Nonvascular Plants

Forest floors, streambeds, and cracks in the sidewalk: what commonality do these places share? Often, you will see mosses growing, barely an inch tall, out of these different environments, covering the earth in a shade of green. Mosses, liverworts, and hornworts are all examples of the group of land plants known as nonvascular plants (Fig. 1), which lack vascular systems but have other adaptations for living on land. Keep reading to learn more about Nonvascular Plants, their life cycle, characteristics, and more.

What Is a Nonvascular Plant?

Land (vascular and nonvascular) plants share one common feature, which plant biologists believed was essential for surviving on land. That characteristic is that their embryos contain tissues from the mother plant to prevent desiccation (drying out). There isn't an abundance of water on land to protect growing plants from drying out, meaning the development of this tissue was essential for survival on land. Both nonvascular plants and vascular plants share this trait.

Figure 1. Photograph of a liverwort.

The Definition of Nonvascular Plants

The nonvascular plants are the mosses, liverworts, and hornworts. They also have other characteristics which have helped them survive on land.

Characteristics of Nonvascular Plants

Despite not having a vascular system, nonvascular plants still have many important features that have allowed them to succeed on land. Life on land means nonvascular plants need mechanisms to prevent drying out, find water and nutrients and have structures that provide support.

Preventing Desiccation

To prevent desiccation or drying out, plants have evolved stomata. Stomata are special openings that allow gas exchange between the plant and its environment. They are controlled by guard cells that will open to let CO₂ in for photosynthesis and expel oxygen as a byproduct of photosynthesis. The mosses and hornworts have stomata, but the liverworts do not.

Another way to prevent drying out is by having a cuticle or a waxy covering. Some species of nonvascular plants have thin cuticles to help prevent drying out, while others do not. Still, their cuticles are quite thin, meaning it is important for nonvascular plants to live in moist environments.

Finding Water and Nutrients

Although nonvascular plants lack true leaves, stems, and roots, they still have a dominant haploid gametophyte that can photosynthesize. These gametophytes are often leaf-like. The diploid sporophyte relies on the gametophyte for nutrients and is not free-living (Fig. 2).

Figure 2. Moss gametophyte (haploid) and sporophyte (diploid) generations.

Nonvascular plants have root-like structures called rhizoids that help with water absorption. Water can be absorbed through the leaf-like and stem-like structures in the plant, and nonvascular plants often live in water-rich environments. They rely on osmosis to absorb water through their cells to transport water from cell to cell. Osmosis is the movement of water from areas of low solute concentrations to areas of higher solute concentrations.

Providing Support

The root-like structures, rhizoids, absorb water and provide support for the plant. The cell walls of these plants contain cellulose, which helps provide structure outside of a watery, buoyant environment.

The Life Cycle of Nonvascular Plants

One quality of all land plants is the life cycle mechanism known as the alternation of generations. In the alternation of a generation's life cycle, a plant goes through both a multicellular diploid stage (sporophyte) and a multicellular haploid stage (gametophyte). The life cycle of the nonvascular plants follows this basic structure. More detail is provided for each step below (Fig. 3).

Figure 3. This diagram shows the life cycle of moss.

1. The diploid multicellular plant, called a sporophyte, will produce spores via meiosis in a sporangium structure. These spores are haploid and unicellular.

2. The spores will be released and grow into a haploid plant called a gametophyte. The gametophyte is multicellular and has chloroplasts for photosynthesis.

3. The gametophytes of nonvascular plants have sex organs called gametangia. On the gametangia are archegonium- the female sex organs that make eggs- and antheridium- the male sex organs that make sperm. Eggs and sperm are made via mitosis.

4. When the sperm are released, they must swim to the egg. Nonvascular plants rely on water for fertilization.

5. Once the sperm reaches the egg, the diploid sporophyte plant will grow out of the archegonium. This diploid plant relies on the gametophyte for nutrients and is NOT independent.

6. The cycle restarts when the sporophytes produce spores via meiosis and releases them to grow into new gametophytes.

Reproduction of Nonvascular Plants

As discussed above, in the life cycle of a moss, the sexual reproduction of nonvascular plants has a few key features. First, the production of gametes happens in the gametophyte, meaning that they are produced via meiosis, NOT mitosis. The male sex organ is the antheridium and produces sperm or male gametes. The archegonium is the female sex organ that contains the egg, or the female gamete.

That sperm must swim to the egg and thus requires water or a damp environment to complete fertilization. This requirement is important because it limits the environments that nonvascular plants can live in.

The fertilized egg will grow into a sporophyte that makes haploid spores via meiosis. The production of the gametophyte from spores is a method of asexual reproduction. That means that the alternation of a generation's life cycle of nonvascular plants includes sexual and asexual reproduction at the different stages.

Some nonvascular plants may also reproduce asexually in the gametophyte stage. Gametophytes produce " gemma " buds that can fall off and become new plants.

Nonvascular vs Vascular Plants

Let's take a look at some of the similarities and differences between nonvascular and vascular plants.

Similarities

Nonvascular and vascular plants, although sharing many differences, also share many commonalities. Both nonvascular and vascular plants have:

• Embryo protection: a way to protect their embryo from drying out using maternal tissue.

• Alternations of generations: a diploid and haploid stage in their life cycle.

• Stomata: Besides the liverworts, both nonvascular and vascular plants have these openings to allow for gas exchange.

• Relationships with fungi: Mutualistic relationships with fungi have allowed both groups of plants to access nutrients more readily.

• Cuticles: Although more developed in vascular plants, cuticles protect from drying out.

Differences

See the table below comparing nonvascular and vascular plant differences (Table 1).

Table 1: Differences between nonvascular and vascular land plants.

The Importance of Nonvascular Plants

Although nonvascular plants like mosses, liverworts, and hornworts are small and do not have flowers, seeds, or other flashy appendages, they play an important role in our environment.

Have you ever seen a riverbank covered in mosses? Those mosses are more than just slippery, pillowy residents on that riverbank. They help hold the riverbank together and prevent erosion. Mosses, liverworts, and hornworts, growing in dense mats, also do this in other places like the arctic tundra or tropical rainforests.

They are also often called "pioneer species" because they will be some of the first plants to colonize an area after a natural disaster or other extinction events. They help make the soil more hospitable for other plant and animal life.