from The American Heritage® Dictionary of the English Language, 4th Edition
- n. The green filamentous growth that arises from spore germination in liverworts and mosses and eventually gives rise to a mature gametophyte.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A filament of cells that forms following the germination of the spores of mosses and liverworts
from the GNU version of the Collaborative International Dictionary of English
- n. 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.
from The Century Dictionary and Cyclopedia
- n. In Muscineæ, a pluricellular, confervoid or filamentous, usually chlorophyllose, structure upou which the leafy plant which bears the sexual organs arises as a lateral or terminal shoot. Also protoneme.
The spores are set free by the bursting of their chamber, and each germinates, putting out a branched thread of cells called a protonema, which may perhaps properly be termed a third generation in the cycle of the plant; for it is only from buds developed on this protonema that the leafy sexual plant arises.
The protonema is not persistent, and the plants are well developed, resembling those of _Pleuridium_.
The moss-plants arise from single projecting cells, and numerous plants may spring from the protonema developed from a single spore.
The spore on germination forms a short filament which soon broadens out into the thalloid protonema.
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.
In addition to the leaves the stem often bears hair-like structures of different kinds, some of which correspond to modified branches of protonema.
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 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.
The protonema forms a flat, lobed, thalloid structure attached to the soil by rhizoids, and the plants arise from marginal cells.
The sexual generation is always a leafy plant, which is not developed directly from the spore but is borne on a well-marked and usually filamentous protonema.
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