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Bryophytes

Bryophytes

Bryophytes are seedless plants without specialized water-conducting tissues. Bryophytes include mosses (phylum Bryophyta), liverworts (phylum Marchantiophyta Hepatophyta), and hornworts (phylum Anthocerophyta). They are plants that virtually everyone has seen, but many have ignored. The most commonly encountered group is the green mosses that cover rotting logs, anchor to the bark of trees, and grow in the spray of waterfalls, along streams and in bogs. Even though mosses often thrive in wet habitats, many mosses and some liverworts can survive in relatively dry environments such as sandy soils and exposed rock outcrops.

The liverworts can take leafy forms, which are very similar superficially to mosses, but differ in the details of leaf size and arrangement. Other liverwort genera are characterized by a thallus made up of relatively small, flattened, ribbonlike segments of photosynthetic tissue, which have the general appearance of short, branched pieces of rich dark green egg noodles or linguini.

The leafy liverworts and the mosses differ in the appearance of their spore-forming structures. The mosses have thin stalks called seta extending from the ends of leafy branches. Seta bear capsules, which produce spores. The leafy and thalloid liverworts have very small, balloon-shaped spore-producing stages that remain virtually hidden within, and totally dependent upon, the photosynthetic plant tissues. The third major group of bryophytes is the hornworts. They received this common name because their spore producing structures, called sporangia, are generally long, slender, hornlike, and without capsules. More than eighteen thousand different bryophyte species have been identified throughout the world, and there are perhaps ten thousand species of moss, approximately eight thousand liverwort species, and only a little more than one hundred species of hornworts.

Characteristics of Bryophytes

There are several characteristic features of bryophytes. First, the green tissue that makes up most of the plant body is not vascularized; it does not have xylem and phloem cells. This absence of specialized tissues for transporting water and dissolved food throughout the organism limits terrestrial forms to being very short plants, since the only way to move substances through the plant body is by osmosis and diffusion from surface moisture.

Second, bryophytes do not have roots, but have rhizoids, which are relatively simple, sometimes multicellular filaments of thin-walled cells that extend from the photosynthetic tissue into the soil or other substrate . They anchor the plant somewhat and in some cases facilitate water and nutrient uptake.

Sexual Reproduction

The third characteristic of bryophytes is something that one could not guess by just looking at the conspicuous green tissue. Unlike other plants (and indeed most other multicellular organisms), the conspicuous portion of bryophytes is composed of haploid cells, containing only one set of chromosomes .

Sexual reproduction in animals involves the union of an egg and a sperm to form a fertilized egg (zygote). This diploid (2n) cell divides mitotically to produce an embryo, and ultimately a mature adult organism. These adults have specialized cells, which divide meiotically to produce haploid (n) sperm or eggs depending on the sex of the individual. In the plant kingdom, this cycle of fertilization and meiosis involves an alternation of generations between the haploid gamete -producing stage (gametophyte) and the diploid organism (sporophyte).

Vascular plants, including flowering plants, conifers, and many, such as ferns, that do not produce seeds, have life cycles with the diploid sporophyte being the predominant generation. In the bryophytes, it is the haploid gametophyte that produces the leaves and thali and therefore predominates. This change from predominant gametophyte to sporophyte was a major evolutionary advancement, which along with the development of vascular tissue facilitated the ultimate success of plants in a diversity of terrestrial habitats.

In order to accomplish sexual reproduction, bryophyte gametophytes produce eggs (n) in the archegonium, a vase-shaped structure that is the female reproductive organ. The sperm (n) are produced in antheridia, which may occur on the same gametophyte, but are often located on separate male plants. Water is generally required for them to swim to the eggs for fertilization. The resulting zygote (2n) develops into the sporophyte (2n). The sporophytes remain attached to and dependent on the female gametophyte. These parasitic sporophytes produce spores (n) by meiosis that then divide mitotically to produce the obvious multicellular gametophyte.

see also Alternation of Generations; Angiosperms; Plant; Pteridophytes; Seedless Vascular Plants; Translocation; Water Movement in Plants

Dean Cocking

Bibliography

Conard, Henry Shoemaker, et al. How to Know the Mosses and Liverworts, 2nd ed. New York: McGraw-Hill, 1980.

