Final answer:
The part of the eudicot embryo that develops into the stem of the plant is the hypocotyl, which is found between the cotyledons and the future root known as the radicle.
Step-by-step explanation:
In eudicot (or true dicot) embryos, the structure that develops into the stem of the plant is known as the hypocotyl. The hypocotyl is the part of the embryonic axis that lies between the attachment point of the cotyledons and the radicle, which will become the root. As the seed germinates and the plant begins to grow, the hypocotyl extends and forms the stem in most dicotyledonous plants.
This contrasts with monocotyledonous plants where the hypocotyl does not extend above the ground because they do not demonstrate stem elongation. It is also important to distinguish between different components of the embryonic axis, such as the plumule, which is part of the epicotyl found above the cotyledons and includes young leaves and the shoot apical meristem, the site where new cells for stem growth are produced.