in Thoreau’s Journal:

There is no ripeness which is not, so to speak, something ultimate in itself, and not merely a perfected means to a higher end. In order to be ripe it must serve a transcendent use. The ripness of a leaf, being perfected, leaves the tree at that point and never returns to it. It has nothing to do with any other fruit which the tree may bear, and only genius can pluck it. The fruit of a tree is neither in the seed nor in the full-grown tree, but it is simply the highest use to which it can be put.