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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. 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 rise to gametophores, stems and leaf like structures (bryophytes do not have true leaves (megaphyll)). These gametophores are the adult form of the gametophyte.〔Ralf Reski (1998): Development, genetics and molecular biology of mosses. Botanica Acta 111, 1-15.〕 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. ==References== 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「protonema」の詳細全文を読む スポンサード リンク
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