We’ve done bats. We’ve done rats. We’ve done creatures of the deep and we’ve done toad maggots that creep. But in all honesty, do you know what scares me more than any of that?
Ok, hear me out. I mean, you probably don’t spend a lot of time thinking about plants, much less being terrified of them. After all, they’re stationary, and they don’t have any mouths or eyes or brains, and any fool with an axe can go chop down a tree.
But listen… there’s a major difference between plant biology and animal biology. I’m not talking about stamens and pistils and all that, I’m talking more fundamental. See, what with the whole stem cell research debate, most people generally understand that we are born with stem cells that turn into particular types of cells, like liver or bone marrow. But once those stem cells determine what they can be, they don’t ever change back.*
(*There are some exceptions to this. I own axolotls, after all.)
Plants don’t have to follow that rule. When you take a cutting from a branch of one plant and replant it, it can grow root cells from former branch cells. That’d be like cutting off someone’s leg and having a head grow out of it.
This distinction is important, because it means plants and animals play by very different rules. Most animals are screwed if they lose, say, their heart, but plants can survive incredible amounts of damage to all different parts of their bodies. The tradeoff for this is that plants can’t have parts that are too specialized. They can’t really have centralized brains, because then they would have a single vulnerable spot that they wouldn’t survive losing.
Because they can’t specialize too much, it’s hard for plants to develop things like locomotion. But not being able to walk doesn’t mean that plants don’t move. They just do it very… very… slowly.
The difference in plant and animal motion isn’t just in terms of speed. Animals move by lugging their entire bodies from place to place. Plants move by simply… growing. Getting larger. And larger.
“A flower does not think of competing to the flower next to it. It just blooms.” -Sensei Ogui
The above is a quote that I have seen passed around both facebook and tumblr for quite a while. It is a nice sentiment, and it is entirely wrong. The essentials of life for all plants are sunlight, water, and nutrients, and not one is limitless. No, not even sunlight; unless you grow enough to tower over all other plants in the area, you’ll be stuck struggling in the shade.
Like animals, plants have also come up with crafty ways to compete with one another. One way is to simply kill the competition, or prevent it from ever growing. Many plants utilize a tactic called allelopathy, which essentially boils down to “let’s use chemicals to fuck with our neighbors.”
The black walnut tree is rather jealous over its root space, and will secrete a chemical called juglone into the soil. Not all plants are affected- some have evolved defenses- but for the ones that have not, juglone inhibits enzymes that are necessary for respiration. In other words, it stop plants from breathing.
But that isn’t even the most sinister effect allelopathy can bestow. Botanists are beginning to find that the secret to the success of many invasive plants lies in their aggressive allelopathy. Spotted knapweed, a European species that is now invasive to the US, utilizes a chemical called (-)-catechin to interfere with its neighbors. When another plant takes up (-)-catechin through its roots, the chemical causes a signalling cascade that actually turns off a a number of the genes in each cell it contacts. This means that the cell can no longer produce the proteins it needs to survive, and it dies within the hour.
Allelopathy is not always harmful to neighboring plants, however; in some cases, whether through mutually beneficial co-evolution or one plant sneakily taking advantage of another’s defenses, sometimes plants produce chemicals that help others grow. Sometimes a plant will produce a chemical that has a negative effect on one plant species, but a positive effect on another. It’s a tricky balance.
Some plants prefer a more medieval way of one-upping the competition than by using chemicals, however. These would be the climbing vine species, including the strangler figs. I’ll let Sir David describe what those do.
‘Strangler fig’ really is an apt name.
Perhaps plants very slowly fighting with other plants does not seem particularly frightening to you. But plants are not just affected by plant competitors- they are equally harassed and attacked by animal parasites and predators. A sheep or cow is about as vicious as it gets from a plant’s point of view. And, of course, they have means with which to fight back.
