Which chemical is known to cause plant cell elongation towards or away from a stimulus?

Auxin is a plant hormone that plays a crucial role in the regulation of cell elongation, particularly in response to directional stimuli such as light and gravity. This process is known as differential growth. When auxin is unevenly distributed in plant tissues, it causes the cells on one side of the plant to elongate more than those on the opposite side. For example, in phototropism, when a plant is exposed to light from one direction, auxin accumulates on the shaded side, leading those cells to grow longer and causing the plant to bend toward the light. Similarly, in gravitropism, auxin moves to the lower side of the plant root or shoot, resulting in downward growth of roots and upward growth of shoots. This ability of auxin to promote cell elongation in response to stimuli is essential for various growth patterns in plants.

The other options refer to different biological processes; for instance, circadian rhythms involve the regulation of physiological cycles based on an organism's internal clock, while endogenous rhythms are related to internal timing mechanisms that influence biological processes throughout the day. Vernalisation is the process by which flowering in some plants is promoted by exposure to prolonged cold, but it does not directly relate to the concept of cell elong