Characteristics of Mosses and Other Non-Vascular Plants

Non-Vascular Plant Characteristics

The major characteristic that separates non-vascular plants from others in the Kingdom Plantae is their lack of vascular tissue. Vascular tissue consists of vessels called xylem and phloem. Xylem vessels transport water and minerals throughout the plant, while phloem vessels transport sugar (product of photosynthesis) and other nutrients throughout the plant. The lack of features, such as a multi-layered epidermis or bark, means that non-vascular plants don't grow very tall and typically remain low to the ground. As such, they don't need a vascular system to transport water and nutrients. Metabolites and other nutrients are transfered between and within cells by osmosis, diffusion, and cytoplasmic streaming. Cytoplasmic streaming is the movement of cytoplasm within cells for the transport of nutrients, organelles, and other cellular materials.

Non-vascular plants are also distinguished from vascular plants (flowering plants, gymnosperms, ferns, etc.) by the lack of structures that are normally associated with vascular plants. Genuine leaves, stems, and roots are all missing in non-vascular plants. Instead, these plants have leaf-like, stem-like, and root-like structures that function similarly to leaves, stems, and roots. For example, bryophytes typically have hair-like filaments called rhizoids that, like roots, help to hold the plant in place. Bryophytes also have a lobed leaf-like body called a thallus.

Another characteristic of non-vascular plants is that they alternate between sexual and asexual phases in their life cyles. The gametophyte phase or generation is the sexual phase and the phase in which gametes are produced. Male sperm are unique in non-vascular plants in that they have two flagella to aid in movement. The gametophyte generation appears as green, leafy vegetation that remains attached to the ground or other growing surface. The sporophyte phase is the asexual phase and the phase in which spores are produced. Sporophytes commonly appear as long stalks with spore-containing caps on the end. Sporophytes protrude from and remain attached to the gametophyte. Non-vascular plants spend most of their time in the gametophyte phase and the sporophyte is completely dependent upon the gametophyte for nutrition. This is because photosynthesis takes place in the plant gametophyte.

Mosses

Mosses are the most numerous of the non-vascular plant types. Classified in the plant division Bryophyta, mosses are small, dense plants that often resemble green carpets of vegetation. Mosses are found in a variety of land biomes including the arctic tundra and tropical forests. They thrive in moist areas and can grow on rocks, trees, sand dunes, concrete, and glaciers. Mosses play an important ecological role by helping to prevent erosion, aiding in the nutrient cycle, and serving as a source of insulation.

Mosses acquire nutrients from the water and soil around them through absorption. They also have multicellular hair-like filaments called rhizoids that keep them firmly planted to their growing surface. Mosses are autotrophs and produce food by photosynthesis. Photosynthesis occurs in the green body of the plant called the thallus. Mosses also have stomata, which are important for gas exchange needed to acquire carbon dioxide for photosynthesis.

Reproduction in Mosses

The moss life cycle is characterized by the alternation of generation, which consists of a gametophyte phase and sporophyte phase. Mosses develop from the germination of haploid spores that are released from the plant sporophyte. The moss sporophyte is composed of a long stalk or stem-like structure called a seta with a capsule at the tip. The capsule contains plant spores that are released into their surrounding environment when mature. Spores are typically dispersed by wind. Should the spores settle in an area that has adequate moisture and light, they will germinate. The developing moss initially appears as thin masses of green hairs that eventually mature into the leaf-like plant body or gametophore.

The gametophore represents the mature gametophyte as it produces male and female sex organs and gametes. The male sex organs produce sperm and are called antheridia, while the female sex organs produce eggs and are called archegonia. Water is a 'must-have' for fertilization to occur. Sperm must swim to archegonia in order to fertilize the eggs. Fertilized eggs become diploid sporophytes, which develop and grow out of the archegonia. Within the capsule of the sporophyte, haploid spores are produced by meiosis. Once mature, the capsules open releasing spores and the cycle repeats again. Mosses spend the majority of their time in the dominant gametophyte phase of the life cycle.

Mosses are also capable of asexual reproduction. When conditions become harsh or the environment is unstable, asexual reproduction allows mosses to propagate faster. Asexual reproduction is accomplished in mosses by fragmentation and gemmae development. In fragmentation, a piece of the plant body breaks off and eventually develops into another plant. Reproduction through gemmae formation is another form of fragmentation. Gemmae are cells that are contained within cup-like discs (cupules) formed by plant tissue in the plant body. Gemmae are dispersed when raindrops splash into the cupules and wash gemmae away from the parent plant. Gemmae that settle in suitable areas for growth develop rhizoids and mature into new moss plants.

