Which way is down??

Sometimes I find myself wondering about a phenomenon so ordinary that I had taken it for granted all my life. Mushrooms always orient themselves so that the cap is horizontal, with the spore-producing surface pointing down.

Chicken mushroom

Up to this point, I’m OK. I know that they disperse their spores by simply opening up the gills or tubes to let the spores fall down, and then the wind will catch them. And the upper surface acts to keep the spores dry, essential to their proper dispersal. But here is the question: how does the mushroom know which way is down.

Perhaps it works like in plants. The stalk goes straight up, towards the sun, and the cap just ends up horizontal, at right-angles to the stalk?? But plants have a reason to reach for the sun, they need it to photosynthesize. If they grow in the shade, they may lean sideways towards the sun, and sunflowers turn their entire flower not upwards, but at an angle to face the sun. This one had fallen over in a storm, and contorted itself around back to the sun’s rays.

Mushrooms don’t do this. The cap just faces up and the gills down. And they can grow in very dark places (although some do in fact need light and react to it.)

The urge for the cap to face down is very strong. If the stalk emerges sideways to avoid a rock or a root, it will bend upwards as soon as it can, so that the cap faces down.

Sometimes the mushroom emerges straight, and then gets knocked over by a passing creature, like a beagle (!).

But the following day it has begun to compensate:

Leccinum sp.

and in one more day it is splendidly resurgent:

This by the way is an 8 inch chunky bolete; there is an acorn at the base of the baby one, for scale.

Bracket fungi, with no stalks, grow straight out of the tree at right-angles, like these polypores on the right edge of the still-standing remnant of the trunk below:

And when the tree falls, the new growth simply rotates itself through 90 degrees so it is facing down again. It’s a little hard to explain in photos, so I’ve given you three attempts below. In each image the old fungi that grew before the fall are vertical now, seen thin edge-0n, and the new ones are horizontal.

So how DO they do it?? Remarkably, it appears that they sense gravity. Just like us. Our ears contain tiny calcium beads called otoliths (‘ear stones’), which float around and brush against tiny hairs and thus remind us of which way is up. If they get loose, our sense of gravity goes wrong and we feel dizzy. It seems that fungal cells may have a similar way of sensing gravity, so they always know which way is down. The nuclei in each cell act a little like otoliths, and the whole fungus responds.

The phenomenon is called gravitropism*, and here are two more successful examples.

If you’d like to know more, here is an excellent article.

*The shelf mushrooms that restart new versions at right angles to the previous ones are technically examples not of gravitropism, but of gravimorphogenesis, since the original doesn’t reorient itself, only the new growth is sensitive to gravity.

PS Rather wonderfully, fungi have been taken into zero gravity in space to see how this affects them. Many of them not surprisingly get too confused to form proper fruiting bodies at all!

4 thoughts on “Which way is down??”

  1. And maybe the mushrooms don’t like their gills to get wet, if it rains, because it would make them rot & they wouldn’t be able to reproduce. So they protect their gills.


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