The wheel-tree, also spoken of as the seed-tree or seed-pod tree, was a species of plant native to the mulefa world.
The wheel-trees had deeply-ridged bark of a red-gold colour and were about half-again as tall as the redwood trees in California.
The wheel-trees produced flowers which were fertilised by Dust.
The seed-pods were perfectly circular and about as thick as the width of a human palm. The pods were extremely hard but not heavy. They were covered in fibrous hairs so that one could run their hand along the surface easily in one direction, but not the other. Inside the seed-pod were many flat, oval-shaped, pale seeds about the size of a human's little fingernail.
The wheel-tree plants had a symbiotic relationship with the mulefa. The mulefa used the seed-pods of these trees as wheels, which allowed them to move around with greater ease. By using them in this manner, the mulefa wore down the seed-pods until they broke open, releasing the seeds and carefully planting them.
A further effect of this symbiotic relationship was that the oil released by the seed-pod during its use as a wheel gave the mulefa Dust and consciousness.
Mary Malone used the wheel-tree oil on the amber lenses she had created to allow her to view Dust. The mulefa turned this into the amber spyglass by adding a bamboo tube to keep the lenses the correct distance apart.
When Mary Malone left the mulefa world to return to her own, Atal gave her a bag of wheel-tree seeds. She gave three of the seeds to a scientist at the Botanic Garden in Oxford where they grew well.