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By Regina Bailey

Liverworts

Liverworts are non-vascular plants that are classified in the division Marchantiophyta. Their name is derived from the lobe-like appearance of their green plant body (thallus) that looks like the lobes of a liver. There are two main types of liverworts. Leafy liverworts closely resemble mosses with leaf-like structures that protrude upward from the plant base. Thallose liverworts appear as mats of green vegetation with flat, ribbon-like structures growing close to the ground. Liverwort species are less numerous than mosses but can be found in almost every land biome. Though more commonly found in tropical habitats, some species live in aquatic environments, deserts, and tundra biomes. Liverworts populate areas with dim light and damp soil.

Like all bryophytes, liverworts do not have vascular tissue and acquire nutrients and water by absorption and diffusion. Liverworts also have rhizoids (hair-like filaments) that function similarly to roots in that they hold the plant in place. Liverworts are autotrophs that require light to make food by photosynthesis. Unlike mosses and hornworts, liverworts do not possess stomata that open and close to obtain carbon dioxide needed for photosynthesis. Instead, they have air chambers below the surface of the thallus with tiny pores to permit gas exchange. Because these pores can not open and close like stomata, liverworts are more susceptible to drying out than other bryophytes.

Reproduction in Liverworts

As do other bryophytes, liverworts exhibit alternation of generations. The gametophyte phase is the dominant phase and the sporophyte is totally reliant on the gametophyte for nutrition. The plant gametophyte is the thallus, which produces male and female sex organs. Male antheridia produce sperm and female archegonia produce eggs. In certain thallose liverworts, archegonia reside within an umbrella-shaped structure called an archegoniophore.

Water is required for sexual reproduction as sperm must swim to archegonia to fertilize the eggs. A fertilized egg develops into an embryo, which grows to form a plant sporophyte. The sporophyte consists of a capsule that houses spores and a seta (short stalk). Spore capsules attached to the ends of seta hang below the umbrella-like archegoniophore. When released from the capsule, spores are dispersed by wind to other locations. Spores that germinate develop into new liverwort plants. Liverworts can also reproduce asexually through fragmentation (plant develops from a piece of another plant) and gemmae formation. Gemmae are cells attached to plant surfaces that can detach and form new individual plants.

Hornworts

Hornworts are bryophytes of the division Anthocerotophyta. These non-vascular plants have a flattened, leaf-like body (thallus) with long, cylindrically shaped structures that look like horns protruding from the thallus. Hornworts can be found around the globe and typically thrive in tropical habitats. These small plants grow in aquatic environments, as well as in moist, shaded land habitats.

Hornworts differ from mosses and liverworts in that their plant cells have a single chloroplast per cell. Moss and liverwort cells have many chloroplasts per cell. These organelles are the sites of photosynthesis in plants and other photosynthetic organisms. Like liverworts, hornworts have unicellular rhizoids (hair-like filaments) that function to keep the plant fixed in place. Rhizoids in mosses are multicellular. Some hornworts have a blue-green color that can be attributed to colonies of cyanobacteria (photosynthetic bacteria) that live inside the plant thallus.

Reproduction in Hornworts

Hornworts alternate between a gametophyte phase and a sporophyte phase in their life cycle. The thallus is the plant gametophyte and the horn-shaped stalks are the plant sporophytes. Male and female sex organs (antheridia and archegonia) are produced deep within the gametophyte. Sperm produced in male antheridia swims through the moist environment to reach eggs in the female archegonia.

After fertilization takes place, spore containing bodies grow out of archegonia. These horn-shaped sporophytes produce spores that are released when the sporophyte splits from tip to base as it grows. The sporophyte also contain cells called pseudo-elaters that help to disperse spores. Upon spore dispersal, germinating spores develop into new hornwort plants.

Summary of Key Points

• Non-vascular plants, or bryophytes, are plants that lack a vascular tissue system. They have no flowers, leaves, roots, or stems and cycle between sexual and asexual reproductive phases.
• The primary divisions of bryophytes include Bryophyta (mosses), Hapatophyta (liverworts), and Anthocerotophyta (hornworts).
• Due to the lack of vascular tissue, non-vascular plants typically remain close to the ground and are found in moist environments. They are dependent upon water to transport sperm for fertilization.
• The green body of a bryophyte is known as the thallus, and thin filaments, called rhizoids, help to keep the plant anchored in place.
• The thallus is the plant gametophyte and produces male and female sex organs. The plant sporophyte houses spores that, when germinated, develop into new plants.
• The most abundant of the bryophytes are mosses. These small, dense mats of vegetation often grow on rocks, trees, and even glaciers.
• Liverworts resemble mosses in appearance but contain lobed, leaf-like structures. They grow in dim light and damp soil.
• Hornworts have a leaf-like body with long horn-shaped stalks that extend from the plant body.

Sources

• "Bryophytes, Hornworts, Liverworts, and Mosses - Australian Plant Information." Australian National Botanic Gardens - Botanical Web Portal, www.anbg.gov.au/bryophyte/index.html.
• Schofield, Wilfred Borden. "Bryophyte." Encyclopædia Britannica, Encyclopædia Britannica, Inc., 9 Jan. 2017, www.britannica.com/plant/bryophyte.