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Pontifications on Poison

Being some ramblings on events associated with poisonous plants.

Saturday 29th October 2011

Yesterday, I wrote that I drove home on Thursday evening through some patches of mist that I took to be a clear sign that the seasons were changing. So, when it was a bright sunny afternoon on Friday, I thought I would go out for a drive and have a look at the autumn colours before the next period of strong wind or the first period of sub-zero temperatures took all the leaves off the trees.

The many colours of autumn trees

The many colours of autumn trees

It’s a little strange that there can be such beauty in death. Though there are many things about the spring that are attractive and encouraging and heart-warming, we tend not to make as much fuss about the beauty of the green leaves appearing on a tree as we do of the brown and golden and yellow dying leaves just before they drop off in the autumn.

Death in plants is also interesting when it comes to poisonous plants. As far as I know, there are very few plants that remain poisonous after death. The process of deterioration and decay that is dying as far as plants are concerned mostly produces deterioration and decay in the poisonous components.

Two well-known exceptions to this are Jacobaea vulgaris, common ragwort, and Taxus baccata, the yew tree. The problems of ragwort in hay are well-known and there are numerous stories of cattle being poisoned by the dead foliage of a yew tree either blown into pasture from an adjacent churchyard or dropped into a field by someone thinking they were usefully recycling it.

This is an area of plant science that I find very interesting for reasons that are fundamental to the subject of poisonous plants.

Dying leaves stand out against green grass

Dying leaves stand out against green grass

Most of the time, when someone tries to explain why a plant is poisonous you will hear talk of ‘defence mechanisms’ or ‘deterring grazing’ but I don’t think those sorts of explanations stand up to scrutiny.

For a start, many plant poisons are quite slow acting. A slow-acting poison offers no defence against ingestion because, on a one to one basis, by the time the creature doing the eating finds out it has made a mistake, the plant has been eaten. The notion that there is some sort of racial memory in animals so that today’s creature knows not to eat a particular plant because its great aunt died after doing so is one that I reject.

And, if being poisonous is about defending yourself against being eaten then what possible purpose is served by still being poisonous after you are dead?

I realise that, when discussing evolution, some people use a sort of shorthand for what evolution has brought about but, to me, doing so sometimes gives the impression that plants are sentient beings. You hear things like ‘this plant, ‘X’, produces a sap in order to stop this insect, ‘Y’, from eating it’.

Bare branches soon replace the autumn colours

Bare branches soon replace the autumn colours

The way that I understand evolution is to look from history forward rather than looking from here back. There’s a danger of looking at the flora and fauna we have and assuming that this was always where nature was heading. It can seem like an objective has been achieved that the flesh of the aril of Taxus baccata is harmless and the skin of the seed is indigestible meaning that birds can distribute them.

What that approach ignores is the incomprehensible number of alternatives that have appeared and failed over time. Over time, all the versions of the yew tree with sour flesh or digestible seed casings have appeared and failed to reproduce. Equally, all the birds with stomach acid strong enough to digest the seed casings and release the toxins have failed to live long enough to continue breeding.

What that means is that we have, at the moment, a situation where, for many plants not just the yew, berries being eaten and undigested seeds being excreted is the mechanism for spreading the plant further.

There’s a very human tendency to assume that the present is a destination; that things have evolved to reach this conclusion. That, of course, is absolutely not the case. Evolution continues and maybe, millions of years in the future, plants that remain poisonous when dead may have evolved away from that.