CONCEPTS IN BIOLOGY
PART V. THE ORIGIN AND CLASSIFICATION OF LIFE
22. The Plants Kingdom
22.4. Nonvascular Plants
The nonvascular plants, which include the mosses, hornworts, and liverworts, are commonly known as bryophytes. These three kinds of plants share the following characteristics.
1. They lack vascular tissue.
2. They do not have true roots or leaves.
3. The gametophyte generation is the most prominent part of the life cycle.
4. The sperm swim to the egg.
Because they lack vascular tissue and roots for the absorption and transportation of water, they must rely on the physical processes of diffusion and osmosis to move dissolved materials through their bodies. They have sperm that swim to the egg, so they are similar to their algal ancestors and must have water to reproduce sexually. Thus, most of these organisms are small and live in moist habitats. In other words, they are only minimally adapted to a terrestrial environment.
The Moss Life Cycle
Although there are differences in details of the life cycle of nonvascular plants, we will use the moss life cycle to represent the group. The moss plant that you commonly recognize is the gametophyte generation.
Recall that the gametophyte generation is haploid and is the gamete-producing stage in the plant life cycle. Although the cells of the gametophyte have the haploid number of chromosomes (the same as gametes), not all of them function as gametes. At the top of the moss gametophyte are two kinds of structures that produce gametes: the antheridium and the archegonium (figure 22.4).
FIGURE 22.4. The Life Cycle of a Moss
The haploid gametophyte generation produces eggs in a structure called an archegonium and sperm in a structure called an antheridium. Sperm swim to the egg and fertilization occurs within the archegonium. The zygote is diploid and is the first stage of the sporophyte generation. The sporophyte grows and protrudes from the top of the gametophyte. Meiosis takes place in the capsule of the sporophyte, producing haploid spores. The spores are released and give rise to new gametophyte plants.
The antheridium is a sac-like structure that consists of the developing sperm surrounded by a layer of cells called jacket cells. The archegonium is a flask-shaped structure that produces the egg; it has a tubular channel leading to the egg at its base. There is usually only one egg cell in each archegonium. When the sperm are mature, the outer jacket of the antheridium splits open, releasing the flagellated sperm. The sperm swim through a film of dew or rainwater to the archegonium and continue down the channel of the archegonium to fertilize the egg. When the sperm and egg nuclei fuse, a diploid zygote is produced. The zygote is the first cell of the sporophyte generation. The zygote divides by mitosis and the sporophyte generation begins to grow within the archegonium and eventually develops into a mature sporophyte, which grows out of the top of the gametophyte.
The sporophyte consists of a long stalk with a capsule on the end of it. Within the capsule, meiosis takes place, producing haploid spores. These tiny spores are released and distributed by wind and water. When they germinate, they give rise to a multicellular filament known as a protonema. The protonema develops into a mature gametophyte plant. Thus, in mosses and other nonvascular plants, the gametophyte generation is the dominant stage and the sporophyte is dependent on the gametophyte.
Kinds of Nonvascular Plants
The mosses are the most common nonvascular plants. Mosses grow as a carpet of many individual gametophyte plants. Each moss plant is composed of a central stalk less than 5 centimeters tall, with short, leaflike structures that are the sites of photosynthesis. There are over 15,000 species of mosses, and they are found anywhere there is adequate moisture.
The gametophytes of liverworts and hornworts are flat sheets only a few layers of cells thick. The name liverwort comes from the fact that these plants resemble the moist surface of a liver. There are about 8,000 species of liverworts. Hornworts derive their name from the presence of a long, slender sporophyte, which protrudes from the flat gametophyte plants. Their cells are unusual among plants, because they contain only one, large chloroplast in each cell, whereas other plants have many chloroplasts per cell. There are about 100 species of hornworts found throughout the world. Figure 22.5 shows examples of nonvascular plants.
FIGURE 22.5. Nonvascular Plants
There are three kinds of nonvascular plants: mosses, liverworts, and hornworts. Because they lack vascular tissue, they are generally small and are usually found in moist environments.
22.4. CONCEPT REVIEW
7. What sex cells are associated with antheridia? With archegonia?
8. List three characteristics shared by nonvascular plants.
9. What are the three major kinds of nonvascular plants?