Final answer:
Auxin transport in a plant is polar, which facilitates directional growth responses to environmental stimuli such as light and gravity by promoting cell elongation on one side of the plant.
Step-by-step explanation:
The direction of auxin transport in a plant is best described as polar. This refers to the one-way, directional movement of auxins from where they are synthesized, typically in the shoot or apical meristem, towards the base of the plant. This movement is essential for various growth processes, such as cell elongation in phototropism and gravitropism, vascular tissue differentiation, leaf development, and the regulation of apical dominance. Auxins influence the direction of plant growth by promoting cell elongation on one side of the plant, causing it to bend toward or away from stimuli such as light and gravity.