In similarity with other green plants, the mosses of the phylum Bryophyta are eukaryotic, utilize chlorophyll a and b, as well as xanthophylls and carotenoid pigments for photosynthesis, store starch, possess pectin-cellulose cell walls, and display open mitosis. Mosses have a sporic (diplohaplontic) life cycle that is oogamous, and may represent the link between thallophytes (flat, stemless and rootless plants such as liverworts) and cormophytes (plants with distinct leaves, stems, and roots).

View a high magnification image of moss bulbils.

The haploid gametophyte is the dominant phase of the moss life cycle and is the familiar small green plant occurring in the woods in a lush bed of green moss. Germination into the gametophyte plant is triggered by incident light on the haploid moss spore when surrounded by a moist environment. The protonema is a developing filament of haploid cells that sprouts from the spore wall and resembles a filamentous green alga. Protonema is of Latin origin meaning early worm, perhaps because of its long, thin form. This stage of the bryophyte life cycle typically lasts from a few days to many months.

From the tangle of branching filaments, bulbils (baby bulbs: clumps of cells that develop from the protonemata) form as brown structures that contrast dramatically against the verdant green beds of many mosses. Bud-like, the aerial bulbs or bulbils will develop into typical parenchymatous moss gametophytes when they reach a favorable environment. Although the parasitic moss sporophyte typically does not reproduce asexually, the host moss gametophyte does, via gemma in gemmae cups, bulbils budding from the gametophyte surfaces, or fragmentation of the branching protonema. Some moss species also exhibit small tuber-like structures on the rhizoids that allow vegetative reproduction. Interestingly, more advanced plants, such as lilies and wild onions, have also adopted the strategy of using aerial bulbils as an alternative means of asexual reproduction.

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