The Structure and Reproduction of Bryophytes
The Form and Reproduction of Liverworts, Hornworts, and Mosses
Aug 14, 2009
Bryophytes are restricted to moist areas both structurally and reproductively. However, they are especially common in moist areas both in the tropics and in temperate zones and in polar areas they are the most abundant plants of all.
Bryophytes are Small and Flat
Structurally, bryophytes either totally lack water and food conducting vascular tissue (tubes) or possess only rudiments of such tissue. This accounts for the nonvascular label we attach to this group of plants. The lack of such conducting tissue deters them from ever growing very large. In fact, few exceed 2 centimeters (0.8 inches) in length and most are no thicker than several sheets of paper.
In form and appearance liverworts appear to be strips of green ribbon (known as the thallus) attached to damp soil while mosses resemble masses of green, fuzzy velvet growing on damp soil, rocks, and trees.
On the underneath side of mature bryophytes are slender, elongated single cells or filaments of cells called rhizoids. Superficially resembling roots, rhizoids function only to anchor the plant, not to absorb water. In fact, bryophytes completely lack true roots, stems, and leaves.
Bryophytes Undergo Alternation of Generations
Like all sexually reproducing plants, bryophytes exhibit an alternation of generation between the diploid generation (full set of genes) known as the sporophyte (phase that forms the spores) and the haploid generation (half set of genes) known as the gametophyte (phase that forms sperm and eggs known as gametes).
In the complex vascular seed plants the diploid or sporophyte is what we commonly regard as the mature plant possessing a full set (2n) of genes. The haploid or gametophyte is a set of specialized organs such as flowers or cones produced on the sporophyte forming the gametes or sex cells – sperm and egg which possess only half a set (n) of genes. However, in bryophytes the roles are reversed. What we regard as the body of the plant is the gametophyte phase with the sporophyte phase being represented in specialized organs (spore case) growing out from the gametophyte.
Sperm are produced in male gametophytes. Through a drop of dew they swim to the egg cell in the female gametophyte, where fertilization occurs and a diploid (2n) zygote is produced. In some species, both egg and sperm are produced on the same gametophyte.
The zygote divides by mitosis and develops into a diploid embryo (hollow ball of cells). Eventually, the embryo grows out of the female gametophyte into a stalk-like structure, the sporophyte. Meiosis (cell divisions in which the number of genes are reduced by half) occurs within the capsule at the end of the sporophyte and haploid spores are formed.
The capsule helps protect the spores. Once the spores are released, they are carried wherever wind or water transports them. Many fall onto unfavorable habitats and die. A lucky few reach the shady, moist confines they require, burst open, and begin to grow through mitosis (cell divisions in which the normal number of genes is maintained) into another gametophyte plant; another turn of the life cycle is completed.
Liverworts can also reproduce asexually by means of gemmae. Gemmae are small cup-shaped structures that contain many haploid cells. When washed out of the gemma, these cells can divide by mitosis to produce a new individual.
Life Cycle of Bryophytes
The life cycle of moss illustrates the basic mode of reproduction for all bryophytes.
When an asexual moss spore lands in a moist place, it germinates (breaks open) and grows into a mass of tangled green threads which look remarkably like filamentous green algae. As growth continues rhizoids are formed and tiny shoots begin to reach upward. These shoots grow into the familiar green moss plants, which are the gametophyte stage of its life cycle.
Growing out of the tip of the gametophyte phase are female structures known as archegonia and male structures known as antheridia. Some species produce both sperm and egg on the same plant, whereas other species produce sperm and eggs on separate plants.
Once released, sperm from the antheridia swims to the female archegonia through rain drops or heavy dew and fertilizes the eggs there forming a zygote. This zygote is the sporophyte stage of the life cycle. It grows directly out of the body of the gametophyte and actually depends on it for water and nutrients.
The mature sporophyte, known as a sporangium or spore case, is a long stalk ending in a capsule resembling a saltshaker. Inside the sporangium spores are produced. When the capsule matures and ripens, it splits open, scattering the spores to the wind to begin the life cycle anew.
The bryophytes are represented by three small groups of plants that are primitive both in structure and reproduction. Relegated to the shade and shadow of moist areas, liverworts, hornworts, and mosses are not well known and may not even be recognizable as true plants to most people
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