Protonema

A protonema (plural: protonemata) is a thread-like chain of cells that forms the earliest stage (the haploid phase) of a bryophyte life cycle. When a moss first grows from the spore, it grows as a protonema which develops into a leafy gametophore.

Hornworts do not have a protonemal stage, the spores germinate directly into a new Gametophyte generation.