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 Mosses are small, non-vascular flowerless plants in the taxonomic division Bryophyta (, ) ''sensu stricto''. Bryophyta ('' sensu lato'', Schimp. 1879) may also refer to the parent group bryophytes, which comprise liverworts, mosses, and h ...

. When a moss first grows from a

spore In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, ...

, it starts as a ''germ tube'', which lengthens and branches into a filamentous complex known as a ''protonema'', which develops into a leafy

gametophore Gametophores are prominent structures in seedless plants on which the reproductive organs are borne. The word gametophore and ‘-phore’ (Greek Φορά, "to be carried"). In mosses, liverworts and ferns (Archegoniata), the gametophores support ...

, the adult form of a

gametophyte A gametophyte () is one of the two alternation of generations, alternating multicellular organism, multicellular phases in the life cycles of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has on ...

bryophyte The Bryophyta s.l. are a proposed taxonomic division containing three groups of non-vascular land plants ( embryophytes): the liverworts, hornworts and mosses. Bryophyta s.s. consists of the mosses only. They are characteristically limited ...

s. Moss spores germinate to form an

alga Algae ( , ; : alga ) are any of a large and diverse group of photosynthetic, eukaryotic organisms. The name is an informal term for a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from u ...

-like filamentous structure called the protonema. It represents the juvenile

. 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

s, stems and leaf like structures. Bryophytes do not have true leaves ( megaphylls). Protonemata are characteristic of all mosses and some liverworts but are absent from hornworts. Protonemata of mosses are composed of two cell types: chloronemata, which form upon germination, and caulonemata, which later differentiate from chloronemata and on which buds are formed, which then differentiate to

s.the very best life offers on the same page+differentiate+to+caulonemata+on+which+buds+are+formed+which+then+differentiate+to+gametophores%22&hl=no&sa=X&ved=0ahUKEwjFztSuodLeAhXFkCwKHZV9Cy8Q6AEIKTAA#v=onepage&q=%22Later%20in%20development%2C%20chloronemata%20differentiate%20to%20caulonemata%20on%20which%20buds%20are%20formed%20which%20then%20differentiate%20to%20gametophores%22&f=false Photoreceptors and Light Signalling
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References

Bryophytes Plant morphology {{botany-stub