The Integral Trees
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The Integral Trees is a 1984 science fiction novel by Larry Niven. Like much of Niven's work, the story is heavily influenced by the setting: a ring or torus of air around a neutron star. Its sequel is The Smoke Ring.
Setting
The story is set at the fictional neutron star Levoy's Star (abbreviated "Voy"). The gas giant Goldblatt's World (abbreviated "Gold") orbits this star within its Roche limit. Thus, Gold's gravity is insufficient to hold its atmosphere, which is pulled loose into an independent orbit around Voy. This orbiting air forms a ring known as the Gas Torus. The Gas Torus is huge—one million kilometers thick—but most of it is too thin to be inhabitable. The cenral part of the Gas Torus, where the air is thicker, is known as the Smoke Ring. The Smoke Ring supports a wide variety of life.
There is no "ground" in the Smoke Ring; it is a world consisting entirely of sky. Thus, most animals can fly, even the fish. Furthermore, since the Smoke Ring is in orbit, it is in free fall. There is no "up" or "down". Most animals have trilateral symmetry, allowing them to see in all directions.
Most plants in the Smoke Ring are quite fragile, as they don't have to support their own weight. A notable exception to this rule are the eponymous Integral Trees. These are trees that are up to 100 kilometers long. Tidal locking causes them to be oriented radially, with one end pointing in toward Voy and one end pointing out. The ends of the tree feel a tidal force of up to 1/5 g. Each end consists of a leafy "tuft", which is where photosynthesis occurs.
Each tuft of a tree is 50 kilometers from the tree's center of mass. Thus, a tuft is either orbiting too slowly (the in tuft) or too quickly (the out tuft). The ends of the tree are subject to a constant hurricane-force wind. This wind bends the ends into an ∫, the integral symbol.
The Smoke Ring was colonized 500 years ago by a crew of 20 astronauts. Their descendants have adapted to the free-fall environment by growing taller and developing prehensile toes.
Plot outline
Quinn Tribe inhabits the "in" tuft of Dalton-Quinn tree. They normally subsist on the tree's cottony foliage, augmented by hunting and a flock of domesticated turkeys. But ever since the tree came near Gold six years ago (four local years, which are 7/5 of a standard year), it has been falling in toward Voy, out of the Smoke Ring. The tribe is suffering from a severe drought. The tribe's leader, the Chairman, decides to send a hunting party of nine up the trunk, including the Chairman's son-in-law Clave and Jeffer, the Scientist's apprentice.
When the party reaches the midpoint of the tree, they are attacked by a group from the out tuft, and two of them are killed. During the battle, the tree splits in half, causing the in tuft to fall farther in toward Voy (and killing its inhabitants), but allowing the out tuft to return to the Smoke Ring proper. The seven remaining members of Quinn Tribe and one of the attackers jump clear of the exploding tree, and are left adrift in the sky with no method of propulsion.
Before dying of thirst, they manage to hook a passing Moby (a flying whale), which takes them to a jungle. They cut loose and crash, and find themselves in the middle of a battle between Carther States, who live in the jungle, and slave runners from London Tree. The group is split when six of them are captured by the slavers; the other two remain in the jungle.
Carther States counter-attacks some weeks later, and the Quinn Tribe group is reunited. In the battle, they manage to steal the London Tree CARM (cargo and repair module, a miniature spacecraft relic from the original settlers). Not knowing how to pilot the CARM, they accidentally engage its main motor, which sends them thousands of miles away before running out of fuel.
The tribe, unable to find their way back, settle in a new tree, which they dub Citizens Tree.