Biology 200 - Chapter 20: Bryophytes
Introduction to the Plant Kingdom and Shared Ancestry
Plants and green algae (Chlorophyta) share a specific suite of physiological and structural characteristics, which strongly suggests they evolved from a common ancestor:
Presence of Chlorophylls and , and carotenoids.
Utilization of starch as a primary food reserve.
Presence of cellulose within the cell walls.
Development of a phragmoplast and cell plate during the process of cell division.
The chronological appearance of land plants is dated to approximately million years ago.
Evidence suggests that the common ancestor of modern plants progressed from aquatic environments to land habitats even earlier than the -million-year mark.
Evolutionary Adaptations for Terrestrial Life
By the time plants became firmly established on land, they evolved several specialized features to prevent desiccation (drying out):
Cuticle: Plant surfaces developed a fatty, waxy layer known as a cuticle to retard water loss.
Multicellular Gametangia: Gamete-producing structures became multicellular and were surrounded by a protective jacket of sterile cells.
Multicellular Sporangia: Spore-producing structures also became multicellular and protected by a jacket of sterile cells.
Embryo Development: Zygotes developed into multicellular embryos within the parental tissues that originally surrounded the egg, providing protection and nourishment.
Overview of Bryophyte Phyla
There are approximately species of bryophytes recognized today.
Bryophytes are divided into three distinct phyla:
Phylum Hepaticophyta: Known as the liverworts.
Phylum Anthocerophyta: Known as the hornworts.
Phylum Bryophyta: Known as the mosses.
None of these phyla appear to be closely related to other living plants, suggesting that bryophyte lineages may have arisen independently from ancestral green algae.
General Characteristics and Ecology of Bryophytes
Habitat and Distribution:
Bryophytes occupy a vast range of habitats including damp banks, trees, logs, bare rocks in scorching sun, and frozen alpine slopes.
They are found at elevations ranging from sea level to over .
Symbiotic Associations:
- Bryophytes often maintain associations with mycorrhizal fungi, which are found within their rhizoids.
Diversity in Form and Function:
Peat mosses: Ecologically significant in bog environments.
Luminous mosses: Found in caves and dark, damp places. They possess specialized lens-like cells that concentrate meager light onto their chloroplasts.
Internal Structure and Conduction:
Bryophytes lack true xylem and phloem (vascular tissues).
Many species possess hydroids, specialized cells for water conduction.
A few species possess leptoids, specialized cells for food (organic nutrient) conduction.
Most water is absorbed directly through the plant surface.
Reproductive Requirements:
- Bryophytes must have external water (such as film or droplets) to reproduce sexually.
Alternation of Generations in Bryophytes
Bryophytes exhibit a clear alternation of generations.
In mosses and other bryophytes, the leafy plant is the major part of the gametophyte generation.
The gametophyte generation (haploid, ) is responsible for producing gametes.
The sporophyte generation (diploid, ) grows out of the gametophyte and is responsible for producing spores through meiosis.
Phylum Hepaticophyta: The Liverworts
Structure and Form:
The most common liverworts possess flattened, lobed bodies called thalli (singular: thallus).
Thalloid liverworts make up about of the species, while the remaining are leafy.
Gametophytes develop from spores; upon germination, some produce a protonema (an immature gametophyte consisting of short filaments).
Growth is generally prostrate, and one-celled rhizoids on the lower surface anchor the plant.
Genus Marchantia:
This is the best-known genus of thalloid liverworts.
The thallus forks dichotomously (divides into two equal branches) as it grows, with each branch featuring a notch at the apex and a central groove.
Meristematic cells located in the notch continue to divide for growth.
The bottom layer of the thallus consists of the epidermis from which rhizoids and scales arise.
The upper surface is divided into diamond-shaped segments marking internal air chambers. Each segment has a bordered pore for gas exchange. Rows of chloroplast-containing cells sit on the floor of these chambers.
Marchantia Asexual Reproduction:
Occurs via gemmae (singular: gemma).
Gemmae are tiny, lens-shaped pieces of tissue that detach from the thallus.
They are produced in gemmae cups scattered over the upper surface of the thallus.
