Specifically, the growth of walnut shell is controlled by the shell growth layer. When the shell growth layer produces different numbers or speeds of cell proliferation in several different places, it will form a triangular or quadrangular shape. This proliferation may be caused by changes in plant hormone levels, light, temperature, water and other environmental factors.
Therefore, the formation of three-sided walnut and four-sided walnut is the result of adaptive response and genetic variation of plants to environmental changes.