Dyson tree
 Artist's concept of a spherical Dyson tree
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A Dyson tree is a hypothetical genetically-engineered plant (perhaps resembling a tree) capable of growing in a comet, suggested by the physicist Freeman Dyson. Plants could produce a breathable atmosphere within hollow spaces in the comet (or even within the plants themselves), utilising solar energy for photosynthesis and cometary materials for nutrients, thus providing self-sustaining habitats for humanity in the outer solar system analogous to a greenhouse in space or a shell grown by a mollusc.
A Dyson tree might consist of a few main trunk structures growing out from a comet nucleus, branching into limbs and foliage that intertwine, forming a spherical structure possibly dozens of kilometers across.
Dyson trees are mentioned a number of times in science fiction, beginning in the 1980s:
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