Marchantia Sexual Reproduction:
Gametangia are formed on specialized stalks called gametophores.
Antheridiophore: The male gametophore. Antheridia containing multi-flagellated sperm are found on the upper surface.
Archegoniophore: The female gametophore. Archegonia containing eggs are arranged in rows and hang down beneath the spokes of the archegoniophore.
The embryo is dependent on the gametophyte for sustenance.
Sporophyte Anatomy in Marchantia:
Foot: Anchors the sporophyte to the archegoniophore.
Seta: A short stalk.
Capsule: The sporangium where meiosis occurs to produce haploid spores.
Elaters: Specialized cells inside the capsule with spiral thickenings. They do not undergo meiosis. They are sensitive to humidity and twist/untwist rapidly to help break up spore masses and aid dispersal.
Calyptra: A cap-like tissue derived from the gametophyte that protects the immature sporophyte.
Leafy Liverworts
Leafy liverworts possess two rows of partially overlapping leaves.
Unlike mosses, these leaves lack a midrib and often feature folds or lobes.
The cells of leafy liverworts commonly contain oil bodies.
Phylum Anthocerophyta: The Hornworts
Structure and Form:
Mature sporophytes resemble miniature greenish-blackish rods.
Gametophytes are thalloid.
A unique feature is that cells typically contain only one large chloroplast.
Asexual Reproduction:
Primarily through fragmentation.
Occasionally through lobes that separate from the main thallus or the formation of tiny tubers that develop into new gametophytes.
Sexual Reproduction:
Archegonia and antheridia are produced in rows just beneath the upper surface of the gametophytes.
Sporophytes have numerous stomata.
Growth is unique: a meristem located above the foot continually increases the length of the sporophyte from the base.
Meiosis produces spores, which are intermingled with diploid elaters (similar in function to liverwort elaters).
Phylum Bryophyta: The Mosses
Structure and Diversity:
Approximately species are known.
Divided into three classes: Peat mosses, True mosses, and Rock mosses.
Moss Anatomy:
Gametophyte leaves are typically one-cell thick (except at the midrib) and are never lobed or divided.
Peat moss (Sphagnum) leaves have a unique structure: large, transparent, empty cells that absorb and store water, with small, green photosynthetic cells sandwiched between them.
The axis is stem-like and lacks true xylem/phloem, though it may have a central strand of hydroids.
Sexual Reproduction in Mosses:
Gametangia (archegonia and antheridia) are located at the apices of leafy shoots.
Archegonium: Cylindrical with a swollen base (venter) containing the egg and a neck containing a narrow canal.
Antheridium: Found on short stalks with walls one cell thick. They produce sperm cells with a pair of flagella.
Paraphyses: Multicellular filaments scattered among the archegonia and antheridia.
Fertilization and Spore Production:
Archegonia release chemical substances to attract sperm. Sperm must swim through external water down the neck of the archegonium.
The zygote grows into a spindle-shaped embryo.
The top of the archegonium splits off to form the calyptra (cap) on top of the sporophyte.
The mature sporophyte consists of a capsule, seta, and foot.
Inside the capsule, meiosis produces spores.
Operculum: The lid at the tip of the capsule.
Peristome: One or two rows of teeth under the operculum that open or close in response to humidity to regulate spore release.
Spores germinate into a filamentous protonema, which produces buds that develop into new leafy gametophytes.
Asexual Reproduction in Mosses:
Primarily occurs through fragmentation.
In arctic regions, wind dispersal of fragments is a routine mode of propagation.
Human and Ecological Relevance
Successional Pioneers: Bryophytes are often the first species to colonize bare rock after volcanic eruptions or geological upheavals.
Soil and Moisture Management:
They accumulate mineral and organic matter for other organisms.
They retain moisture and help reduce flooding and erosion.
Act as indicators of surface water.
Industrial and Domestic Uses:
- Used as packing material.
Specific Uses of Peat Moss (Sphagnum):
Soil conditioner due to its high water-absorptive capacity.
Poultice material for wounds due to its natural antiseptic properties and high absorbency.
Used as a fuel source in many parts of the world.