Bryophytes

Categories: Nonvascular plants; paleobotany; Plantae; taxonomic groups

Bryophytes (from the Greek word bryon, meaning “moss”) were once grouped together into one large phylum. Many botanists today recognize that these organisms belong to at least three distinct phyla: phylum Hepatophyta (the liverworts), phylum Anthocerophyta (the hornworts), and phylum Bryophyta (the mosses).

Origin and Relationships

Bryophytes are thought to have originated more than 430 million years ago, during the Silurian period. Many botanists speculate that bryophytes arose from an ancestor in the green algal order Charales or Coleochaetales based on biochemical, morphological, and life history comparisons. For example, Chara has a flavonoid biosynthesis pathway that is similar to that of higher plants, while Coleochaete retains its zygote inside parental tissue, similar to higher plants. These characteristics, along with similarities in cell division patterns, photosynthetic pigment contents, and the use of starch as a storage material, all suggest ancestry in the Charales or Coleochaetales orders.

Historically, bryophytes were thought to represent a group that formed a separate lineage from that of vascular plants. By the late 1990’s a growing body of evidence suggested that bryophytes and vascular plants were derived from a common green algal ancestor. Some botanists suggest that the earliest land plants may have been members of the phylum Anthocerophyta. One of the key arguments in this theory is that the structure of some hornwort chloroplasts is virtually identical to the chloroplast structure of the presumed algal ancestors.

Studies conducted in the 1990’s involving the presence or absence of certain portions of noncoding deoxyribonucleic acid (DNA) called introns in the genetic information of several groups of algae, bryophytes, and vascular plants revealed that members of the Hepatophyta, the liverworts, were likely among the first land plants. Like the algae, they lack the introns that are found in groups that are presumed to be more derived. Thus, based on the assumption that introns are derived characters, ancestors of modern liverworts may have given rise to vascular plants.

Anatomy

The dominant phase of the bryophyte life cycle is the haploid gametophyte phase. The gametophyte is photosynthetic and is usually small because of the lack of efficient vascular tissues.

Bryophytes possess rootlike rhizoids that anchor the plant to the soil and aid in nutrient uptake. A waxy cuticle, which helps prevent water loss, covers the body. Liverworts have pores for gas exchange, while hornworts and mosses have stomata to regulate gas movement. Some liverworts and hornworts have a thalloid body type, which is not differentiated into leaf and stem. The thallus may be simple, composed of a ribbonlike, flattened body of relatively undifferentiated tissues, or complex, in which there is a distinct differentiation of tissues. The flat body may aid in the uptake of water and minerals and in gas exchange. The bodies of some liverworts and the mosses are divided into leaf and stem. These terms are used for convenience even though xylem and phloem are not present.

Some mosses possess tissues that have functions similar to xylem and phloem. Hydroids are water-conducting cells that make up a tissue called hadrom. Leptoids are food-conducting cells that make up a tissue called leptom. These tissues appear similar to the conducting tissues in a group of fossil plants called protracheophytes, which are thought to be an intermediate group between the bryophytes and the vascular plants.

The diploid sporophyte of liverworts and mosses consists of a foot, which is attached to a stalklike seta. The seta connects the foot to the spore-producing organ called the sporangium, or capsule. The hornwort sporophyte, however, lacks a seta and possesses a long, cylindrical sporangium. The foot of the bryophyte sporophyte contains specialized transfer cells, which bring materials from the maternal gametophyte to the sporophyte. The sporophyte is totally dependent on the maternal gametophyte for its survival. A layer of sterile tissue called the calyptra covers the capsules of liverworts and mosses. When the spores are mature, the sporophyte may die, allowing the release of spores as the capsule decays (as in some thalloid liverworts). Alternatively, the capsule may rupture, allowing spores to be released through pores (as in mosses and leafy liverworts), or the capsule may split along the side to release the spores (as in the hornworts). Liverworts and hornworts often have specialized structures in the capsules called elaters that aid in dispersing spores from the capsules.

Reproduction and Life Cycle

Bryophytes may reproduce either asexually or sexually. Asexual reproduction primarily occurs by fragmentation. Some of the liverworts also reproduce asexually by the production of small masses of vegetative tissue called gemmae in special structures called gemma cups. Water drops disperse the gemmae.