The most obvious of these means is poison- plants will produce phytotoxins (literally, plant toxins) that can cause anything from mild stomach upset or itchy skin all the way to asphyxiation or cardiac arrest. While this is good enough for us to leave certain plants alone permanently, there is a downside for the plants. Firstly, these chemicals are often complex and expensive to produce and build up in the cells; and secondly, animals may evolve resistance.
Some plants have come up with a slightly evil compromise.
I spoke before about how it doesn’t bother a plant so much to lose a few branches, roots, or leaves. So a bit of nibbling by a few herbivores will not cause a great deal of harm to a large bush. But when the number of leaves begins to dip dangerously low, this is another matter, and the plant may have ways to detect this and to take action- and more.
One example of this is oak trees. When caterpillars begin to eat oak leaves, the tree responds by ramping up the amounts of tannin and phenol in their tissue, making the stuff harder for caterpillars to ingest. In fact, the reason why herbivores have to eat so much plant matter- seriously, think about the amount of time a cow spends grazing, then sitting down and chewing cud- is because of this type of defense. As soon as the plant detects that it is being eaten, it turns on its less digestible compounds.
This can cause a normally harmless plant to become deadly. There is a famous case that occurred in the 1980s in which roughly 3,000 kudu inexplicably dropped dead in South African game reserves.
No predator attacked these kudu, and none of them looked sick; indeed, they seemed completely healthy. The culprit was, of course, a plant; acacia trees, which kudu can usually eat without repercussion. The problem was that a drought had killed many of the kudu’s other food sources, forcing them to feed almost exclusively from acacia trees. The trees did not like this, and when they started losing too many leaves, they went on the defensive.
Not only did the trees that were being attacked raise their tannin content to deadly levels (preventing the kudu from digesting any of the food that sat uselessly in their stomachs), but they also released an ethylene gas into the air to communicate with other acacias. When the other trees picked up the signal, they too increased their tannin levels, leading to mass murder of the hapless kudu.
In lieu of a nervous system or the ability to communicate with sound or movement, plants regularly speak to other plants- and animals- via chemicals like these. I am sure most of us are familiar with the phenomenon of sealing fruit in a bag to make it ripen faster. This is because fruit releases ethylene gas as it ripens that signals itself and the rest of the fruit on the tree to keep ripening. Apples and bananas are particularly strong ethylene producers, which is why slow ripening fruit like kiwis ripen faster if kept in close proximity to either of those.
Like the acacia, plants also produce ethylene when they are wounded to stimulate the healing process. Other plants in the area may “listen in” and ramp up their chemical defenses in preparation, whether or not they are the same species.
As I said before, plants can also use these chemicals to communicate with animals in rather surprising way. The tobacco hornworm is a familiar garden pest, and a voracious plant-eater. The tobacco plants that this insect preys on do not waste much time trying to poison their predator. Instead, they send out a cry for help via a volatile chemical compound. And who should answer but the lovely braconid wasp?
Yes, as a matter of fact, the plant calls the wasp over to kill the caterpillars. And this is not the only time that plants call for backup; it’s been documented many times, generally with insect predators. And the plants know who to call, too, because they can tell who’s eating them. Different types of herbivore saliva actually enact different defenses. This even extends as far as mechanical damage- if you rip a plant’s leaf without putting any of your saliva on it, the plant may not behave as though it’s being attacked. Those defensive compounds are expensive to produce, after all, and the plant doesn’t want to waste time over a freak accident.
The idea of a plant knowing who’s attacking it can seem ridiculous, considering that plants lack a brain or even a simple nervous system. But plants may know a hell of a lot more than we think they do; and it may be because we are too used to looking at things from an animal mindset. So a plant can’t afford to have a centralized organ for thinking, right? That may not mean it can’t think. It looks as though it definitely doesn’t mean that the plant can’t learn.
M. pudica, known as the “sensitive plant,” is one of the few plants that can display animal-like behavior. This plant actually responds to touch by closing its leaves. It does this not with muscles but via a system of pressure sensors that cause the cells in the leaves to lose their turgidity (firmness).
