“The Supreme Lord said: There is a banyan tree which has its roots upward and its branches down, and the Vedic hymns are its leaves. One who knows this tree is the knower of the Vedas.” (Bg. 15.1)

Here the material world is described as a tree whose roots are upwards and branches are below. We have experience of a tree whose roots are upward: if one stands on the bank of a river or any reservoir of water, he can see that the trees reflected in the water are upside down. The branches go downward and the roots upward. Similarly, this material world is a reflection of the spiritual world. The material world is but a shadow of reality. In the shadow there is no reality or substantiality, but from the shadow we can understand that there is substance and reality. In the desert there is no water, but the mirage suggests that there is such a thing as water. In the material world there is no water, there is no happiness, but the real water of actual happiness is there in the spiritual world. (from Introduction to the Bhagavad-gita As It Is)