A friend of mine, a talented painter, disputed a suggestion that all forms, no matter how complex, can ultimately be constructed out of simple geometric shapes. I'd been introduced to this comforting idea in a free, online drawing class. As soon as I said it out loud, it sounded too good to be true.

At a London Underground station, somewhere on the Northern Line, on a wall facing the platform, damp has caused dark, fungal marks to grow in an approximation of human arms. In the middle of these 'arms' sits a lighter, oval space, potentially caused by some poster or signboard having been removed, revealing a temporary, unspoiled surface. This oval, in the context of the surrounding, anthropomorphic mould, looks, of course, like a head.

I waited for the train and traced the mushrooming complexity of this organic figure. Shades of muscle could be imagined where the damp had most firmly taken hold. The arms were uneven, bent into uncomfortable positions. Like all figures stretched out like awkward birds, this one could have been an attempt to figure Christ. How long will it last, I wonder. At some point it will be covered with a billboard, or scraped from the wall, pressure-hosed until nothing but spores remain.

I bet these mouldy lines have given many people pleasure, or pause for thought, though I doubt many people will have spoken about them. Increasingly, I'm coming to suspect that anything you record deliberately - anything at all - is worth the recording. Worth is invested by the act of writing something down, just as framing invests a picture with significance.

But if someone had been standing there with me, I don't know if I would have pointed any of this out. I record it now, after I'm no longer there, because I want to be reminded, today and in the future, and because I want to share the experience. At the time, I was content to experience it alone. I can't even remember which station I was in. South of the river, I think.

Beneath this apparition, under the electric rail, was a pool of rust-brown mud. I couldn't work out where this water had come from. There was no obvious leaking from the roof. The rest of the track was filthy but dry. Perhaps tunnels this far down are always piercing the water table. Where does the water table start and end? Are we in it, above it, or below it? I don't understand how the earth works.

If I had to build that figure out of simple lines, I'd start with the head, or the vacant space that, in the midst of patchwork mould, makes us think of a head. Then, I'd trace the upper edges of the arms, although in the original there are no obvious edges. The form is suggested by vague contours of clots, riddling fronds, and bulbous off-shoots. It's the mind that draws the line, running from the featureless head all the way down to the lack of human hands.