The Integral Trees is a 1984 science fiction novel by American writer Larry Niven (first published as a serial in Analog in 1983). Like much of Niven's work, the story is heavily influenced by the setting: a gas torus, a ring of air around a neutron star. A sequel, The Smoke Ring, was published in 1987. It was nominated for the Nebula Award for Best Novel in 1984, and the Hugo Award for Best Novel in 1985.
Setting The story occurs around the fictional neutron star Levoy's Star (abbreviated "Voy"). The gas giant Goldblatt's World (abbreviated "Gold") orbits this star just outside its Roche limit and therefore its gravity is insufficient to keep its atmosphere, which is pulled loose into an independent orbit around Voy and forms a gas torus around Voy. The gas torus is huge—one million kilometers thick—but most of it is too thin to be habitable. The central 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. Furthermore, the Smoke Ring is in orbit and therefore in free fall: there is no "up" or "down". Most animals have trilateral symmetry that allows them to see in all directions. The majority of Smoke Ring animals have evolved to fly on at least an occasional basis—even the fish. The Smoke Ring contains numerous "ponds", globs of water of various sizes which float free like everything else. While there are aquatic and amphibious animals in the Smoke Ring that live the majority of their lives in such ponds, these animals may find their habitat unsuitable at any moment. Whether their home pond drifts too far out of the habitable center of the Smoke Ring and into the gas torus, becomes too large and breaks up due to tidal forces, or impacts a large object such as an integral tree, aquatic animals must be able to propel themselves through the air to find a new place to live. Most plants in the Smoke Ring are quite fragile because they do not need to support their own weight. A notable exception to this rule are the eponymous Integral Trees, which can grow up to 100 kilometers long. Tidal locking causes them to be radially oriented, with one end pointing in toward Voy while the other points toward space. The ends of the tree feel a tidal force of up to 1⁄5 g0 on the largest trees. Each end of a tree is a green, leafy tuft. An integral tree tuft is up to 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) compared to the atmosphere, which is in orbit at all points. The ends of the tree are therefore subject to a constant gale-force wind that causes the ends to curve into the shape of an integral symbol (∫) and pushes water and food onto the tufts, or (less forcefully) onto the trunk, where the gravity-like tidal forces pull the material out towards the tufts. Each tuft serves several main purposes for the tree. First, the green foliage is photosynthetic, providing the tree with energy from sunlight; second, the tufts are where the tree produces its seeds; and third, various plants, animals, and sundry other objects can become trapped in the tighter branches, which gradually migrate toward the "treemouth". The treemouth, a pit at the top of each tuft on the lee side of the trunk, is the integral tree's root; water collected along the trunk flows down due to tidal forces and is ingested by the treemouth. Tuft branches catch and capture various things and animals in the wind and gradually, over the course of years, migrate to the treemouth, where they and their catch are absorbed by the tree for nourishment. Many other large Smoke Ring plants have a scavenger/carnivore aspect similar to this, though with somewhat different mechanisms.
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