The transition of radial vascular bundles in the root to conjoint vascular bundles in the stem occurs in the hypocotyl zone.

1. Radial vascular bundles in the root:
- In the root, the vascular bundles are arranged in a radial pattern.
- The xylem and phloem tissues are arranged alternately in separate bundles.
- Each vascular bundle consists of xylem towards the center and phloem towards the periphery.

2. Transition to conjoint vascular bundles in the stem:
- As the root transitions into the stem, the arrangement of vascular bundles changes.
- In the hypocotyl zone, the vascular bundles become conjoint, where the xylem and phloem are no longer separate but are present together within the same bundle.
- The xylem and phloem tissues are now arranged in a concentric manner.
- The xylem is present towards the center and is surrounded by phloem tissue.
- This arrangement allows for more efficient transport of water and nutrients throughout the stem.

3. Hypocotyl zone:
- The hypocotyl is the region of the stem located below the cotyledons (embryonic leaves) and above the root.
- It is the transition zone between the root and the stem.
- In this zone, the vascular tissue undergoes structural changes to adapt to the different requirements of the stem.

4. Significance of the transition:
- The transition from radial vascular bundles in the root to conjoint vascular bundles in the stem allows for more efficient transport of water, minerals, and nutrients.
- Conjoint vascular bundles provide a more organized and compact arrangement of xylem and phloem tissues, facilitating the movement of substances.
- This transition is an important adaptation for the growth and development of the plant as it transitions from the underground root system to the aboveground stem system.

In conclusion, the transition from radial vascular bundles in the root to conjoint vascular bundles in the stem occurs in the hypocotyl zone. This transition allows for more efficient transport of water and nutrients and is an important adaptation for the growth and development of the plant.