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How is the protonema stage different in mosses and liverworts?
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Bryophyte
Bryophyte moss growing on oak trees.
In bryophytes, the protonema represents a developmental stage that grows directly from a germinating spore before the formation of the gametophore. The structure of this stage differs significantly between mosses and liverworts:
- Mosses: The protonema typically exists as an extensive, highly branched system of multicellular, chlorophyll-rich filaments. This system can proliferate under favorable conditions to increase the population of leafy shoots, which arise as buds from the protonema branches.123
- Liverworts and Hornworts: The protonema is generally reduced, often limited to a short, unbranched filament. This structure rapidly initiates a three-dimensional cell mass known as a sporeling, which is rich in chlorophyll and soon develops an apical cell to give rise to the gametophore.12
While the moss protonema functions as a persistent, spreading network, the liverwort protonema is more transient and quickly transitions into the sporeling stage.12