Nick Baker’s hidden Britain
King Alfred’s cakes fungus
You’ve seen them for sure but have you even wondered what they are? These hard, black, hemispherical lumps stuck to dead wood? The various common names don’t really help much: cramp balls, King Alfred’s cakes, coal knobs. They are in fact a fungus, but unlike those productions of damp, mouldering mulches, they are specially adapted to exploit a very different kind of habitat.
Like most fungi, Daldinia concentrica are not obvious to the eye for much of their lifecycle. Living primarily inside dead or dying ash trees, when conditions are right and they are ready to set spore, their multitudinous and microscopic filaments, called hyphae, come together. Like strands that make up a twist of rope, they knot up and form the distinctive stroma: the body of the fungus that produces spores. Starting as a small, flesh-coloured protuberance that pushes through the surface tissue or bark of the infected dead limb, it forms knobbly cushions and clusters that eventually harden and darken to form the purpleblack signature creations.
Cut down through one and you will see the formation that gives rise to the second part of their scientific name, concentrica (the first part, Daldinia, comes from the mycologist monk Agostino Daldini): a beautiful arrangement of alternating concentric rings of silver and black. The pattern is reminiscent of the annual growth rings found in trees. Here, though, the whole stroma is produced in around a year and each has in the region of 20 zones. Each zone corresponds to times when conditions were moist and favourable for fast growth.
Dry survivor
When those conditions are right, the tube-like hyphae grow outwards from the centre. This growth is represented by the thicker, lighter silver rings. Then the hyphae change orientation and grow at 90 degrees to the original growth. This forms the dense but thinner, darker bands. This is an adaptation to water conservation in these highly drought-tolerant fungi, and gives them the competitive edge.
The bands are gelatinous as they form and it is thought that this moisture is fed to the fungus where and when it is needed. When conditions dry up, the horizontal growth, like the outer surface, forms a dense, waxy layer, further protecting the fungus from dehydration.
The point of this stroma and the reason the fungi breaks cover like it does is all about the production of spores, of which there are two kinds. The first are difficult to see and are produced while the fungus is in its pinkish, rubbery growth phase. These are asexual spores. Once the stroma is ripe and blackened, they produce sexual spores.
Look at the surface through a hand lens and you’ll see a landscape that looks like the texture of ostrich skin leather. Each bump is the swollen entrance to a flask-shaped pore, or perithecium. The inner surface of this ruptures and, by a process of unequal water pressure, fires up the black spores. Place your specimen on a white sheet and overnight you’ll see that a halo of black dust up to 3cm away appears all around it. Each is a microscopic future Daldinia, fired out into the woodland world. Often the wood surrounding a ripe stroma will be similarly stained by the ejected spores.