Sindarin, singular, from mal meaning gold and orn meaning tree. Plural mellyrn. (Sil., pp. 361-2)

The mallorn tree is a forest giant which grows only in Lórien*, in and amongst which the Galadhrim built their tree city Caras Galadhon.

The branches of the mallorn-tree grew out nearly straight from the trunk, and then swept upward; but near the top the main stem divided into a crown of many boughs, and among these they found that there had been built a wooden platform, or flet as such things were called in those days: the Elves called it a talan. (LotR, pp. 333-4)

Digging through The Lord of the Rings yields information in scattered bits. The mellyrn are of enormous girth and untold height. They possess smooth silvery grey bark that seems to glimmer in the dark and enormous leaves shaped like those of a beech tree**. The leaves are green on top and silver below, turning golden in the autumn. The leaves do not fall during the winter, and so the forest is canopied with gold throughout that season. In the spring, new leaves unfurl and the old leaves fall, carpeting the forest ever deeper in gold as the years pass. However, the tree tops remain golden in the spring, profuse with yellow flowers. The fruit of the tree is described as a small nut with a silver shale.(LotR, pg. 999)

These trees grow only in Lórien with one notable exception. Galadriel's gift to Sam included a single mallorn nut. This he planted where the Party Tree had stood, and it is noted as ''the only mallorn west of the Mountains and east of the Sea, and one of the finest in the world.'' (LotR, pg. 1000)

Although the mallorn resembles the beech tree in many respects (smooth grey bark, trunk and leaf shape, the way the leaves turn golden in the fall and do not drop until the new leaves come out), the mallorn is not a beech tree. Tolkien had beech trees in Middle Earth as well. The most notable is Hírilorn. The giant beech tree of Doriath with three trunks in which Thingol had a house built and there imprisoned Lúthien. (Sil., pg. 172)


* Lórien here refers to Lothlórien which is in Middle Earth, rather than the garden of the Vala of dreams in Valinor.

** I have extrapolated the shape of the leaf from the description of the leaf brooches that the Fellowship were gifted. The one Pippin dropped which was recovered by Aragorn is described in detail in The Two Towers.

Sources:
Tolkien, J.R.R.: The Lord of the Rings, Houghton Mifflin, Co., NY, 1994.
Tolkien, J.R.R.: The Silmarillion, Houghton Mifflin, Co., Boston, 1977.

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