Mike Boom of the Mycological Society of San Francisco has shed some light on the brain that grew on the tree in front of my house. Sadly, Mike agrees with my wife that it is not a brain...

Hi Steve,
It's almost certainly a sulfur shelf, aka Laetiporus gilbertsonii in the western U.S. unless it's growing on a pine tree, in which case it's probably Laetiporus conifericola, still a sulfur shelf in common parlance. In the eastern U.S. it's Laetiporus sulphureus. It's a decomposer that usually grows on dead trees, but is sometimes parasitic on live trees and does them no good. Check out a description here.

If there's any message from the tree in this case it's "help me!" There's not a whole lot you can do, though, other than hope the tree is in good enough health to survive it. If it's not, at least the sulfur shelf will eventually decompose the dead body and turn it into good fertilizer.

What you see is the fruiting (the mushroom) of the resident fungal organism, the mycelium, which invisibly skulks around year-round inside the tree digesting parts of it. Once a year it fruits a mushroom to reproduce, which it does by dropping lots of spores that can each start a new mycelium.

The best defense: eat it! Make sure of your identification first, because mushroom ID via tiny web pictures is never a sure thing. But if it is a sulfur shelf, it's quite tasty when young and fresh, with a consistency of tofu. Saute it, serve it in a salad, and congratulate yourself for protecting other trees. If it gets even slightly old, it gets tough and bitter, something like an old idealist.

Bon appetit,
Mike Boom

Mike is very cool, and makes a very convincing argument. But I can play that game too, and I am still convinced it is a "humanus brainius." But that's okay. We'll see who gets the last laugh when the Tree Creatures arrive.