Which plant growth process allows for the upward growth of stems?

The process that allows for the upward growth of stems is phototropism. Phototropism refers to the growth response of a plant to light, whereby stems and leaves grow towards the light source. This behavior is driven by the differential distribution of plant hormones called auxins, which promote cell elongation on the side of the plant that is furthest from the light. As the cells on the shaded side elongate more than those on the light-exposed side, the stem bends towards the light and grows upward.

In contrast, gravitropism (often associated with roots, demonstrating positive gravitropism by growing downward) refers to how plants respond to gravity. Transpiration is the process of water movement through a plant and its evaporation from aerial parts, primarily leaves, and does not directly influence stem growth direction. Hydrotropism involves a plant's growth response to moisture gradients in the soil, generally affecting root growth rather than stem elevation.