The part of an embryo that grows into a stem is the plumule.

Here's a breakdown:

* Plumule: This is the embryonic shoot, located above the cotyledons (seed leaves). It contains the apical bud, which is responsible for the growth of the stem and leaves.

* Radicle: The embryonic root, located below the cotyledons, develops into the primary root.

* Cotyledons: These are seed leaves that provide nourishment for the developing embryo.

So, while the whole embryo develops into the plant, it's the plumule that specifically contributes to the formation of the stem.