Some rather recent research on this plant came up with stunning results: the plant appeared capable of learning when a touch was not harmful. The scientists achieved this by dropping water on the plant’s leaves in both high and low light environments. In each case the plant seemed to figure out that it wasn’t worth closing its leaves for the water droplets in a matter of seconds; furthermore, it learned better in the high light condition than in the low light condition. In the same way, food-deprived animals will have more difficulty learning than well-fed animals.
M. pudica didn’t just remember about the water for a few seconds, though. Even a month later, it still appeared to remember that water droplet = not harmful. But when given another stimulus that it wasn’t familiar with, such as a vigorous shaking, the plant closed right up.
How does the plant store memory without a nervous system…? We just don’t know. But this is far from the first experiment to suggest that plants are capable of learning and memory. Other experiments have suggested that plants can remember being tilted sideways after spending a few days in a fridge (the plants respond to tilting by growing in the direction that they think is up).
Plants can actually navigate mazes much more efficiently than many animals can; they do this by growing towards a light or nutrient source. Plant roots ignore nutrient-poor soil patches but spend a lot of time feeding and getting hairier in nutrient-rich patches, just like a foraging animal. Plants will also move (grow) to avoid contact with other plants, and may even be able to tell when other plants are related to them. Some trees will pass on more carbon via fungal networks to their seedlings than other plants, and some plants avoid competing for soil nutrients with relatives but battle with strangers.
There is, in fact, a vast and terrifying body of research that suggests that not only may plants have a kind of intelligence, but it is a kind of intelligence that we are just beginning to tap in to. We are too used to associating ‘behavior’ with mechanical movement to really understand what plants are doing. Because they are behaving- slowly, yes, but with no less drive and intention than many animals. What we store in our brains might be spread out throughout the entire body of a plant, but it is still there, and it is capable of learning.
Imagine this: a movie monster that can basically regenerate any part of its body, that can change its chemical composition on a whim to become toxic, that can call in an army of wasps, for god’s sake. We live with these monsters. They are plants.
To view a list of all my animal articles, head to the Nonfiction section.
Resources to Learn More
“Aspects of Plant Intelligence” by A. Trewavas (warning: it is very, very dense)
Allmann, S., & Baldwin, I. T. (2010). Insects betray themselves in nature to predators by rapid isomerization of green leaf volatiles. Science, 329(5995), 1075-1078.
Bais, H. P., Vepachedu, R., Gilroy, S., Callaway, R. M., & Vivanco, J. M. (2003). Allelopathy and exotic plant invasion: from molecules and genes to species interactions. Science, 301(5638), 1377-1380.
Cooper, S. M., & Owen-Smith, N. (1985). Condensed tannins deter feeding by browsing ruminants in a South African savanna. Oecologia, 67(1), 142-146.
Dudley, S. A., & File, A. L. (2007). Kin recognition in an annual plant. Biology Letters, 3(4), 435-438.
Gagliano, M., Renton, M., Depczynski, M., & Mancuso, S. (2014). Experience teaches plants to learn faster and forget slower in environments where it matters. Oecologia, 175(1), 63-72.
HOVEN, W. V. (1984). Tannins and digestibility in greater kudu. Canadian Journal of Animal Science, 64(5), 177-178.
Ishikawa, H., Hasenstein, K. H., & Evans, M. L. (1991). Computer-based video digitizer analysis of surface extension in maize roots. Planta, 183(3), 381-390.
Jose, S., & Gillespie, A. R. (1998). Allelopathy in black walnut (Juglans nigraL.) alley cropping. II. Effects of juglone on hydroponically grown corn (Zea maysL.) and soybean (Glycine maxL. Merr.) growth and physiology. Plant and soil, 203(2), 199-206.
Kessler, A., & Baldwin, I. T. (2001). Defensive function of herbivore-induced plant volatile emissions in nature. Science, 291(5511), 2141-2144.
Thellier, M., & Lüttge, U. (2013). Plant memory: a tentative model. Plant Biology, 15(1), 1-12.
Trewavas, A. (2003). Aspects of plant intelligence. Annals of Botany, 92(1), 1-20.