Curious, I went outside to investigate the prodigious lemon. I squatted down to examine it, then poked a finger at it, and only then did I realize that I was looking at the cap of a mushroom and not a real lemon. Regrettably, I did not have the presence of mind to take a picture. The resemblence was truly uncanny. The photo I've included here gives an idea, perhaps, of how this mushroom, which is lepiota lutea, could be mistaken for a lemon in terms of color and general shape, but the one I saw in the flowerpot that morning was much more convincing.
At the time, I was astounded, convinced this was a deliberate evolutionary strategy by the mushroom--growing at the bases of trees and mimicking fallen fruit in order to trick herbivores into eating them and thus spreading the spores in their stool. As it turns out, lepiota lutea is commonly known as the "yellow houseplant mushroom" because it commonly turns up in all kinds of potted plants, for some reason. So expert opinion is against my hypothesis which, as the old Time-Life Books commercial used to say, is "dismissed as coincidence." But part of me still wants to believe.