One of the simplest concepts to grasp, when embarking on the study of trees, is the difference between the bark and the sap. Indeed it is so simple that I am not going to bother explaining it to you. You almost certainly know the difference yourself, and have known it for so long that I strongly suspect you cannot remember when first you learned it. Cast your mind back. Can you honestly say, with any precision, when your conscious mind became aware that bark was bark and sap was sap? No, you cannot. I told you so.

This does leave me with something of a quandary, however. In keeping with the abstruse yet brilliant method I have devised to select a daily topic for these essays, today, October the nineteenth, was always going to be devoted to the bark and the sap of trees. I could have chucked out my scheme, of course, and chosen a completely different topic at random, such as the Merovingian kings or the Munich Air Disaster or Peter, Paul, and Mary or the Oxford comma. But I was, I am, very reluctant to do so, for fear of inviting the blind toads of mental havoc into my head. Sticking with the method keeps those fearsome creatures at bay. Through the constraint imposed on me by the method, not only do I not have to think for a second what I am going to write about, but my brain remains in a state of sweet and lovely calm. If once I allow those blind toads in, God and all his angels help me, that’s all I can say.

So first thing this morning I got the bucket and the pliers and the tarpaulin and the pebble, and I did the thing with the dainty teaspoon, and then I ran my finger along the row and down the column, and scribbled the number in my jotter, and checked the number against the print-out, and returned the bucket and the pliers and the tarpaulin and the pebble to their respective places, and gazed into the frosted glass, and after a few further procedures I arrived at the bark and the sap as today’s topic. So far so good.

But then I thought, everybody knows the difference between the bark and the sap, don’t they? I can’t tell them anything they don’t know already. So closely interknit are the lives of human beings and trees, at least in this land, green and pleasant, that we learn to know bark as bark and sap as sap, and bark from sap and sap from bark, very early, so early that we cannot remember when it lodged within our brains. I suppose it might well be different for people who grow up in treeless landscapes, deserts for example, or ice-girt polar wastelands. They might find out about trees at a comparatively later stage in their development, late enough to recall. But even those people still know, so me banging on about the subject would still be pointless.

I briefly amused myself by inserting a hidden ‘bark’ into my opening sentence, within the word ’embarking’, and pondered doing the same with ‘sap’ before realising that such tomfoolery might open a door or slit or vent or duct through which the blind toads of mental havoc could come hopping into my head. That I must avoid at all costs.

The topic determined through the method, though, made no mention of trees. The bark and the sap may not be tree bark and tree sap but quite other barks and saps entirely. The bark, for instance, might be the barking of a dog, and the sap might be the lead-weighted leather thumping instrument with which malefactors fell innocents in certain episodes of criminality in alleyways in the night-time. Such saps are often deployed in the film noir genre. Thinking of crime, and more precisely of crime fiction, we might posit that a malefactor is sapping an innocent in a night-time alleyway while, just a few yards away, a dog is not barking. This is of course a reference to the Conan Doyle story “Silver Blaze” (1892), in which Sherlock Holmes alludes to “the curious incident of the dog in the night-time”.

We might make the malefactor Babinsky, the lumbering, walrus-moustached, psychopathic serial killer. He usually goes rampaging about with an axe or a big slicing thing rather than a sap, but we can assume on this occasion he is varying his modus operandi to throw the coppers off the scent. Who should we choose for our innocent? It could be anybody from Ned Slop – newly installed with an alien brain from outer space – to Tiny Enid, though it is unlikely that the plucky fascist tot would ever allow herself to be outwitted by Babinsky. Still, we are getting somewhere.

The dog, though, remains something of a problem. It is only here because it is a beast that barks, and if it does not bark then, according to the method, we have failed to address the topic. One can sense the blind toads gathering at the edges of our reason.

In the circumstances, it might be best to ditch Babinsky and the innocent and the silent dog, the dark alleyway in the night-time, and to return to daylight, and trees, tree bark and tree sap, a copse, a spinney, a forest, or a lone pine or plane tall upon a promontory, silhouetted against a blazing sky, windswept and forlorn. We would have to concede the near-impossibility of bashing out a thousandish words on the bark and the sap, but we could rest easier knowing that the blind toads of mental havoc had been banished from our head, on today of all days, Friday the nineteenth of October in the year of Our Lord MMXII.