Final answer:
A protonema is a mass of green, branched, one-cell-thick filaments that is produced by germinating moss spores. It is the haploid stage of a moss and serves as the initial growth stage for the moss.
Step-by-step explanation:
A protonema is a mass of green, branched, one-cell-thick filaments that is produced by germinating moss spores. It is the haploid stage of a moss and forms from the haploid spore. The protonema serves as the initial growth stage for the moss and eventually gives rise to the gametophyte, which is the haploid form of moss.