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Non-vascular plants are plants without a vascular system consisting of xylem and phloem. Instead, they may possess simpler tissues that have specialized functions for the internal transport of water.

Non-vascular plants include two distantly related groups:

• Bryophytes, an informal group that taxonomists now treat as three separate land-plant divisions, namely: Bryophyta (mosses), Marchantiophyta (liverworts), and Anthocerotophyta (hornworts). In all bryophytes, the primary plants are the haploid gametophytes, with the only diploid portion being the attached sporophyte, consisting of a stalk and sporangium. Because these plants lack lignified water-conducting tissues, they cannot become as tall as most vascular plants.
• Algae, especially green algae. The algae consist of several unrelated groups. Only the groups included in the Viridiplantae are still considered relatives of land plants.

These groups are sometimes called "lower plants", referring to their status as the earliest plant groups to evolve, but the usage is imprecise since both groups are polyphyletic and may be used to include vascular cryptogams, such as the ferns and fern allies that reproduce using spores. Non-vascular plants are often among the first species to move into new and inhospitable territories, along with prokaryotes and protists, and thus function as pioneer species.

Mosses and leafy liverworts have structures called phyllids that resemble leaves, but only consist of single sheets of cells with no internal air spaces, no cuticle or stomata, and no xylem or phloem. Consequently, phyllids are unable to control the rate of water loss from their tissues and are said to be poikilohydric. Some liverworts, such as Marchantia, have a cuticle, and the sporophytes of mosses have both cuticles and stomata, which were important in the evolution of land plants.

All land plants have a life cycle with an alternation of generations between a diploid sporophyte and a haploid gametophyte, but in all non-vascular land plants, the gametophyte generation is dominant. In these plants, the sporophytes grow from and are dependent on gametophytes for supply of water and mineral nutrients and photosynthate, the products of photosynthesis.

Non-vascular plants play crucial roles in their environments. They often dominate certain biomes such as mires, bogs and lichen tundra where these plants perform primary ecosystem functions. Additionally, in bogs mosses host microbial communities which help support the functioning of peatlands. This provides essential goods and services to humans such as global carbon sinks, water purification systems, fresh water reserves as well as biodiversity and peat resources. This is achieved through nutrient acquisition from dominant plants under nutrient-stressed conditions.

Non-vascular plants can also play important roles in other biomes such as deserts, tundra and alpine regions. They have been shown to contribute to soil stabilization, nitrogen fixation, carbon assimilation etc. These are all crucial components in an ecosystem in which non-vascular plants play a pivotal role.

References

1. Copeland, H.F. (1956). The classification of lower organisms. Palo Alto: Pacific Books.
2. Adl, S.M.; et al. (2005). "The new higher level classification of eukaryotes with emphasis on the taxonomy of protists". Journal of Eukaryotic Microbiology. 52 (5): 399–451. doi:10.1111/j.1550-7408.2005.00053.x. PMID 16248873. S2CID 8060916.
3. Glime (April 19, 2015). "WATER RELATIONS: PLANT STRATEGIES" (PDF). Bryophyte Ecology. Archived (PDF) from the original on March 28, 2016. Retrieved December 8, 2016.
4. Bragina, Anastasia; Oberauner-Wappis, Lisa; Zachow, Christin; Halwachs, Bettina; Thallinger, Gerhard G.; Müller, Henry; Berg, Gabriele (2014). "The Sphagnum microbiome supports bog ecosystem functioning under extreme conditions". Molecular Ecology. 23 (18): 4498–4510. Bibcode:2014MolEc..23.4498B. doi:10.1111/mec.12885. PMID 25113243.
5. St. Martin, Philippe; Mallik, Azim U. (2017). "The status of non-vascular plants in trait-based ecosystem function studies". Perspectives in Plant Ecology, Evolution and Systematics. 27: 1–8. Bibcode:2017PPEES..27....1S. doi:10.1016/j.ppees.2017.04.002.

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