We can learn Dhamma from trees. A tree is born due to a cause, and it grows following the course of nature until it buds, flowers and bears fruit. Right here the tree is discoursing Dhamma to us, but we don't understamd this. We're unable to bring it within and contemplate, so we don't know hat the tree is teaching us Dhamma. The fruit appears and we merely eat it without investigating : Sweet, sour or bitter, it's the nature of the fruit. And this is Dhamma, the teaching of the fruit. Then the leaves grow old, they wither, die and fall from the tree. All we see is that the leaves have fallen down. We step on them, we sweep them up, that's all. We don't know that nature is teaching us. Later on, the new leaves sprout, and we merely see that, without taking it further. This is not the truth that is known through internal reflection.
If we can bring all this inward and investigate it, we will see that the birth of the tree and our own birth are no different. This body of ours is born and exists, dependent on conditions, on the elements of earth, water, wind and fire. Every part of the body changes according to its nature. It's no different from the tree. Hair, nails, teeth and skin, all change. If we know the things of nature, then we will know ourselves.
(The Teachings of Ajahn Chah)