Final answer:
The part of the plant to replant when dividing is the roots, due to their indeterminate growth and potential to re-establish the plant.
Step-by-step explanation:
When dividing a plant, the part that should be replanted is the roots. This is because plant roots contain the necessary structures to re-establish the plant in a new location. Roots generally exhibit indeterminate growth, meaning they can continue to grow throughout a plant's life. Some plants also have meristematic tissue within their roots, which is a region of actively dividing cells essential for plant growth.
Examples of parts that are not typically replanted are the leaves and flowers, as they tend to exhibit determinate growth, ceasing to grow once they reach a certain size. While the stem contains important structures, such as nodes and internodes, and can sometimes be used to propagate plants (depending on the species), it is generally the roots that are most crucial when dividing plants.