Final answer:
The pith is the structure that isn't found in the leaf of a dicot or the stem of a monocot, as it's only a central feature in the stems of dicots.
Step-by-step explanation:
The structure that isn't present in the leaf of a dicot and the stem of a monocot is the pith. In dicots, the vascular bundles of the stem form a distinct ring with the pith occupying the center of the ring. In monocots, however, the vascular bundles are scattered throughout the ground tissue, and the pith is typically absent or very small. When examining leaves, dicot leaves often have networked veins, while monocot leaves typically have parallel veins. However, regarding the pith, it is a central feature in the stems of dicots but not in the stems of monocots, which is why the pith is the structure that differentiates them.