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.[citation needed]
Non-vascular plants include two distantly related groups:
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.[citation needed]
Non-vascular plants do not have a wide variety of specialized tissue types.[3] 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 cuticleorstomata, 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.[4]
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.[5]
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.[6]