pots.
What Plant Defenses Can Do for You
Some plant defenses have benefits to humans. For example, certain plants have developed chemical defenses that repel would-be attackers but appeal to our taste buds. A class of chemicals called terpenes inhibit the growth of attacking bacteria and fungi—and they also give mints, like the native wild mint (Mentha arvensis) and garden mints like peppermint (Mentha piperita) and spearmint (Mentha spicata), their minty aroma and taste.
In nature, most true herbivores either avoid toxic plants or have evolved specialized biochemical methods to neutralize the toxins in their food. Humans, through the process of domestication, have reduced the levels of toxins in many edible plants, saving our bodies the trouble of changing our own biochemistry. We have also used the science of biochemistry to turn many of those toxins created by plants with attitude into medicines. Aspirin, for example, is made from salicylic acid, a chemical that is also found in the bark of the willow tree. Extracts of willow bark have been used to alleviate headaches, pains, and fevers since antiquity. Many cancer-fighting drugs are based on chemicals used by plants as defense mechanisms.
Mechanical Means. Trees are examples of plants with good mechanical defenses. To eat a tree, an herbivore must expend a lot of energy to digest cell walls. The woodier and thicker the cell wall, the harder it is to eat and digest and the less nutrition the herbivore gets from the tree. A tree's cell walls contain calcium, cellulose, and lignin—substances that give the walls their solid structure and toughness. Pine trees also produce a sap called resin that traps insects. Amber, which is found in other parts of the world, is fossilized pine resin.
Maple, oak, birch, willow, and many other trees supplement their unpalatable mechanical defense with chemical defenses. They produce bitter tannins in their leaves and fruit. Tannins from oak wood were used to tan animal skins into leather. Tannin is also responsible for the dry, puckery feeling you get in your mouth when you eat unripe fruit and the slight bitterness when you drink tea or wine. Tannin interferes with the digestion of some herbivores. Younger tree leaves have lower levels of tannin and thus are more often used for food.
Chemical Warfare. Many other plants engage in chemical warfare against non-insect attackers—even ordinary garden celery. When this domesticated plant is under attack by pink rot fungus, it produces chemicals that bind with the DNA and the cell membranes of the fungus, destroying invading fungal cells.
These toxins, called furocoumarins, are also present in high concentrations in some wild plants. When lightly pigmented humans and other mammals come into skin contact with the juice of the stems, leaves, and fruit of the nonnative, invasive wild parsnip (Pastinaca sativa), they can develop intense, localized sunburns, known as phyto-photo dermatitis, a sensitivity to sunlight caused by the plant's furocoumarins. The same effect can occur if the plant is ingested. As a result, light-skinned animals such as white horses, goats, and cows should not be grazed on wild parsnip; they may become ill and die. This nonnative species has been spreading from the southeast corner of the state, toward the north and west. You certainly don't want to add wild parsnip's large clusters of small yellow flowers to your summer bouquets.
More Is Better. Some plants use two or more defense mechanisms. The nonnative common mullein (Verbascum thapsus) produces chemicals called irridoid glycosides that decrease consumption of leaves by worms. The plant also has trichomes, tiny hairlike structures, on its large oval leaves. The trichomes deter most grazers, though children love to touch the leaves for their fuzziness.
Stinging nettles (Urtica dioica) are native plants that use both chemical and physical defense mechanisms. The skin of the nettle stems and leaves is protected by glandular trichomes, rigid hairlike cells that break off and inject a poisonous substance when they are touched. The trichomes pack a triple punch of poison, a combination of formic acid (a venom also found in ants), histamine (responsible in part for itching), and serotonin (which causes pain).
Stinging nettles are found in open woods and fields. The plants can be several feet high and are easily brushed against, causing a burning that may last several hours. Washing affected skin with soap and water can neutralize the sting of stinging nettles. Other animals learn to avoid the nettle.
Plant vs. Plant. A few plants even defend themselves against other plants. Spotted knapweed (Centaurea stoebe) is a biennial or short-lived invasive perennial with beautiful deep pink flowers similar in look to the garden bachelor button (Centaurea cyanis). Spotted knapweed, however, is much more aggressive than the garden flower. It was probably introduced to North America in the 1890s in contaminated hay or alfalfa seed. The plant has a long tap root that helps the knapweed survive in seasonally dry areas. Spotted knapweed roots also release toxins into the soil that prevent the growth of neighboring plant species—a phenomenon known as allelopathy. This gives the spotted knapweed an advantage by reducing competition. It also reduces the diversity of plant species in the area, and as a result increases runoff and sedimentation.
Black walnut trees (Juglans nigra) produce an allelopathic chemical called hydrojuglone. It is given off by all parts of this native tree and can affect the plants around it through root contact and decay of wood and leaves into the soil. Some plants under or near walnut trees, especially those growing in poorly aerated or drained soil, will yellow, wilt, and die.
On your next nature outing, try to spot some of the spines, hairs, or rigid stems that make up the physical and mechanical defensive weapon systems of plants with attitude. Notice which plants grow under a walnut and which begin growing further away. Healthy plants are the victorious results of the largely invisible life-and-death struggle taking place all around us. The plants may seem aggressive, especially when you develop a rash from wild parsnip or an irritation from stinging nettles, but after all, they are only protecting themselves.