Dicot stems have a circular arrangement of vascular tissues, whereas the stems of monocots have vascular-tissue bundles scattered throughout. In monocots the vascular bundles also tend to be toward the outside of the stem. Dicot vascular tissue forms a cylinder near the center of the stem.
Although not all dicots can produce secondary thickening, no monocots can. Therefore, flowering plants with thickening, woody stems are dicots.
Another difference between monocot and dicot stems involves the role played in root development. In dicots, the lower part of the embryo forms a radicle, and the roots grow from there. In monocots, the radicle doesn't form, and the roots grow out of nodes on the stem.