Nonvascular Plants Defined

Nonvascular plants belong to the division Bryophyta, which includes mosses, liverworts, and hornworts. These plants have no vascular tissue, so the plants cannot retain water or deliver it to other parts of the plant body. The bryophytes do not possess true roots, stems, or leaves, although the plant body is differentiated into leaflike and stemlike parts. In some species, there are rootlike structures called rhizoids. With no vascular tissue, the bryophytes cannot retain water for long periods of time. Consequently, water must be absorbed directly from the surrounding air or another nearby source. This explains the presence of mosses in moist areas, such as swamps and bogs, and on the shaded sides of trees.

The life cycle of the moss is typical of the bryophytes. Flask-shaped archegonia, located among the top leaves of the female gametophytes, produce one egg cell each. Antheridia, located similarly on the male gametophyte, produce many sperm cells that swim in drops of rainwater or dew into the neck of the archegonium to fertilize the egg cell.

The zygote that results from the fertilization develops into a young sporophyte within the archegonium. The sporophyte grows out of the archegonium, taking its nourishment from the gametophyte, and differentiates into a slender stalk with a spore capsule near the tip. Haploid spores are produced by meiosis in this capsule, and when the tip of the capsule opens, the spores are freed. The spores settle in the soil and germinate into gametophytes, which represent the next stage in the alternation of generations.

The life cycles of all bryophytes are uniform, and although the gametophyte generation is the most familiar aspect of the life cycle, neither the sporophyte nor the gametophyte generation is dominant.