A protonema (plural: protonemata) is a thread-like chain of cells that forms the earliest stage of development of the gametophyte (the

haploid Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...

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 horn ...

. 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, f ...

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

s. Moss spores germinate to form an

alga Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mi ...

-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 Plant hormone (or phytohormones) are signal molecules, produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, from embryogenesis, the regulation of organ size, pat ...

cytokinin Cytokinins (CK) are a class of plant hormones that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and le ...

bud In botany, a bud is an undeveloped or embryonic shoot and normally occurs in the axil of a leaf or at the tip of a stem. Once formed, a bud may remain for some time in a dormant condition, or it may form a shoot immediately. Buds may be spec ...

s 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 (

megaphylls A leaf (plural, : leaves) is any of the principal appendages of a vascular plant plant stem, stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", wh ...

). 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 gametophores.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