Protonema

A protonema (plural: protonemata) is a thread-like chain of cells that forms the earliest stage of development of the gametophyte (the haploid phase) in the life cycle of mosses.

[1] Protonemata are characteristic of all mosses, are present in some liverworts under certain conditions [2] but are absent from hornworts.

In comparison, caulonema cells are longer, have less chloroplasts and have cross walls that are situated at an oblique angle to the growth axis.

This is distinct from the three-faced apical growth of the mature gametophyte, which similarly divides at the tip, but forms three daughter cells.

[3] A bud typically forms on caulonema cells triggered by the plant hormone cytokinin (particularly 6-(Δ2isopentenyl)adenine, the native cytokinin of mosses), but they can also form on chloronema if the hormone is present at lower concentrations.