Introduction:
Auxin is a plant hormone that plays a significant role in controlling the growth and development of plants. It is primarily produced in the tips of stems and roots, and its movement throughout the plant helps regulate various physiological processes.

Explanation:
The movement of auxin within the plant is primarily unidirectional, from the tip of the stem towards the base. This movement is known as basipetal transport. Let's understand why this is the correct answer:

1. Origin of Auxin:
Auxin is primarily synthesized in the apical meristem, located at the tip of the stem. It is produced in specialized cells called the apical meristem cells.

2. Role in Growth:
Auxin controls various growth processes in plants, such as cell elongation, apical dominance, and tropisms. It promotes elongation in stem cells and inhibits growth in lateral branches, leading to upward growth.

3. Polar Transport:
Auxin exhibits polar transport, which means it moves in a specific direction within the plant. The movement of auxin occurs through specialized cells called parenchyma cells, which form continuous strands known as vascular tissues.

4. Basipetal Movement:
The movement of auxin within the plant is primarily basipetal, meaning it moves from the tip of the stem towards the base. This movement occurs through the phloem, which is responsible for transporting organic compounds throughout the plant.

5. Redistribution and Regulation:
Once auxin reaches a particular region, it can be redistributed to other parts of the plant. This redistribution allows auxin to control growth in various regions and coordinate different growth processes.

Conclusion:
Auxin originates at the tip of the stem and controls growth in other parts of the plant through basipetal movement. Understanding the directional movement of auxin is crucial in comprehending its role in plant growth and development.

Auxin is a prominent plant hormone which has several functions in plants. It is usually present at the growing tips like shoot or root tips. They are transported primarily toward the roots. It moves from apex to base, polar transport. Thus it is called the basipetal movement.