Malcolm, Bill, and Nancy Malcolm. Mosses and Bryophytes: An Illustrated Glossary. Portland, OR: Timber Press/Micro-Optics Press, 2000.

Shaw, A. Jonathan, and Bernard Goffinet, eds. Bryophyte Biology. New York: Cambridge University Press, 2000.

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Bryophytes

Bryophytes

Plant scientists recognize two kinds of land plants: bryophytes (nonvascular land plants) and tracheophytes (vascular land plants). Bryophytes are small, herbaceous plants that grow closely packed together in mats or cushions on rocks or soil or as epiphytes on the trunks and leaves of forest trees. Bryophytes are distinguished from tracheophytes by two important characteristics. First, in all bryophytes the ecologically persistent, photosynthetic phase of the life cycle is the haploid , gametophyte generation rather than the diploid sporophyte ; bryophyte sporophytes are very short-lived, are attached to and nutritionally dependent on their gametophytes, and consist of only an unbranched stalk, or seta, and a single, terminal sporangium. Second, bryophytes never form xylem tissue, the special lignin-containing, water-conducting tissue that is found in the sporophytes of all vascular plants. At one time, all bryophytes were placed in a single phylum, intermediate in position between algae and vascular plants. Modern studies of cell ultra-structure and molecular biology, however, confirm that bryophytes comprise three separate evolutionary lineages , today recognized as mosses (phylum Bryophyta), liverworts (phylum Marchantiophyta), and hornworts (phylum Anthocerotophyta). Following a detailed analysis of land plant relationships, Paul Kenrick and Peter R. Crane proposed that the three groups of bryophytes represent a structural level in plant evolution, identified by their monosporangiate life cycle. Within the bryophytes, liverworts are the geologically oldest group, sharing a fossil record with the oldest vascular plants (Rhyniophytes) in the Devonian era.

Mosses

Of the three phyla of bryophytes, greatest species diversity is found in the mosses, with up to fifteen thousand species recognized. A moss begins its life cycle when haploid spores, which are produced in the sporophyte capsule, land on a moist substrate and begin to germinate. From the one-celled spore a highly branched system of filaments , called the protonema, develops. Cell specialization occurs within the protonema to form a horizontal system of reddish-brown anchoring filaments and upright green filaments. Each protonema, which superficially resembles a filamentous alga, can spread over several centimeters to form a fuzzy green film over its substrate. As the protonema grows, some cells of the specialized green filaments form leafy buds that will ultimately form the adult gametophyte shoots. Numerous shoots typically develop from each protonema so that, in fact, a single spore can give rise to a whole clump of moss plants. Each leafy shoot continues to grow apically , producing leaves in spiral arrangement on an elongating stem. In many mosses the stem is differentiated into a central strand of thin-walled water-conducting cells, called hydroids, surrounded by a parenchymatous cortex and a thick-walled epidermis. The leaves taper from a broad base to a pointed apex and have lamina that are only one-cell-layer thick. A hydroid-containing midvein often extends from the stem into the leaf. Near the base of the shoot, reddish-brown multicellular rhizoids emerge from the stem to anchor the moss to its substrate. Water and mineral nutrients required for the moss to grow are absorbed, not by the rhizoids, but rather by the thin leaves of the plant as rain water washes through the moss cushion.

As is typical of bryophytes, mosses produce large, multicellular sex organs for reproduction. Many bryophytes are unisexual, or sexually dioicous . In mosses male sex organs, called antheridia, are produced in clusters at the tips of shoots or branches on the male plants; female sex organs, the archegonia, are produced in similar fashion on female plants. Numerous motile sperm are produced by mitosis inside the brightly colored, club-shaped antheridia

