Final answer:
The major difference between monocot and eudicot stems is in the organization of the vascular bundles, where they are scattered in monocots and arranged in a ring in eudicots.
Step-by-step explanation:
The major distinguishing feature between monocot and eudicot (often referred to as "dicot") stems is the organization of the vascular bundles. In monocots, the vascular bundles are scattered throughout the ground tissue of the stem, without a particular pattern. However, in eudicots, the vascular bundles are characteristically arranged in a ring around the periphery of the stem, often with the xylem facing inward and the phloem facing outward. This distinct difference in the arrangement of vascular tissue contributes to different stem anatomies between these two plant groups.