The mallorn (pl. mellyrn) was a kind of large tree growing in Valinor, Númenorand the Westlands.
Description
Mellyrn are trees of immense size, especially when growing on Númenor. They are also described as similar to birches: their bark was smooth and silver-grey, and the leaves were green on top and silver underneath; they turned golden in autumn and remained on the tree through the winter to fall to cover the ground in the spring, when new leaves sprouted. Golden flowers bloomed on the branches.
The trunk of the tree divided to a crown of many branches, and the Galadhrim used the space to build telain (a sort of platform) or buildings on the top of the trees. The leaves also had practical uses, such as for packing rations of lembas.
Frodo Baggins explained that he could feel the life of the tree when he touched a mallorn in Lothlórien.

History
Mallorn Tree in The Lord of the Rings Trading Card Game
Mallorn-trees originally grew in Valinor and from there were spread to Tol Eressëa by the Elves once the Lonely Isle was moved to its final position by Ulmo. The Elves of Tol Eressëa brought mallorn-trees to the Men of Númenor and they grew on the shores around the Bay of Eldanna in that land. Even Tar-Aldarion, the great Ship-King, did not cut down these trees.
Mallorn-nuts were given by Tar-Aldarion to his friend Gil-galad, the Noldorin King of Lindon. The mellyrn did not grow in Lindon, but Galadrieltook some nuts with her to Lothlórien, where they grew to immense heights (but not as great as the groves of Númenor).Lothlórien became known as the Golden Woodbecause of them.
Caras Galadhon, the city of Galadriel and Celeborn in Lothlórien, was built in the branches of huge mallorn-trees. The Fellowship spent the night in a flet in a mallorn-tree nearby, and were later given lembas wrapped in mallorn-leaves.
Galadriel gave Samwise Gamgee a box of soil containing a single silver mallorn nut. After the War of the Ring and the Scouring of the Shire, Sam planted the nut in the Party Field where the Party Tree had stood before its felling. It was the only mallorn-tree in Middle-earth outside of Lórien. When the tree bloomed in the next summer it was said that all the Shire became golden from the flowers.

Etymology
The word comes from malt ("gold") and orn ("tree"). In Gondor Sindarinthe same word was pronounced Malthorn.
Malinornë is the Quenya translation of Mallorn.
Uses outside the Legendarium
By 1956 Tolkien had been informed about "a new house far away [that] has been called The Mallorns" — regrettably, Tolkien noted, as the correct Sindarin plural would be mellyrn.
Similar trees
It seems redwood (sequoia giganteum) giant eucalyptus (eucalyptus regnans) and taxodium (taxus bacchata) are like to mallorn. (By author)



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