Seen with Goethe's eyes, the plant kingdom as a whole appears to be a single mighty plant. In it the ur-plant, while pressing into appearance, is seen to observe the very rule which we have found governing its action in the single plant - that of repeated expansion and contraction.⁷ Taking the tree in the sense already indicated, as the state of highest expansion along the ur-plant's way of entering into spatial manifestation, we note that tree-formation occurs successively at four different levels - as fern-tree (also the extinct tree-form of the horsetail) among the cryptogams, as coniferous tree among the gymnosperms, as palm-tree among the monocotyledons, and lastly in the form of the manifold species of the leaf-trees at the highest level of the plant kingdom, the dicotyledons. All these levels have come successively into existence, as geological research has shown; the ur-plant achieved these various tree-formations successively, thus giving up again its state of expansion each time after having reached it at a particular level.