POISONOUS PLANTS

toxicity severe

toxicity high

toxicity medium

toxicity lowish

This lists those plants which are particularly poisonous or harmful in one way or another.

It must be remembered that most plants are poisonous in some way, even the humble potato (if green). These are not listed here.

Only the plants that are particularly nasty and very poisonous are herein listed.

It must be remembered that for millions of years plants have endured a constant battle from predators: things from the animal (and insect) kingdom that are trying to devour them. They have had to develop a defence against this onslaught. Some plants have come up with mild poisons or distasteful bitterness that deter animals from eating them whilst others have gone the whole hog and produced toxic poisons that will kill any animal that even dares nibble on a leaf.

They have accomplished this by natural selection: a plant that, by random chance, has happened to produce a distasteful or poisonous substance gets to propagate another day, proliferating that trait, whilst those same plants that have not invented this trait lose out (are eaten into oblivion) and eventually die off over millennia.

But then the question must arise how did the plant just happen to invent a substance that was in some way either injurious (or beneficial) to those animals that were consuming it? Well, it can happen by chance, and this seems to be the conventional view: The plant, by chance (using its genetic material), produces multitudinous new and novel compounds, many having no effect whatsoever on either the plant or the potential devourer: these possess neutral selective pressure. They may or may not propagate, the plant could easily lose the ability to produce that chemical. Some invented compounds benefit the plant directly and exhibit some useful property of advantage to the plant (positive selection pressures), these compounds will be welcomed and even amplified by the plant. Some compounds are harmful to predators, and thus also of advantage to the plant, these too will be kept or amplified by the plant. Then other compounds may be injurious to the plant itself; these will die out naturally. Whilst others still may produce some nutrient or chemical that is beneficial to its predator, so the predator, rather than eating it all, farmed it, thus propagating it. You could classify that as natural selection too. Moreover, some plants may produce a chemical that makes the imbibers' brain feel very happy or pleasantly strange; the propagation and protection of these too would have been encouraged by 'farmers' (The word 'farmer' is here used in the broadest possible sense; it could even include other animals beside hominids - after all, if a compound affects their brains, it can affect their behaviour too - much as Toxoplasma does to rats (they lose their fear of cats and are eaten by them. The cats become infected, and pass on the Toxoplasma in their faeces to other mammals including rats, thus propagating Toxoplasma - a sinister but clever ploy that seems to have been evolved by Toxoplasma).

But can it just be chance alone that a plant just happens to produce a toxic chemical out of pure luck, out of the millions of compounds it could have produced by chance? Did plants really try inventing trillions of different compounds and just kept the 100,000 or so that were beneficial in some way to its survival, shedding the 99.9999% of the rest in natural selection? I think not, and this is where I probably diverge from the conventional viewpoint. Not only would such a shotgun blunderbuss approach have been a huge waste of (synthesizing) energy, but it could even be counter-productive - many of the chemicals it produced might have been poisonous to itself!

When the chemical structures of a lot of the substances produced in plants (that are required by the plants) are compared with those in animals, some remarkable similarities and relationships become apparent.

