Introduction To The Bryophytes

In the midst of heavy battles in France during World War I, nurses, at what is now referred to as a M.A.S.H. unit, on one occasion ran out of bandages for the wounded soldiers. In desperation, they substituted some soft green plant material they found growing in the water at the edge of a nearby lake. To their surprise, the material turned out to be a great substitute for the bandages; there were fewer infections in the wounds with the plant bandages than in those with the cotton bandages.

The material the nurses used was a species of Sphagnum moss (bog or peat moss), which has since been experimentally demonstrated to have antiseptic properties. This moss has specialized water-absorbing "leaves" (see Fig. 20.10) and has been used as a packing material in the past. It is still widely used as a soil conditioner. The "bandage" Sphagnum is one of about 23,000 species of bryophytes that include mosses, liverworts, and hornworts, many of which frequently make a soft and cool-looking green covering on damp banks, trees, and logs that are shaded for at least a part of the day (Fig. 20.1).

Introduction to the Plant Kingd om: Bryophytes 383

Introduction to the Plant Kingd om: Bryophytes 383

Plants For Slopes And Sun

In contrast, some bryophytes can withstand long periods of desiccation and are also found on bare rocks in the scorching sun (Fig. 20.2), while others occur on frozen alpine slopes. The habitats of bryophytes range in elevation from sea level near ocean beaches up to 5,500 or more meters (18,000 or more feet) in mountains.

Some bryophytes are restricted to very specific habitats. For example, a few species are found only on the antlers and bones of dead reindeer. Others are confined to the dung of herbivorous animals, while still others grow only on the dung of carnivores. A few tropical bryophytes thrive only on large insect wing covers. The pygmy mosses, which appear annually on bare soil after rains, are only 1 to 2 millimeters (0.04 to 0.08 inch) tall and can complete their whole life cycle in a few weeks.

Bryophytes of all phyla often have mycorrhizal fungi associated with their rhizoids. In some instances, the fungi apparently are at least partially parasitic. One species of completely colorless liverwort that lives underneath mosses is totally dependent nutritionally on its fungal associate. The gametophytes of more advanced plants are completely dependent on their sporophytes for their nutrition.

The widespread peat mosses are ecologically very important in bogs and in the transformation of bogs to dry land. Peat mosses sometimes form floating mats over water and keep conditions acid enough to inhibit the growth of bacteria and fungi. Organisms that die in such waters or bogs are often preserved for hundreds or even thousands of years.

The luminous mosses are found in caves near the entrances and in other dark, damp places. They are called luminous because they glow an eerie golden-green in reflected light. The upper surfaces of their cells are slightly curved, and each cell functions as a tiny magnifying glass, concentrating the dim light on the chloroplasts at the base.

384 Chapter 20

Luminous Plants Moss
Figure 20.2 Grimmia, a rock moss that survives on bare rocks, often in scorching sun.

This allows photosynthesis to take place in light otherwise too faint for it to occur (Fig. 20.3).

None of the bryophytes have true xylem or phloem, and to be able to reproduce, all bryophytes must have external water, usually in the form of dew or rain. Many mosses do have special water-conducting cells called hydroids in the centers of their stems, and a few have food-conducting cells called leptoids surrounding the hydroids. Neither type of cell, however, conducts as efficiently as vessel elements or tra-cheids of xylem and sieve tube members of phloem, and most water is absorbed directly through the surface. The absence of xylem and phloem makes most bryophytes soft and pliable, and it is not surprising, therefore, that birds often use them to line their nests. In one study in the Appalachian Mountains of Virginia in 1975, for example, David Breil and Susan Moyle found birds native to the area used at least 65 species of bryophytes in the construction of their nests.

Alternation of Generations is more conspicuous in bryophytes and ferns than in most other organisms. In mosses, the "leafy" plant is a major part of the gametophyte generation that produces the gametes. The sporophyte generation, which grows from a "leafy" gametophyte, produces the spores. It usually resembles a tiny can with a rimmed lid at the tip of a slender upright stalk.

All bryophytes have similar life cycles, chromosomes, and habitats. However, based on their structure and reproduction, they are separated into three distinct phyla. None of the bryophytes appear closely related to other living plants, and fossils provide little evidence that members of each phylum are related to those of the other phyla. Botanists speculate that the three lines of bryophytes may have arisen independently from ancestral green algae.

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