Of the many kinds of plants in the world, they can be classified as either vascular or nonvascular. Nonvascular plants are the most similar to the earliest land plants.
Definition of Nonvascular Plants
Nonvascular plants do not have the specialized structure know as xylem, which is found in vascular plants. Xylem aids in the movement of water and nutrients throughout a plant.
Nonvascular plants have existed for millions of years, and they can be aquatic or land plants. Nonvascular land plants, called bryophytes, likely diverged from aquatic plants such as algae about 450 million years ago.
The nonvascular characteristic is similar to that of distant green algae ancestors. Since nonvascular plants lack circulatory systems or tracheids, nutrients and water must move between cells.
Bryophytes include algae, mosses (the phylum Bryophyta), liverworts (the phylum Marchantiophyta) and hornworts (the phylum Anthocerotophyta).
Liverworts represent the first bryophytes, dating as far back as the Ordovician Period. The fossil record is limited due to the fact that bryophytes do not contain lignin.
Over 25,000 species of bryophytes exist.
Characteristics of Nonvascular Plants
Bryophytes must live in moist environments because they do not have vascular systems. This way they can directly absorb nutrients into cells.
Bryophytes do not have traditional kinds of leaves, stems and true roots like the more evolved land plants. Because of this, bryophytes tend to be low-growing. Individual shoots are densely packed into cushions, tufts or mats. They spread across their substrate of ground, trees or rocks as mats and mounds.
Two broad types of nonvascular plants are the leafy shoots with flattened organs such as mosses and leafy liverworts, and the thalloid plants, such as hornworts (and some types of liverworts).
Nonvascular plant features include leaf-like structures that are photosynthetic, stems, the thallus and rhizoids to anchor to the available substrate. The thicker the shoots, the better water retention they have.
Nonvascular plants alternate their generations for reproduction. Their haploid gametophyte generation (sexual reproduction form) is long, while their sporophyte generation (asexual reproduction form) is brief. Water is required for their sperm to fertilize gametes.
The main form of nonvascular plants is that of the gametophyte, with a less prominent sporophyte. The sporophyte relies on the gametophyte form for its water and nutrition.
Nonvascular plants do not reproduce in the same manner as vascular plants. Instead of using seeds, flowers or fruit, bryophytes grow from spores. These spores germinate and become gametophytes. Gametes of nonvascular plants use flagella and require a wet environment.
The resulting zygote stays attached to the main plant and makes a sporophyte to release spores. Spores then yield new gametophytes. Most bryophytes possess a sporangium, although algae do not. The sporangium houses spores produced by the plant.
Cytoplasmic streaming: Nonvascular plants use cytoplasmic streaming to move nutrients within conducting cells.
Advantages of Nonvascular Plants
Nonvascular plants have provided and continue to provide numerous benefits. Nonvascular plants helped make the oxygen in the Earth’s atmosphere, allowing the advancement of other plants and animals.
Nonvascular plants also provide microhabitats for many species of animals. Worms and insects that benefit soil quality reside among bryophytes. Other animals can obtain prey and even nesting material from bryophytes.
Nonvascular plants work to break down rocky terrain into beneficial soil for other plants. Bryophyte mats also work as nature’s little purifying and stabilizing powerhouses. They absorb runoff, and they filter groundwater.
Bryophytes also posses antimicrobial and antifungal qualities.
Bryophytes react quickly to environmental changes, making them valuable indicators for air and water quality. While most of them prefer moist environments, some species evolved in deserts. They can live in harsh environments such as tundra.
Bryophytes can withstand drying out or desiccation, giving them an advantage over vascular plants. In fact, one type of desert moss, Syntrichia caninervis, can rehydrate in a matter of seconds by changing its surface area.
Nonvascular plants serve as excellent models for evolutionary and ecological studies. They provide great models for intraspecific and interspecific variation.
Examples of Nonvascular Plants
The three major types of nonvascular land plants include the previously mentioned liverworts, hornworts and mosses.
Liverworts (Marchantiophyta) have spread across most of the land in the world. Over 7,000 species of liverworts exist. Liverworts are distinguished by their leaflets, which look like liver lobes, hence their name. Sporophytes in liverworts are short and small plants. The sporophytes of liverworts do not contain stomata.
Liverworts release haploid spores form their sporangia. These travel via wind or water, germinate and then attach to substrate. Liverworts can be thalloid, growing in thalloid mats, or leafy, with leaf-like photosynthetic structures.
Hornworts (Anthocerotophyta) make up about 160 species in the pantheon of nonvascular plants. Hornworts grow longer sporophytes (spore producers) that resemble pipes. These hornlike sporophytes rupture to spread their spores.
In contrast to liverworts, hornworts possesses stomata. They tend to stay close to moisture sources. Their gametophytes are blue-green in color and grow as a flat thallus.
Their sperm travel to the archegonia to fertilize eggs. After the zygote grows into the long sporophyte, it splits and propels spores into the environment via structures called pseudo-elaters.
Both liverworts and hornworts can also fragment their leaves and branches to reproduce asexually. Such fragments are called gemmae. Raindrops can carry them, and when they land they grow into gametophytes.
Mosses (Bryophyta) make up over 10,000 species of nonvascular plants, and therefore they are the most diverse.
Mosses possess short, flat green leaves; root-like structures; and in some varieties, even branches. The stomata or openings on moss stems allow them to adapt to dry environments.
The rhizoids of mosses arise from the base of their gametophytes. Rhizoids work in a similar manner to roots, allowing the plant to anchor to a substrate. This is especially helpful in areas such as tundra, where frozen soil makes it difficult for other kinds of plants to take root.
Mosses live in tundra, in rainforests and in vastly different locations. They serve as storage for both moisture and sill nutrients. They make food and shelter for animals. Moss makes new habitats for other organisms, especially after disturbances to the environment.
Their stemlike setae have cells for transferring nutrients from the sporophyte to their sporangium. The peristome is a structure in moss that helps release spores under the right moisture conditions.
Moss cushions can be either hemispherical or flattened. The size of the cushions helps determine the hydration of the plant. Mosses also follow the alternation of generations. In addition to their environmental importance, mosses provide excellent landscaping plants for moist areas.
Scientists have recently found evidence that mosses and hornworts may be more closely related to vascular plants than to liverworts.
As ecologists learn more about nonvascular plants, it becomes clear how important they are to ecosystems around the world. Nonvascular plants provide interesting case studies in the status of the environment. Their unique life cycles and long history prove how enduring these plants remain to this day.