Plants are, predominantly, sessile organisms: a seed lands in suitable conditions, it takes root, and the plant lives out the rest of its days in that spot. Being sessile is a significant disadvantage when you are at the bottom of the food web with hungry herbivores and omnivores prowling about. When it comes to flight or fight against these predators, flight is not an option, so plants have become masters of putting up a fight.

The first line of defense is a plant’s physical traits. One of the most easily identified plant defenses is bark. An example of a tree with a hardy bark defense is the Douglas fir (Pseudotsuga menziesii). The bark on a young Douglas fir is smooth, grey, but as it ages that bark becomes thick, with deep furrows making it an easy identifier of the species. Not only does this bark hold microbes, insects, and other animals at bay, it is able to protect the tree’s delicate interior from low to medium intensity fires.

Rugged and thick Douglas fir bark. photo by John F. Williams

While bark is an excellent barrier between the outside world and the tree’s internal systems, it can be damaged. Bears and cougars will scratch the trunks of trees, deer in the middle of winter will eat bark, and people have an affinity for pruning off undesired limbs. Unlike animals, which heal when wounded, trees compartmentalize. The section of harmed tree is sealed off from the rest of the structure to prevent decay and disease from spreading.

Illustration by Sarah Lorse

Magnified photo of stinging cells on stinging nettle leaf and stem. photo by Sarah Lorse

Stinging nettle (Urtica dioica) is a well-known plant, thanks to its stinging cells that resemble hypodermic needles. The cells are hollow and filled with a chemical compound. When a hungry deer or an unlucky hiker brushes against these cells, the tips break off, and the chemicals irritate the skin of the unfortunate passerby. Stinging nettle is a very nutritious plant for humans and other animals, so it makes sense that it goes all out when trying to avoid being eaten.

Other physical defenses in plants are less obvious to us humans. Some plants take up silica from the soil and deposit them in specialized cells. These silicon rich cells give the leaves an abrasive feeling; most species of grass utilize this defense mechanism. The abrasiveness can irritate a browser’s mouth and even cause wear on an animal’s teeth.

Modified surface cells that resemble hairs, trichomes also offer physical defense. These modified surface cells resemble hairs and act against smaller assailants, such as mites. The hairlike cells, which make the plant soft to the human touch, create obstacles making it difficult for invertebrates to reach the cells of the leaf on which they feed. Trichomes also benefit the plant by assisting with water retention and protection against the sun.

If a plant’s physical defenses are not enough to keep hungry predators or wily pathogens at bay, what other options does it have?

Plant Defense Mechanisms – Against Herbivores

Plant Defenses, Genetic Science Learning Center

An Overview of Plant Defenses against Pathogens and Herbivores

Spines, Prickles, and Thorns

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