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 or liverwort first grows from the spore, it grows as a protonema which develops into a leafy gametophore.

Moss spores germinate to form an alga-like filamentous structure called the protonema. It represents the juvenile gametophyte. While the protonema is growing by apical cell division, at some stage, under the influence of the phytohormone cytokinin, buds are induced which grow by three-faced apical cells. These give raise to leafy gametophores, the adult form of the gametophyte ^[1]. Protonema are characteristic of all mosses and some liverworts but are absent from hornworts. The protonema is also the photosynthetic part of a germinating fern spore.

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cs:Prvoklí?ek de:Protonema fr:Protonema pt:Protonema sv:Protonema zh:???

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