The trees of Green World has their own intention. The one that stands at the top of the great forest is the Great Tree Yggdrasil, but even Great Tree Yggdrasil itself is only a part of the great forest.
Leafer that finished their time as seed takes root in the earth. Life of Leafer is extremely long, and it's only after they take root in the earth that they are considered as adult. The existence of Leafer is indispensable when talking about the growth of the greeneries.