The tropism that describes a stem growing upward is phototropism.

Here's why:

* Phototropism is the growth of a plant in response to a light stimulus. Stems grow towards light sources, maximizing their exposure for photosynthesis.

* Geotropism is the growth of a plant in response to gravity. While stems do exhibit negative geotropism (growing away from gravity), this is a secondary factor to phototropism in determining their upward growth.

So, while geotropism plays a role, the primary reason a stem grows upward is due to phototropism.