Mandala designs in nature can be found in something as simple as a flower. The flower has a center and surrounding that center are beautiful petals. The inside structure of a snail's shell is a spiral that twists into the center. Even the rings found inside tree trunks can be looked upon as a mandala, as each ring graduates toward the center.
Our solar system can be considered a mandala form as each planet is part of and revolves around a center planet. For example the Milky Way galaxy our solar system holds within it our Earth.
Even microscopic cells each have a center or nucleus. The wholeness created in anything with a center that then radiates outward and inward is the basis of the mandala structure.
Flower Mandala
