Nimloth was the White Tree that grew in the courts of Armenelos in Númenor, a descendant of Telperion itself. When Sauron grasped power in that island kingdom, he became determined to destroy that symbol of the Valar and their land. Before the tree could be burned, though, Isildur broke through its guards and rescued one of its fruits, which grew into a seedling.
After the Downfall, Isildur carried that new-grown tree to Middle-earth, and planted it in his own courts in Minas Ithil, on the borders of Mordor. A little over a century after the foundation of the kingdom of Gondor, Sauron's forces descended on Minas Ithil, and the young White Tree of Isildur was destroyed. Once again, Isildur preserved its line, rescuing a seedling of his White Tree before abandoning the city. It was from that seedling that all the White Trees that grew in the courts of Minas Tirith were descended.