DISTINGUISHING CHARACTERISTICS OF MOSSES, LIVERWORTS, AND HORNWORTS
Characteristics Mosses (Bryophyta) Liverworts (Marchantiophyta) Hornworts (Anthocerotophyta)
Protonema Filamentous, forming many buds Globose, forming one bud Globose, forming one bud
Gametophyte form Leafy shoot simple or with air chambers Leafy shoot or thallus; thallus Simple thallus
Leaf arrangement Leaves in spirals Leaves in three rows Not Applicable
Leaf form Leaves undivided, midvein present Leaves divided into two-plus lobes, no midvein Not Applicable
Special organelles None Oil bodies Single plastids with pyrenoids
Water-conducting cells Present in both gametophyte and sporophyte Present only in a few simple thalloid forms Absent
Rhizoids Brown, multicellular Hyaline, one-celled Hyaline, one-celled
Gametangial position Apical clusters (leafy forms) Apical clusters (leafy forms) or on upper surface of thallus Sunken in thallus, scattered
Stomates Present on sporophyte capsule Absent in both generations Present in both sporophyte and gametophyte
Seta Photosynthetic, emergent from gametophyte early in development Hyaline, elongating just prior to spore release Absent
Capsule Complex with operculum, theca, and neck; of fixed size Undifferentiated, spherical, or elongate; of fixed size Undifferentiated, horn-shaped; growing continuously from a basal meristem
Sterile cells in capsule Columella Spirally thickened elaters Columella and pseudoelaters
Capsule dehiscence At operculum and peristome teeth Into four valves Into two valves

while a single egg develops in the base of each vase-shaped archegonium. As the sperm mature, the antheridium swells and bursts open. Drops of rainwater falling into the cluster of open antheridia splash the sperm to nearby females. Beating their two whiplash flagellae , the sperm are able to move short distances in the water film that covers the plants to the open necks of the archegonia. Slimy mucilage secretions in the archegonial neck help pull the sperm downward to the egg. The closely packed arrangement of the individual moss plants greatly facilitates fertilization. Rain forest bryophytes that hang in long festoons from the trees rely on torrential winds with the rain to transport their sperm from tree to tree, while the small pygmy mosses of exposed, ephemeral habitats depend on the drops of morning dew to move their sperm. Regardless of where they grow, all bryophytes require water for sperm dispersal and subsequent fertilization.

Embryonic growth of the sporophyte begins within the archegonium soon after fertilization. At its base, or foot, the growing embryo forms a nutrient transfer zone, or placenta, with the gametophyte. Both organic nutrients and water move from the gametophyte into the sporophyte as it continues to grow. In mosses the sporophyte stalk, or seta, tears the archegonial enclosure early in development, leaving only the foot and the very base of the seta embedded in the gametophyte. The upper part of the archegonium remains over the tip of the sporophyte as a caplike calyptra. Sporophyte growth ends with the formation of a sporangium (the capsule) at the tip of the seta. Within the capsule, water-resistant haploid spores are formed by meiosis. As the mature capsule swells, the calyptra falls away. This allows the capsule to dry and break open at its tip when the spores are mature. Special membranous structures, called peristome teeth, that are folded down into the spore mass now bend outward, flinging the spores into the drying winds. Moss spores can travel great distances on the winds, even moving between continents on the jet streams. Their walls are highly protective, allowing some spores to remain viable for up to forty years. Of course, if the spore lands in a suitable, moist habitat, germination will begin the cycle all over again.

Liverworts and Hornworts

Liverworts and hornworts are like mosses in the fundamental features of their life cycle, but differ greatly in organization of their mature game-tophytes and sporophytes. Liverwort gametophytes can be either leafy shoots or flattened thalli . In the leafy forms the leaves are arranged on the stem in one ventral and two lateral rows or ranks, rather than in spirals like the mosses. The leaves are one cell-layer thick throughout, never have a mid-vein, and are usually divided into two or more parts called lobes. The ventral leaves, which actually lie against the substrate (soil or other support), are usually much smaller than the lateral leaves and are hidden by the stem. Anchoring rhizoids, which arise near the ventral leaves, are colorless and unicellular. The flattened ribbonlike to leaflike thallus of the thallose liver-worts can be either simple or structurally differentiated into a system of dorsal air chambers and ventral storage tissues. In the latter type the dorsal epidermis of the thallus is punctuated with scattered pores that open into the air chambers. Liverworts synthesize a vast array of volatile oils, which they store in unique organelles called oil bodies. These compounds impart an often spicy aroma to the plants and seem to discourage animals from feeding on them. Many of these compounds have potential as antimicrobial or anticancer pharmaceuticals.

Liverworts.

