In some cases, roots look similar to stems. It's tempting to say that stems are above ground, and roots below ground - but that's not entirely accurate. One simple difference is that stems bear leaves, and roots don't. Here are some other, less obvious, ways to distinguish stems from roots.

In herbaceous stems, the vascular tissues are contained in bundles; these bundles sit relatively near the surface of the stem. In roots, the vascular tissues form a central core - a location where they're protected from the harsh activity of pushing through soil. (We are referring to young, non-woody tissues)

In stems, side branches arise from axillary buds. These buds are located at the nodes in the leaf axil (the point where the leaf attaches to the stem).

Lateral roots, on the other hand, arise from deep within the root's tissue, near the central core. Roots don't have nodes, and they don't have buds.

the under ground part of the plant is called root.In dicotyledonous plants th direct elongation of the radicle leads to the formation of primary root.when it bears lateral root of several orders that are referred to as secondary root. Strem is the ascending part of the axis bearing branches,leaves,flowers etc. The stess bears nodes and inter nodes.The region of strem where leaves are born are called nodes while internodes are portions between two nodes.