As the World Tree's very first branches and leaves grew, what Zheng Yichen initially thought of was their historical longevity. It wasn't until after Senluo explained it to him that he truly understood how unique they were.
The World Tree also undergoes a form of 'metabolism.' Despite its ancient age, most of its body is quite new (within a million years), with very few parts being preserved from its early days, and even that preservation isn't simply about keeping them as collectibles.
It's a preservation that coincides with the growth of the World Tree itself. As for the so-called 'metabolism' of the World Tree, it's part of a cycle where the old sections provide nutrients for the new growth, facilitating better development for the newer parts.
The parts that have been specifically preserved from its earliest days are different; even the vast passage of time has made this preservation extraordinarily special.
"But the value is still not as great as that of the World Tree seed."