Thalloid Liverworts

The best-known species of thalloid liverworts are in the genus Marchantía (named in honor of French botanist N. Marchant) (see Fig. 20.4). The most widespread Marchantía species is often found on damp soil after a fire. The thallus, which is about 30 cells thick in the center and 10 cells thick at the margin, forks dichotomously as it grows. Each branch has a notch at the apex and a central groove that extends back lengthwise behind the notch. The thalli grow as meristematic cells at the notches divide. Older tissues at the rear decay as the new growth is added. The upper surface of the thallus is divided into diamond-shaped or polygonal segments, the segment lines marking the limits of chambers below. Each segment has a small bordered pore opening into the interior.

Seen through a microscope, a sectioned liverwort thallus looks like groups of short, erect, branching rows of cells with chloroplasts, sitting on a wall of colorless "bricks" (Fig. 20.5). The "brick wall," which may comprise most of the thallus, consists of parenchyma cells that have few, if any, chloroplasts. The tissue apparently stores substances produced in other cells. The bottom layer of cells is an epidermis from which rhizoids and scales arise. The individual groups of upright chlorenchyma cells are enclosed by vertical walls that are covered by a slightly dome-shaped layer of epidermal cells. A conspicuous pore is located in the center of each "roof" and remains open at all times. It resembles a tiny, short, suspended, open-ended barrel.

Marchantía—Asexual Reproduction

Marchantía reproduces asexually by means of gemmae (singular: gemma). Gemmae are tiny, lens-shaped pieces of tissue that become detached from the thallus. They are produced in small gemmae cups scattered over the upper surface of the liverwort gametophyte (Fig. 20.6). Raindrops may splash the gemmae as much as 1 meter (3 feet) away. While gemmae are in the cup, lunularic acid inhibits their further development, but each is capable of growing into a new thallus as soon as it leaves the cup. In addition, parts of an older thallus may die, isolating patches of active tissue, which may then continue to grow independently.

Marchantía—Sexual Reproduction

The gametangia of Marchantía are produced on separate male and female gametophytes and are more specialized than those of other liverworts. Both types of gametangia are formed on gametophores (umbrellalike structures borne on slender stalks rising from the central grooves of the thallus) (see Fig. 20.6). The top of the male gametophore, or antheridiophore, is disclike with a scalloped margin, while that of the female gametophore, or archegoniophore, looks like the hub and spokes of a wagon wheel.

Club-shaped male gametangia (antheridia) containing numerous sperms are produced in rows just beneath the upper surface of the antheridiophore. Archegonia are flasklike female gametangia, each containing a single egg; they are also produced in rows and hang neck downward beneath the spokes of the archegoniophore. Raindrops sometimes splash the released sperms, which have numerous flagella, more than 0.5 meter (1.5 feet) away. Fertilization may occur before the stalks of the archegoniophores have finished growing.

After fertilization, the zygote develops into a multicel-lular embryo (an immature sporophyte) that is totally dependent on the gametophyte for sustenance. A knoblike foot anchors the sporophyte (the diploid, spore-producing phase) in the tissues of the archegoniophore. The sporophyte hangs suspended by a short, thick stalk called the seta. The main part of the sporophyte, in which different types of tissues develop, is called a capsule. Liverwort sporophytes typically have no stomata.

Liverwort Rhizoids

pore air chamber chlorenchyma cells parenchyma cells rhizoid

Figure 20.5 A section through a portion of a Marchantía thallus.

pore air chamber chlorenchyma cells parenchyma cells rhizoid

Figure 20.5 A section through a portion of a Marchantía thallus.

fertilization fertilization

Thallus Cell

gametophyte spores spores gametophyte

Imagenes Las Marchant

Figure 2Q.Ó Life cycle of the thalloid liverwort Marchantía.

Sporocytes in the capsule undergo meiosis, producing haploid spores. Other capsule cells do not undergo meiosis but remain diploid and develop instead into long, pointed elaters with spiral thickenings. They are sensitive to changes in humidity (Fig. 20.7). Spore dispersal in Marchantía takes place as the elaters twist and untwist rapidly. In the sporophytes of other liverworts, the elaters may aid spore dispersal with a snapping action or by suddenly expanding.

Until the young sporophyte is mature, it is protected by the calyptra, a caplike tissue that grows out from the game-tophyte, and by other membranes covering the capsule. The capsule splits at maturity, and air currents carry the spores away. Under favorable conditions, the spores germinate, producing new gametophytes.

Other thalloid forms, such as the floating or amphibious liverworts, do not produce gametophores. Instead, the archegonia and antheridia develop within the thallus beneath the central grooves, where the sporophytes also are formed. The spores are liberated from the submerged sporo-phytes as the thallus decays.

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Responses

  • Priamus
    How does thalloid growth differ from parenchymatous growth?
    5 years ago
  • Alvisio
    Why are the chlorenchyma cells loacted where they are in liverwort?
    5 years ago
  • Kisanet Yohannes
    Where are the scales on a chambered liverwort?
    5 years ago

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