Humbugs and Humanoid Plants
The Sorcerer of the Mangaboos is named Gwig, and unlike the rest of those people he's not eerily handsome. In fact, he's all covered with thorns.
Gwig, it seems, is a bit of a humbug, because after the first Rain of Stones he promised there would be no more. He gets out of this by pointing out that the second was a Rain of Stones-and-Horse-and-Buggy, which is almost good enough to get him a job as Press Secretary.
Unfortunately for Gwig, a greater humbug arrives, as a balloon descends from the roof of the cavern, carrying in its basket none other than the Wonderful Wizard of Oz himself! According to the Great And Powerful Oz, his balloon happened to touch down just as an earthquake split open the ground, and he wound up descending into the underground world of the Mangaboos. This, to me, sounds like more humbug. I can just barely believe that a whole horse and buggy can fall into a crack in the earth, but a balloon?
This seems to confirm my theory. The Wizard (and Dorothy) both have the ability to teleport themselves from our world into the Oz Continuum, or whatever you care to call it. And while Dorothy's power is stress-activated, the Wizard apparently can only make the shift while airborne.
There follows a spectacular magical duel between Gwig the Sorcerer and the Wizard of Oz. You'd think Oz was the underdog in this contest, since he's a mere humbug pitted against a thorn-covered freak who can actually do real magic. But you'd be wrong, because Oz is a great humbug. His stage illusions are every bit as impressive as Gwig's sorcery.
Finally Gwig loses patience and gets vicious. He uses his magic to stop the Wizard's breath. Oz, never one to let oxygen deprivation keep him down, uses his last few seconds of consciousness to pull several knives from his sleeve and assemble them into a sharp sword, with which he cuts Gwig in half, thereby winning the contest and saving himself from suffocation.
Cutting Gwig in half does reveal something important about him and the rest of the Mangaboos: they are vegetables. Gwig's corpse is solid inside like a turnip, so there is no distressing torrent of blood and viscera.
The leader of the Mangaboos takes the surface folk on a little tour, showing them the garden where new Mangaboos grow on bushes. They start out as babies, and when fully grown are ready to be picked. (And fully-dressed: the Mangaboos' clothing is actually part of their bodies. It's rather odd to see a book, even one for children, in which nudity is absolutely impossible but people get cut in half without much fuss.)
This means the Mangaboos are fruits, and it's actually a pretty ingenious method of plant propagation. The plants evidently pollinate, for while there are men and women Mangaboos that seems to be nothing but a superficial appearance. Once fertilized, the Mangaboo plant creates mobile, intelligent fruit which can spread far and wide, consciously tending and planting the bushes and destroying rival species and predators. It's a great system. They live for five years, and don't eat -- which makes perfect sense, since they have no digestive organs. (How they speak is an interesting question.)
Incidentally, this means Mangaboo must be an extremely good source of nutrition. If an adult Mangaboo can live five years off of stored reserves, their bodies must be simply packed with calories. Assume that half a Mangaboo's weight at picking-time is used for food storage (the other half is neural tissue, muscles, etc.). They probably aren't warm-blooded, so they can get by on, say, 1000 calories per day. That means Mangaboo flesh contains about 40 calories per gram. That's five times the calorie content of butter! Forget about ethanol and switchgrass; we could solve all our energy problems overnight by planting Mangaboos!
The Mangaboo leader (known only as "the man with the star") may be aware of this, because he announces the visitors must be destroyed shortly.
This man doesn't know who he's dealing with, does he?
Dorothy and the Wizard promptly do what they do best: they mount an instant coup d'état against the Man With The Star by picking a fresh Mangaboo princess from the Royal Bush to replace him. He hands over the star emblem and goes off to be planted (or possibly converted into sustainable biofuels).
Unfortunately, their meddling in Mangaboo internal politics blows up in their faces when the Princess eventually decides that her predecessor was right and the surface people must, indeed, be destroyed. The Wizard keeps the Mangaboos at bay with burning kerosene -- and given what we've seen about their energy content, Mangaboos should go up like bombs if they catch fire.
But the Mangaboos are persistent and determined, and in spite of the threat of flaming oil and kicks from Jim the cart-horse, they eventually drive our heroes into the Black Pit, where we shall find them all next time.
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