Protonema

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

Bryophyte

Bryophytes are all embryophytes that are non-vascular: they have tissues and enclosed reproductive systems, but they lack vascular tissue that circulates liquids. They neither have flowers nor produce seeds, reproducing via spores...

 life cycle. When a moss

Moss

Mosses are small, soft plants that are typically 1–10 cm tall, though some species are much larger. They commonly grow close together in clumps or mats in damp or shady locations. They do not have flowers or seeds, and their simple leaves cover the thin wiry stems...

 or liverwort

Marchantiophyta

The Marchantiophyta are a division of bryophyte plants commonly referred to as hepatics or liverworts. Like other bryopeos, they have a gametophyte-dominant life cycle, in which cells of the plant carry only a single set of genetic information....

 first grows from the spore

Spore

In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions...

, it grows as a protonema which develops into a leafy gametophore

Gametophore

The word gametophore, also known as gametangiophore, is composed of gametangium and "phore" . In moss and fern the gametophore is the bearer of the sex organs , the female archegonia and the male antheridia. If the archegonia as well as the antheridia are arranged at the same plant, they are...

.

Moss

Moss

Mosses are small, soft plants that are typically 1–10 cm tall, though some species are much larger. They commonly grow close together in clumps or mats in damp or shady locations. They do not have flowers or seeds, and their simple leaves cover the thin wiry stems...

 spores germinate

Germination

Germination is the process in which a seed or spore emerges from a period of dormancy. The most common example of germination is the sprouting of a seedling from a seed of an angiosperm or gymnosperm. However, the growth of a sporeling from a spore, for example the growth of hyphae from fungal...

 to form an alga-like filamentous structure called the protonema. It represents the juvenile gametophyte

Gametophyte

In plants and algae that undergo alternation of generations, a gametophyte is the multicellular structure, or phase, that is haploid, containing a single set of chromosomes:...

.

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

Bryophyte

Bryophytes are all embryophytes that are non-vascular: they have tissues and enclosed reproductive systems, but they lack vascular tissue that circulates liquids. They neither have flowers nor produce seeds, reproducing via spores...

 life cycle. When a moss

Moss

Mosses are small, soft plants that are typically 1–10 cm tall, though some species are much larger. They commonly grow close together in clumps or mats in damp or shady locations. They do not have flowers or seeds, and their simple leaves cover the thin wiry stems...

 or liverwort

Marchantiophyta

The Marchantiophyta are a division of bryophyte plants commonly referred to as hepatics or liverworts. Like other bryopeos, they have a gametophyte-dominant life cycle, in which cells of the plant carry only a single set of genetic information....

 first grows from the spore

Spore

In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions...

, it grows as a protonema which develops into a leafy gametophore

Gametophore

The word gametophore, also known as gametangiophore, is composed of gametangium and "phore" . In moss and fern the gametophore is the bearer of the sex organs , the female archegonia and the male antheridia. If the archegonia as well as the antheridia are arranged at the same plant, they are...

.

Moss

Moss

Mosses are small, soft plants that are typically 1–10 cm tall, though some species are much larger. They commonly grow close together in clumps or mats in damp or shady locations. They do not have flowers or seeds, and their simple leaves cover the thin wiry stems...

 spores germinate

Germination

Germination is the process in which a seed or spore emerges from a period of dormancy. The most common example of germination is the sprouting of a seedling from a seed of an angiosperm or gymnosperm. However, the growth of a sporeling from a spore, for example the growth of hyphae from fungal...

 to form an alga-like filamentous structure called the protonema. It represents the juvenile gametophyte

Gametophyte

In plants and algae that undergo alternation of generations, a gametophyte is the multicellular structure, or phase, that is haploid, containing a single set of chromosomes:...

. While the protonema is growing by apical cell division, at some stage, under the influence of the phytohormone cytokinin

Cytokinin

Cytokinins are a class of plant growth substances that promote cell division. They are primarily involved in cell growth, differentiation, and other physiological processes. Their effects were first discovered through the use of coconut milk in the 1940s by a scientist at the University of...

, bud

Bud

thumb|right|Flower buds have not yet bloomed into a full-size flower.In botany, a bud is an undeveloped or embryonic shoot and normally occurs in the axil of a leaf or at the tip of the stem...

s are induced which grow by three-faced apical cells. These give raise to gametophore

Gametophore

The word gametophore, also known as gametangiophore, is composed of gametangium and "phore" . In moss and fern the gametophore is the bearer of the sex organs , the female archegonia and the male antheridia. If the archegonia as well as the antheridia are arranged at the same plant, they are...

s, stems and leaf like structures (bryophytes do not have true leaves (megaphyll)). These gametophores are the adult form of the
gametophyte. Protonema are characteristic of all mosses and some liverworts but are absent from hornwort

Hornwort

Hornworts are a group of bryophytes, or non-vascular plants, comprising the division Anthocerotophyta. The common name refers to the elongated horn-like structure, which is the sporophyte. The flattened, green plant body of a hornwort is the gametophyte plant.Hornworts may be found world-wide,...

s.
The protonema is also the photosynthetic part of a germinating fern

Fern

A fern is any one of a group of about 20,000 species of plants classified in the phylum or division Pteridophyta, also known as Filicophyta. The group is also referred to as Polypodiophyta, or Polypodiopsida when treated as a subdivision of tracheophyta...

spore.