What are Bryophytes? – Definition, Characteristics and Examples
Bryophytes are a group of non-vascular plants that include mosses, liverworts and hornworts. They are among the oldest and simplest land plants, having no true roots, stems or leaves. They rely on rhizoids, which are hair-like structures, to anchor them to the substrate and absorb water and minerals. Bryophytes have a life cycle that alternates between a haploid gametophyte stage and a diploid sporophyte stage. The gametophyte is the dominant and photosynthetic phase, while the sporophyte is dependent on the gametophyte for nutrition. Bryophytes reproduce sexually by producing sperm and eggs in specialized organs called antheridia and archegonia, respectively. They also reproduce asexually by fragmentation or by producing gemmae, which are small pieces of tissue that can grow into new plants.
Bryophytes are widely distributed in various habitats, ranging from cold arctic regions to hot deserts. They can grow on rocks, soil, tree bark, logs, roofs and walls. They play important roles in the ecosystem, such as preventing soil erosion, retaining moisture, providing shelter and food for animals, and cycling nutrients. Bryophytes are also useful for humans, as they have been used for medicinal purposes, peat production, soil improvement and decoration.
There are about 20,000 species of bryophytes in the world, classified into three divisions: Bryophyta (mosses), Marchantiophyta (liverworts) and Anthocerotophyta (hornworts). Each division has distinctive features and characteristics that will be discussed below.
Bryophyta – Mosses
Mosses are the most diverse and abundant group of bryophytes, with about 12,000 species. They have a leafy appearance, with small green leaves arranged spirally around a thin stem. The leaves are usually one cell thick, except at the midrib, and have no veins or stomata. The stem may have a central strand of water-conducting cells called hydroids, but it is not true vascular tissue. Mosses have rhizoids that attach them to the substrate, but they do not absorb nutrients from it.
Mosses can be found in moist and shady places, such as forests, bogs, swamps and streams. Some mosses can also tolerate dry and exposed conditions, such as rocks, deserts and rooftops. Mosses can absorb water and nutrients directly from the air or rainwater through their leaves. They can also store water in their cells and tissues to survive droughts.
Mosses have a gametophyte-dominant life cycle. The gametophyte consists of two forms: a protonema and a gametophore. The protonema is a filamentous or platelike structure that develops from a spore. It produces buds that grow into gametophores. The gametophore is the leafy part of the moss that bears the sex organs. Mosses can be either monoicous (having both male and female organs on the same plant) or dioicous (having separate male and female plants). The male organ is called an antheridium and produces sperm cells that swim in water to reach the female organ called an archegonium. The archegonium produces an egg cell that is fertilized by a sperm cell to form a zygote.
The zygote develops into a sporophyte that grows out of the archegonium. The sporophyte consists of a foot, a seta and a capsule. The foot anchors the sporophyte to the gametophyte and absorbs nutrients from it. The seta is a stalk that elevates the capsule above the gametophyte. The capsule is a spore-producing organ that has a lid called an operculum and a ring of teeth called a peristome. The capsule produces spores by meiosis that are released when the operculum falls off and the peristome opens due to dryness. The spores germinate into new protonemata, completing the life cycle.