Liverwort sporophytes develop completely enclosed within gametophyte tissues until their capsules are ready to open. The seta, which is initially very short, consists of small, thin-walled hyaline cells. Just prior to capsule opening, the seta cells lengthen, thereby increasing the length of the seta up to twenty times its original dimensions. This rapid elongation pushes the darkly pigmented capsule and upper part of the whitish seta out of the gametophytic tissues. With drying, the capsule opens by splitting into four segments, or valves. The spores are dispersed into the winds by the twisting motions of numerous intermixed sterile cells called elaters. In contrast to mosses, which disperse their spores over several days, liverworts disperse the entire spore mass of a single capsule in just a few minutes.

Hornworts.

Hornworts resemble some liverworts in having simple, unspecialized thalloid gametophytes, but they differ in many other characters. For example, colonies of the symbiotic cyanobacterium Nostoc fill small cavities that are scattered throughout the ventral part of the hornwort thallus. When the thallus is viewed from above, these colonies appear as scattered blue-green dots. The cyanobacterium converts nitrogen gas from the air into ammonium, which the hornwort requires in its metabolism, and the hornwort secretes carbohydrate-containing mucilage, which supports the growth of the cyanobacterium. Hornworts also differ from all other land plants in having only one large, algal-like chloroplast in each thallus cell. Hornworts get their name from their long, horn-shaped sporophytes. As in other bryophytes, the sporophyte is anchored in the gametophyte by a foot through which nutrient transfer from gametophyte to sporophyte occurs. The rest of the sporophyte, however, is actually an elongate sporangium in which meiosis and spore development take place. At the base of the sporangium, just above the foot, is a mitotically active meristem , which adds new cells to the spore-producing zone throughout the life span of the sporophyte. In fact, the sporangium can be releasing spores at its apex at the same time that new spores are being produced by meiosis at its base. Spore release in hornworts takes place gradually over a long period of time, and the spores are mostly dispersed by water movements rather than by wind.

Mosses, liverworts, and hornworts are found throughout the world in a variety of habitats. They flourish particularly well in moist, humid forests like the fog forests of the Pacific Northwest or the montane rain forests of the Southern Hemisphere. Their ecological roles are many. They provide seed beds for the larger plants of the community , they capture and recycle nutrients that are washed with rainwater from the canopy, and they bind the soil to keep it from eroding. In the Northern Hemisphere peatlands, wetlands often dominated by the moss Sphagnum, are particularly important bryophyte communities. This moss has exceptional water-holding capacity, and when dried and compressed forms a coal-like fuel. Throughout northern Europe, Asia, and North America, peat has been harvested for centuries for both fuel consumption and horticultural uses, and today peat lands are managed as a sustainable resource.

see also Evolution of Plants; Gametophyte; Nitrogen Fixation; Peat Bogs; Reproduction, Alternation of Generations and; Sporophyte.

Barbara Crandall-Stotler

Bibliography

Crandall-Stotler, Barbara. "Morphogenetic Designs and a Theory of Bryophyte Origins and Divergence." BioScience 30 (1980): 580-85.

Hébant, Charles. The Conducting Tissues of Bryophytes. Vaduz: J. Cramer, 1977.

Kenrick, Paul, and Peter R. Crane. The Origin and Early Diversification of Land Plants: A Cladistic Study. Washington, DC: Smithsonian Institution Press, 1997.

Miller, Norton G. "Bogs, Bales and BTUs: A Primer on Peat." Horticulture 59 (1981):38-45.

Schofield, W. B. Introduction to Bryology. New York: Macmillan, 1985.

Shaw, Jonathon A., and Bernard Goffinet, eds. The Biology of Bryophytes. Cambridge, England: Cambridge University Press, 2000.

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bryophyte

bryophyte Group of small, green, rootless non-vascular plants (phylum Bryophyta), including moss and liverwort. Bryophytes grow on damp surfaces exposed to light, including rocks and tree bark, almost everywhere from the Arctic to the Antarctic. There are c.24,000 species. See also alternation of generations

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bryophyte

bry·o·phyte / ˈbrīəˌfīt/ • n. any flowerless, rootless plant of the phylum Bryophyta, including mosses and liverworts.

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