Which histogen of apical meristem is seen in root apex but not in shoot apex?

Concept: Histogen is a derivative of promeristem, which is composed of a group of cells or primary tissue and is thought to give rise to new plant parts or organs. Hanstein, a scientist, proposed the histogen theory.

Calyptrogen is a histogen layer found in the root apical meristem, which is responsible for the formation of the root caps. A group of about 1000 cells in the shape of a hemisphere, these cells face the root tip and are made up of Epidermis, Cortex, and Stele cells.

Hence, the correct answer is option D.