What is the primary role of auxin in plants?

Auxin plays a crucial role in the regulation of plant growth and development, particularly in the process known as phototropism, which is the directional growth of a plant in response to light. When light shines on one side of a plant, auxin is distributed unevenly, with higher concentrations accumulating on the shaded side. This causes the cells on that side to elongate more than those on the light-exposed side, resulting in the bending of the plant towards the light source.

This growth response allows the plant to maximize light exposure for photosynthesis, which is essential for its survival and growth. Phototropism demonstrates how auxin influences not only the growth patterns of plants but also their adaptation to environmental factors.

While stem elongation is indeed a function of auxin, it is part of the broader mechanism involved in phototropism rather than a standalone primary role. Other options like ripening and cell division involve auxin but are not primarily defined by these actions. Therefore, phototropism embodies the significant influence of auxin on how plants adapt and respond to their environment.