"Some of this may sound confusing, sir, but…"
"Tell me," Adams had asked him, "about symbiosis. I don't care about all this other stuff."
"It really is," the robot said, "a very simple thing. Now, take heather, for instance. You know, of course, that it is associated with a certain fungus."
"No," Adams said, "I didn't."
"Well, it is," the robot said. "A fungus that grows inside of it, inside its roots and branches, its flowers and leaves, even in its seed. If it weren't for this fungus, the heather couldn't grow on the kind of soil it does. No other plant can grow on so poor a soil. Because, you see, sir, no other plant has this particular fungus associated with it. The heather gives the fungus a place to live and the fungus makes it possible for the heather to make its living on the scanty soil where it has no opposition."
"I wouldn't call that," Adams had told him, "a very simple business."
"Well," said the robot, "there are other things, of course. Certain lichens are no more than a symbiotic combination of an alga and a fungus. In other words, there is no such a thing as a lichen in this case. It's just two other things."
"It's a wonder to me," said Adams sourly, "that you don't simply melt down in the white heat of your brilliance."
"Then there are certain green animals," said the robot.
"Frogs," said Adams.