原丝体是一个如线一般的细胞串，是苔藓植物(Bryophytes)生命周期的第一个阶段，也就是单倍体(gametophyte)(配子体)阶段。当一个苔类(Liverworts)或藓类(Mosses)开始从孢子生长出来之后，会成长成类似藻类的「原丝体」，其后会发展成叶状的配子体。原丝体是所有苔类和一些藓类的特征，但并不出现在角藓身上。
此外，原丝体亦是发芽的蕨类孢子进行光合作用部份。

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.