CONSIDER THE FOLLOWING: Indole acetic acid. a plant phyto-hormone or auxin, is related to tryptamine. Tryptophan is a tryptamine similar in chemical structure to serotonin, a neurotransmitter required by animals. Bufotenine is a tryptophan-like substance produced by certain toads and mushrooms (note both kingdoms are represented here) that is poisonous to mammals and has hallucinogenic properties. Psilocybin produced by the psilocybin mushrooms is also related to tryptamines and is also a hallucinogen in mammals. Yohimbine, a hallucinogen and indole-alkaloid (note the reference to indole acetic acid) produced by the Yohimbine tree (related to the Madder Family) is a serotonin antagonist in mammals, with a great many effects too numerous to list. Cocaine, produced by the Opium poppy, is a tropane alkaloid, like atropine which is produced by the potato family of plants. Cocaine is a dopamine inhibitor. Dopamine is another neurotransmitter based on tryptophan. Ergotamine is a poisonous ergot-family alkaloid produced by a certain fungus that is parasitic on cereal grains and grasses. Despite ergotamines chemical complexity, it is also plainly based, at least in part, on tryptamine. It possesses similarities to several neurotransmitters, serotonin, domamine and adrenaline and has the remarkable ability to act both as agonist and antagonist on a number of those neurotransitter receptors at the same time. Strychnine, a poisonous alkaloid synthesized in the seeds of the strychnos nux vomica tree, is based on an indole acetic acid core with a confusion of multiply-fused rings, seven in all. It consists of two five-membered, four six-membered, and one seven-membered ring, very high for a 23-atom'd core (it does have extra atoms external to the ring). Strychnine acts on the glycine receptor in the spinal cord and brain. Curiously, Glycine is yet another neurotransmitter. Reserpine, another indole-alkaloid, is synthesized in the roots of Rauwolfia Serpentina and blocks the uptake of noradrenaline and dopamine. Noradrenaline is both another neurotransmitter, and a hormone in mammals. A rather remarkable compound that is synthesized in both plants and animals is beta-carboline, an alkaloid and founder-member of the class of alkaloids called beta-carboline alkaloids. Some beta-carbolines, notably pinoline and tryptoline, are manufactured within the human body and are implicated (along with melatonin) in our sleep/wake pattern. Many are selective inhibitors of enzyme monoamine oxidase type A and thus are monoamine oxidase inhibitors (MOAI's) within the human body. Beta-carboline is directly related to tryptamine. Other beta-carolines, the harmines (a class of compounds synthesized in the non-native Syrian Rue plant and used as a hypnotic drink by rain-forest shamans) are mono-amineoxidase inhibitors, MAOI's. The Monoamine class of compounds include many neurotransmitters. Tryptoline (aka Tetrahydro-β-carboline and Tetrahydronorharmane) is also a naturally produced derivative of Β-carboline. Bryophyllin A is a bufadienolide found in both plant and animal kingdoms which was first found in some species of toads (hence the bufa in the name of the series of compounds). Later it was discovered in a Crassula species from Indonesia and some other members of the Bryophyllum Genus. It is a powerful insecticide, active against the third member of the kingdoms. What is very strange about this particular bufadienolide is the very unusual triangular cage (bottom left) formed by three oxygen atoms straddling a phenyl group bonded at the apex by another carbon atom. This same cage-like structure, but with the cage straddling 1,2,4 positions, is also to be found in the extremely irritant Daphnane-type of Phorbol Resiniferatoxin, which is found in Resin Spurge (Euphorbia resinafera) (a non-native). Resiniferatoxin is a neurotoxin and the most irritant substance known, nearly 100,000 times more irritant that capsaicin the irritant in chili peppers. Other bufadienolides omit this structure. Bufadienolides are characterised by a steroidal frame (the four fused rings) with an added lactone ring (top right). Steroids are, of course, compounds vital to the functioning of mammals, and are found in a variety of (mostly highly poisonous) forms in hundreds of different plants. If ingested, steroid look-e-likees in plants may interfere with the normal operation of resident steroids in mammals.

So, are all these just coincidences? A substance that just happens to be required by a plant has almost the same chemical structure (with variations), as do substances which are synthesized by animals and that happen to have a similar chemical structure to substances required by the animal kingdom? Can it all just be pure coincidence? Or has there, at some time in the distant past, been some collusion between plant and animal kingdoms? A sharing of ancient genes that have since diversified into genes for synthesizing compounds with different functions, but similar structures?

So, how can these remarkable similarities in structure between certain substances shared by the plant and animal kingdom have come about? Well, we (plants and animals) are all of this Earth. [Actually, between you the reader, myself,  Fred Hoyle and a few others, I don't think we are totally of this Earth, but the same argument apples from wherever we ultimately derived].

I think we (the plant and animal kingdoms) shared (either now, or once in the distant past) a lot of useful genes. They could have swapped or shared genes by means of plasmids, circular bits of DNA, delivered by viruses. Or by a multitudinous variety of other methods of gene-swapping yet to be discovered by biologists [they are making revelatory findings about gene-swapping all the time - discoveries that they never suspected, nor predicted. It's all in the DNA. Perhaps only 10% of DNA is understood. What they previously thought was junk DNA turns out to have innovatory properties relevant to this topic that are still being interpreted. And introns (the non-protein-coding DNA between genes) are now recognised as being not so passive after all. DNA is turning out to be somewhat of a Gordian knot, its ancient secrets still to un-fold]. Of course, since DNA has itself been evolving, changing its function over time, we may never be able to fully un-ravel its mysteries [by looking at a car engine, could you, from first principles, deduce that it was once re-melted from Roman armour, and before that was once buried in the ground as yellow ochre?].

Although by now the plant and animal kingdoms seem to have diverged and crystallised into two separate kingdoms, it could be that some gene swapping or sharing is still occurring. We suffer viruses; so too do plants. Might one or two viruses be ambivalent about which kingdom they infect? There are millions of viruses out there. Mankind knows of just some small sub-fraction of the total number of virii. Could these ambivalent viruses (if they exist, and I see no reason why they couldn't) be swapping some genes between both kingdoms even now?

I believe they could.

Why - Hoooooom, why, I could be a prime example...
The Ent, Truebeard.