Mosses and Other Bryophytes

What factor limits the size of bryophytes?

In the cool forests of the northern United States, the moist ground is covered with green. When you walk, the ground almost feels like a soft, spongy carpet. Look closely, however, and you will see that this forest carpet is made of short, soft plants known as mosses. Mosses have a thin waxy coating that makes it possible for them to resist drying, and thin filaments known as rhizoids (RY zoydz) that anchor them to the soil. Rhizoids also absorb water and minerals from the surrounding soil. Figure 22–10 shows the structure of a typical moss.

Mosses belong to a group of plants that is known as bryophytes (BRY oh fyts). Unlike algae, the bryophytes have specialized reproductive organs enclosed by other, nonreproductive cells. The bryophytes show a higher degree of cell specialization than do the green algae and were among the very first plants to become established on land. In addition to mosses, the bryophytes include two other groups, known as hornworts and liverworts. Each of the three groups is generally considered to be a separate phylum. DOL•22

Why Bryophytes Are Small Bryophytes are generally found in damp places where there is plenty of available water, and there are good reasons for this. Most other land plants carry water in a specialized tissue called vascular tissue, which contains tubes hardened with a substance called lignin. Bryophytes, however, do not make lignin and do not contain true vascular tissue. Bryophytes are small because they lack vascular tissue. They can draw up water no higher than a meter above the ground. Without strong cell walls hardened by lignin, bryophytes also cannot support a tall plant body against the pull of gravity. These factors limit the height of bryophytes and confine them to damp environments.

Life Cycle Like all land plants, bryophytes display alternation of generations. In bryophytes, the gametophyte is the dominant, recognizable stage of the life cycle. The gametophyte is also the stage that carries out most of the plant's photosynthesis. The sporophyte is dependent on the gametophyte for its supply of water and nutrients.

Bryophytes produce sperm cells that swim using flagella. For fertilization to occur successfully, these sperm must be released where there is enough water for them to swim to an egg cell. Because of this, bryophytes live only in damp habitats where there is standing water for at least part of the year.

FIGURE 22–10 Structure of Moss In bryophytes, the gametophyte is the dominant, more familiar stage of the life cycle and is the form that carries out photosynthesis.

Table of Contents