Final answer:
The moss protonema is a tangle of single-celled filaments that hug the ground. It is green, photosynthetic, and can live for several years.
Step-by-step explanation:
The moss protonema is the filamentous growth of a moss that is visible under a microscope. It typically appears as a tangle of single-celled filaments that hug the ground. These filaments are often akin to an apical meristem and actively divide, giving rise to a gametophore with a photosynthetic stem and foliage-like structures. The moss protonema can be green, photosynthetic, and can live for several years.