Bryophytes exhibit a typical plant life-cycle pattern called alternation of generations. There are distinct male and female gametophytes in some species, while other species produce both male and female organs in one plant. The reproductive organs are all multicellular. Male organs are called antheridia. Special cells within an antheridium undergo mitotic cell division to produce flagellated haploid sperm cells. The sperm cells are the only flagellated cells produced by bryophytes. As with many other plant groups, the presence of flagella on the sperm indicates that these cells require liquid water to swim to the egg.

The female organs are called archegonia. The archegonium is composed of a slender neck, within which is a canal. The base of the archegonium has a swollen region called the venter, which contains the egg. Special cells within an archegonium undergo mitotic cell division to produce a haploid egg.

If one gametophyte produces both antheridia and archegonia, the organs usually develop at different times, to reduce to likelihood of self-fertilization. When the sperm and eggs are mature, sperm are released from the antheridia in the presence of liquid water. Water drops transfer sperm from an antheridium to an archegonium. Sperm cells swim through the neck canal of the archegonium where fertilization occurs in the venter. The resulting zygote develops into an embryo, which then grows into the diploid sporophyte.

Sporogenous tissues in the sporangium undergo meiosis to produce haploid spores. The spore walls contain a substance called sporopollenin, which is resistant to chemicals and decay. After release, spores germinate and grow into new haploid gametophytes. The early threadlike stage of mosses and some liverworts is called the protonema. Protonemata are very similar to the body form of some algae.

Phylum Hepatophyta

There are between six thousand and eight thousand species of hepatophytes (from the Greek word hepar, meaning “liver”), which are commonly called liverworts. Hepatophytes are divided into three general groups: the simple thalloid liverworts, the complex thalloid liverworts, and the leafy liverworts. More than 85 percent of all hepatophyte species are leafy. Liverworts are usually terrestrial, although some species may be semiaquatic. Thalloid types are found worldwide. Leafy liverworts, which are often similar in appearance to mosses, are abundant in tropical jungles and fog belts. However, they are typically found in habitats that are more moist than those preferred by mosses.

Phylum Anthocerophyta

This phylum, the hornworts, consists of some one hundred species and represents the smallest group of bryophytes. The best-known genus, Anthoceros (from the Greek words anthos, meaning “flower” and keras, meaning “horn”), is found in temperate regions. The gametophyte is similar to thalloid liverworts. The cavities of the gametophyte body are filled with mucilage, a slimy secretion, in which grow nitrogen-fixing cyanobacteria, such as the genus Nostoc.

Phylum Bryophyta

Phylum Bryophyta, the mosses, consists of more than ninety-five hundred species. There are three important classes: class Sphagnidae, which includes the globally distributed, and economically as well as ecologically important genus Sphagnum; class Andreaeidae, which consists of a small group of blackish green to reddish brown tufted rock mosses growing on granitic or calcareous rocks in northern latitudes; and the class Bryidae, which consists of true mosses.

Economic Uses

Bryophytes are ecologically important members of terrestrial ecosystems. They are primary producers, providing food and habitat for animals. Humans have used bryophytes for many purposes. For example, Sphagnum deposits in peat bogs have been used for centuries as fuel for heating and cooking. Dried Sphagnum also has the ability to absorb large amounts of liquid, which makes it ideal to act as a soil conditioner for planting. American Indians used mosses as compresses to dress wounds. The antiseptic quality of Sphagnum, along with its absorptive properties, made its use attractive as bandage material for the British when cotton supplies were low during World War I.

Bibliography

Bell, Peter R., and Alan R. Hemsley. Green Plants: Their Origin and Diversity. 2d ed. New York: Cambridge University Press, 2000. Contains an introduction to major groups of bryophytes. Includes photos, diagrams.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York: W. H. Freeman/Worth, 1999. A standard text for introductory botany courses at the university level. Discusses structure and significance of major groups of bryophytes. Includes photos and diagrams.

Shaw, A. Jonathan, and Bernard Goffinet, eds. BryophyteBiology. New York: Cambridge University Press, 2000. A scholarly study of bryophyte origins, systematics, and biology. Includes tables, photos, and diagrams.