Protonema (pl. protonemata) The young bryophyte gametophyte that develops following spore germination.
苔藓植物中直接由孢子萌发形成的结构称为原丝体。
英语百科
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.