Poisonous plants

Almost all plants contain some substance that, in sufficient quantity, can do harm. Even the common potato, if it turns green, produces a toxic substance (solanine). Usually, however, we do not call it “poisonous”, and indeed plants of this kind do not appear in the list.

Here we are interested only in the species that are truly dangerous or harmful. The English text on which this page is based divides them into four levels of toxicity (low, medium, high and severe); the table on this page, instead, flags the potentially lethal species.

Why do plants produce poisons?

It seems strange that a plant should manufacture substances harmful to mammals: what does it gain from it? The answer lies in a long history of defence. For millions of years plants have been under attack from animals, insects, bacteria and viruses that want to feed on them. Let us see how they defend themselves and how they have “learned” to do so.

A plant cannot run away

An animal in danger can flee or fight back. A plant stands still, with its roots in the ground: it has to defend itself on the spot. The original text adds that plants do not have an immune system able to fight invaders from the inside, as ours does.

Defences: from thorns to poison

Defences are of many kinds, from the mildest to the heaviest:

Physical barriers. Thorns, prickles and bristles keep herbivores away. Examples are bramble, sea buckthorn, blackthorn, thistles, barberry and holly.
Unpleasant or bitter taste. For many plants this is enough: the animal tastes, dislikes it and soon learns never to touch it again. Some bitter or toxic substances also serve to stop bacteria and viruses.
Stinging hairs. The nettle has hollow hairs full of an irritating liquid. In Australia there is an even more fearsome plant, known as “gympie-gympie”.
Sap that irritates the skin. When the stem breaks, a harmful liquid comes out. This happens with hogweeds (genus Heracleum); giant hogweed is the best known for the burns it causes. Spurges, poppies and aconite too contain substances that irritate the skin or that can be absorbed and act inside the body.
Very strong poisons. These are the extreme solution: a few bites of a leaf are enough to kill the animal. But they are rare. For most plants, simply being unappetising is already sufficient.

How a poison arises: natural selection

How did a plant manage to “invent” a substance harmful to whoever eats it? The most widely accepted explanation is natural selection, which works as follows.

The genetic material of a plant can change by chance and, by changing, the plant may produce new substances. Almost always these substances do neither good nor harm. Then one of the following things happens:

the substance is indifferent: the plant can keep it or lose it without consequences;
the substance is useful to the plant: it is kept and, over time, may become more abundant;
the substance harms whoever eats the plant: the predator moves away, the plant survives and reproduces, and the trait spreads, while undefended plants are eaten until they disappear;
the substance harms the plant itself: that plant reproduces less and the trait disappears;
the substance suits the animal: for example a nutritious fruit. The animal does not destroy the plant, indeed it spreads it. It is as if it “cultivated” it, and this too is natural selection;
the substance changes the brain, giving a pleasant or strange sensation: then whoever takes it tends to protect and cultivate it (think of the plants used for their effects on the brain).

The author points out that “cultivators” can also be animals other than humans, because a substance that acts on the brain changes behaviour. As an example he cites Toxoplasma, a parasite that makes rats less afraid of cats: in this way the rats are eaten more easily, the parasite passes into the cat and from there spreads through the faeces to other mammals. It is a sinister but effective strategy.

The author’s doubt

At this point the author says he is not very convinced. In his view chance alone is not enough to explain how a plant managed to find, among millions of possibilities, precisely the useful substances. It would have had to try thousands of billions of different compounds, in order to keep only about a hundred thousand and discard almost everything else. It would be an enormous waste of energy and, what is more, many of those compounds would have been poisonous to the plant itself. For this reason he declares that he departs from the most widespread idea.

Plant substances resemble those of animals

The author notes a curious fact: many substances that serve plants are similar to those produced by animals. A substance useful to the plant has an almost identical structure, with small differences, to a substance made by animals for their own needs. Can it be just chance? Or, in a very remote past, did plants and animals share ancient genes, which later turned into genes for substances with different functions but similar structure?

“We are all of this Earth”

The author observes that we, plants and animals, are all of this Earth. Then he adds, with irony, that perhaps we are not entirely so, and cites the astronomer Fred Hoyle. The reasoning, however, does not change: wherever life comes from, the argument holds just the same.

An exchange of genes between plants and animals?

According to the author, the two kingdoms (the plant and the animal one) shared in the past, or perhaps still share, many useful genes. The exchange may have taken place through plasmids, small rings of DNA (deoxyribonucleic acid, the molecule that contains the instructions for building and running a living being) carried by viruses. Or through other mechanisms that biologists have not yet discovered.

Why does he think that still unknown mechanisms are possible? Because discoveries about DNA keep surprising us. Perhaps only 10% of DNA has been understood. What was once thought to be “junk DNA”, that is useless, has shown important functions that are still being studied. Even introns, parts of DNA that do not contain instructions for building proteins, have turned out to be less passive than expected. DNA is a true Gordian knot, with ancient secrets still to be untied.

Moreover, DNA itself has changed over time, and has also changed its functions. So perhaps we shall never be able to reconstruct its whole history, a little as, looking at a car engine, we could not work out, starting from scratch, that its metal was once the armour of a Roman soldier and, before that, a yellow ochre buried underground.

To clarify. A few points need to be made more precise:

– The “10%” is the author’s estimate. Today we know that only a small part of DNA contains instructions for proteins, but a larger fraction has other roles, for example it regulates the activity of genes. Exactly how much is still debated.

– Introns are not found “between genes”, as the text says, but inside genes: they are stretches that are cut out before the protein is built, and sometimes they have regulatory roles.

– The exchange of genes between different species really exists (it is called horizontal gene transfer) and is well documented, especially among bacteria. There are also cases between bacteria and plants, such as that of the bacterium Agrobacterium. But it has not been shown that an exchange between plants and animals explains the similarity of molecules: it is a hypothesis of the author.

Does it still happen today?

Even though plants and animals now seem two separate worlds, some exchange of genes might still occur. We are struck by viruses, and the same is true of plants. Could there be some “ambivalent” virus, able to infect both kingdoms? There are millions of viruses and humanity knows only a small fraction of them. If such viruses existed (and the author does not see why they should not), they could carry some genes from one kingdom to the other, even now. He thinks they do.

In summary

Plants cannot run away, and so they defend themselves with thorns, unpleasant taste, stinging hairs, irritating sap and, in a few cases, powerful poisons.
According to the most accepted explanation, poisons arose by natural selection: plants that by chance produced substances harmful to predators survived and passed them on.
The author of the original text believes that chance alone is not enough and also proposes an ancient exchange of genes between plants and animals, perhaps through viruses. It is a suggestive idea, but not proven.
In any case it remains true that many plant substances act on us because they resemble our own molecules.

Reworked and simplified, with clarifications, from an English text: https://wildflowerfinder.org.uk/