‘Some people think that buddha nature is like seeds of grass and trees: when dharma rain is abundant, sprouts and stems grow; branches, leaves, flowers, and fruit mature; and their fruit contains seeds. Such a view is an assumption of ordinary people. If you come up with such an assumption, investigate thoroughly that each and every seed, flower and fruit is itself pure mind.
A fruit has seeds that are not visible, but develop roots, stems, and so forth. The elements of the plants are not assembled from outside, but branches and twigs grow. Not limited to inside or outside, the growth of plants is not in vain, past and present. Thus, even if you take up the view of ordinary people, the rooots, stems, branches, and leaves are the all are of buddha nature that arises and perishes simultaneously with all things.’ (Shobogenzo Bussho)
If it isn’t clear, he is basically saying that buddha nature is not a process of becoming, it’s what is behind the process, and everything else. At least, that’s how I see it.