Which part of most terrestrial plants exhibits positive phototropism?

The correct answer is that the stem of most terrestrial plants exhibits positive phototropism. Positive phototropism is the growth of a plant part towards light, which is vital for maximizing photosynthesis. The stem is responsible for transporting nutrients and supporting the plant structure, while also playing a key role in directing leaves towards light sources.

In response to light exposure, hormones such as auxins accumulate on the shaded side of the stem, stimulating those cells to elongate more than the cells on the light-exposed side. This differential growth causes the plant to bend toward the light source, ensuring that leaves are positioned optimally for photosynthesis.

Other parts of the plant, such as the roots and flowers, do not typically exhibit this kind of growth response to light. Instead, roots generally exhibit positive gravitropism, growing downwards in response to gravity rather than light. The vascular bundles are structures within the stem that transport water, nutrients, and food, and they do not exhibit growth responses to light. Flowers may orient themselves to attract pollinators but do not exhibit phototropism in the dynamic way that stems do. Therefore, the stem is indeed the part of plants that exhibits positive phototropism most prominently.