Magellum is a genus of plants native to Pandora. There are two known species of this genus: the delta tree (Magellum triangulare) and the unidelta tree (Magellum deltoides), which grow in mutualistic symbiosis.
The Unidelta tree differs from the Delta tree in that it has non-toxic leaves and poisonous roots, while the Delta tree has the opposite. When a delta tree and a unidelta tree grow side by side, their underground roots connect and toxins from the unidelta tree flow into the delta tree, causing the latter's roots to also become poisonous. The benefit to the delta tree is that it does not have to expend energy to produce a leaf toxin; The benefit to the undelta tree is that it receives nutrients from its companion.
A sign that the roots of a delta tree have become poisonous is a slight change in its bioluminescence. The Na'vi have learned to recognize this change and the animals are careful not to gnaw at the roots of such trees, with the result that the union of the two species guarantees the safety of both.
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Grace Augustine and Norm Spellman observed the interaction of the roots of two trees by scanning in the jungle and displaying the scan result on a multiscreen. This plant experiment was also carried out under laboratory conditions, in glass flasks at Site 26.