It is true that the Buddha said in the Mahayana Parinirvana Sutra "all beings have the Buddha-nature," but it is not true in the way most people believe it is. When a Buddhist doesn't study what the Buddha actually said one may surely believe that everyone, right now, is a fully enlightened Buddha manifesting the 32 marks of a superman. But nowhere, in the Mahayana Mahaparinirvana Sutra, does the Buddha declare that "all beings are perfected Buddhas" far from it.
Buddha-nature means that we, as sentient beings, have the potential to become fully enlightened Buddhas. We are not actual Buddhas. If, as some Buddhists in the past we want to believe that plants and trees, as sentient beings, have the potential to become Buddhas, I guess it's okay. But if we believe plants and trees are fully enlightened Buddhas—it's not true. No Sutra says this. Nevertheless some believed this like Ryôgen (912–985) a Japanese Tendai scholar. According to him the budding is Bodhicitta, i.e., the aspiration to become a Buddha; the growth is the training, i.e., the Bhumis; the fruiting is nirvana; and the dying is parinirvana! Others like Chûjin (1065–1138) believed that trees and plants had the 32 marks of a Buddha! Well, of course, this is taking things a bit too far.
There is an interesting parable the Buddha tells in the Mahayana Parinirvana Sutra which helps us to understand that our Buddha-nature is only potential; not actual. In order to actualize it we have to go on a perilous spiritual journey which ain't easy. It's not for the weak and fainthearted, in other words.
O good man! Do not say that beings do not have the Buddha-Nature just because of the retrogressive mind. For example, two persons hear: "In another land, there is a mountain made of the seven treasures. In the mountain, there is a clear spring, whose water tastes sweet. Should any person reach this spring, he will make away with poverty, anyone who drinks its water will gain a long life. But the way to it is long and steep. Then, both men desire to go there.
One of the men goes equipped with various traveling outfits, whereas the other goes unprepared, not even keeping to the rules of food. They are walking together, when along the way they meet a man who has plenty of treasures, perfect in the seven rarities. The two go up to this person and ask? "Is there any mountain of the seven treasures in that land" The man replies: "There truly is such a land, and it is not false. I have already gained the treasures. I have tasted the water. The only thing to worry one is that the way is steep, and there are robbers, gravel, and thorns; watery[succulent] plants are lacking. Thousands and millions go [set out], but few reach the end.”
On hearing this, one of the men feels regret and says: "The way is long and there is more than one trouble. Innumerable is the number of those who go and few gain the end. And how can I expect to reach this place? At present, I have what I need to have. If I stick to this, I will not lose my life. If life is at stake, where can I look for longevity?" The other man also says: "People indeed go; I too shall go. If I truly gain that place, I shall have rarities and taste the sweet water. If I cannot, let my life end there." Now, the one regrets having started out and draws back, whereas the other goes on, gaining the mountain and rarities, and tastes the water that he desired to taste. Carrying all that he has gained, he comes back to where he lives, serves his parents and his ancestors.
Then, the one who regretted having started out on the journey and who turned back, sees this and goes down with a fever. "He went and is now back. How can I remain here?" And equipping himself, he starts out on the journey again.
The seven treasures can be compared to Great Nirvana, the sweet water to the Buddha-Nature, the two persons to two Bodhisattvas who first aspire to Bodhi, the steep path to birth and death, the man whom they meet on the way to the Buddha-World-Honoured One, the robbers to the four Maras, the gravel and thorns to defilements, the lack of watery plants to the non-practising of the Way of Bodhi, the one who turns back to the retrogressing Bodhisattva, and the one who goes on alone to the non-retrogressing Bodhisattva.
Bringing to perfection and actuality what is only potential, namely, our Buddha-nature, is certainly analogous to a difficult—almost impossible journey. It is not like a trip across town to a Zen center of Dharma center, to do is zazen for a half hour or more. Not even winning Bodhicitta is easy. But then some modern Buddhists want to believe they are already Buddhas or not from from it. This is spiritual hubris somewhat like the Christian suffers from who believes God talks to him.