The Collaborative International Dictionary
Protonema \Pro`to*ne"ma\, n.; pl. Protonemata. [NL., fr. Gr. ? first + ?, ?, a thread.] (Bot.) The primary growth from the spore of a moss, usually consisting of branching confervoid filaments, on any part of which stem and leaf buds may be developed.
Wiktionary
n. (context botany English) A filament of cells that forms following the germination of the spores of mosses and liverworts
Wikipedia
A protonema (plural: protonemata) is a thread-like chain of cells that forms the earliest stage (the haploid phase) of a bryophyte or pteridophyte life cycle. When a moss first grows from the spore, it grows as a protonema which develops into a leafy gametophore.
Hornworts do not have a protonemal stage, the spores germinate directly into a new Gametophyte generation.
Moss spores germinate to form an alga-like filamentous structure called the protonema. It represents the juvenile gametophyte. 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 gametophores, 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 hornworts. The protonema is also the photosynthetic part of a germinating fern spore.
Usage examples of "protonema".
The indications of a serial progression are not so clear in the mosses, but the majority of the forms may be regarded as forming a great phylogenetic group in the evolution of which the elaboration of the moss-plant has proceeded until the protonema appears as a mere preliminary stage to the formation of the plants.
Usually the protonema is filamentous and ceases to be evident after the plants have developed.
The protonema is usually filamentous, and in some of the simplest forms is long-lived, while the small plants borne on it serve mainly to protect the sexual organs and sporogonia.
In this moss the filamentous protonema is capable of assimilation, but the leaves of the small plants are destitute of chlorophyll, so that they are dependent on the protonema.
The protonema forms a flat, lobed, thalloid structure attached to the soil by rhizoids, and the plants arise from marginal cells.
A filamentous protonema is first developed, some of the branches of which are exposed to the light and contain abundant chlorophyll, while others penetrate the substratum as brown or colourless rhizoids.
The moss-plants arise from single projecting cells, and numerous plants may spring from the protonema developed from a single spore.
Not only does a moss-plant never arise directly from the spore, but in all cases of vegetative reproduction, apart from the separation of branches by decay of older regions of the plant, a protonema is found.
Bryales the protonema or some of its branches have the form of flat plates or masses of cells.
The formation of the moss-plant on the protonema is always from a single cell and is similar in all mosses.
The social growth of the plants characteristic of many mosses is a result of the formation of numerous plants on the original protonema and on developments from the rhizoids.
Besides this, gemmae may be formed on the protonema, on the leaves or at the apex, and some mosses have specialized shoots for their better protection or distribution.
Erect branches may arise from the protonema, and gemmae may be developed on it.
Java, the high organization of the sporogonium makes it probable that the persistent protonema is an adaptation to the peculiar conditions of life.