Bryophyta

Bryophyta is a group of non-vascular, spore-producing plants. They are small, soft, and green, and are mostly found in moist, shaded habitats. Unlike higher plants, bryophytes lack true roots, stems, and leaves but have structures that resemble these organs.

General Characters of Bryophyta

• Bryophytes are the first land plants. It is believed that they originated from an aquatic plant.
• Bryophytes are not considered successful land plants because they lack vascular tissue and require water for fertilisation.
• Bryophytes are known as amphibious plants of the plant kingdom, because these plants can live in soil but are dependent on water for fertilisation.
• They lack true roots, stems or leaves. They may possess root-like, leaf-like or stem-like structures.
• Bryophytes are sciophytes; i.e., they prefer to grow in cool, moist (wet) and shady places.
• Bryophytes in general are of little economic importance, but some mosses provide food for herbaceous mammals, birds and other animals.
• The water-conducting tissue in Bryophyta is parenchyma.
• Bryophytes include the various mosses and liverworts commonly found growing in moist, shaded areas in the hills.
• Mosses, along with lichens, are the first organisms to colonise rocks and hence are of great ecological importance. 
• They decompose rock, making the substrate suitable for the growth of higher plants. Since mosses form dense mats on the soil, they reduce the impact of falling rain and prevent soil erosion.

1.0Classification of Bryophyta

The Bryophyta are divided into major groups, or phyla, each with unique characteristics.

Liverworts

• Bryophytes included in this class have a shape like liver (e.g., Marchantia) or flat (e.g. Riccia), so they are known as liverworts.
• The plant body of this group is thallus-like. The thallus is dorsiventral and closely appressed to the substrate. Rhizoids and scales are present on the thallus. Rhizoids are unicellular and unbranched.
• Scales are multicellular and protective.
• The leafy members (e.g., Porella) have tiny leaf-like appendages arranged in two rows along the stem-like structures.
• The sporophyte of Liverworts is completely dependent on the gametophyte; i.e., it depends on the gametophyte for food, water, and habitat.
• The sporophyte of Liverworts is made up of a foot, a seta and a capsule. After meiosis, spores are produced within the capsule. These spores germinate to form free-living gametophytes.
• True elaters are present in the sporophyte of some liverwort species. Elaters are hygroscopic, and they help disperse spores.
• Asexual (vegetative) reproduction in Liverworts takes place by fragmentation of thalli, or by the formation of specialised structures called gemmae (sing. Gemma). Gemmae are green, multicellular, asexual buds that develop in small receptacles called Gemma cups on the thalli. The gemmae become detached from the parental body and germinate to form new individuals. e.g. Marchantia
• During sexual reproduction, male and female sex organs are produced either on the same (e.g. Riccia) or on different thalli (e.g. Marchantia).

Mosses

• The predominant stage of the life cycle of a moss is the gametophyte, which consists of two stages.
• The first stage is the protonema stage, which develops directly from a spore. It is a creeping, green, branched and frequently filamentous stage.
• The second stage is the leafy stage, which develops from the secondary protonema as a lateral bud. They consist of upright, slender axes bearing spirally arranged leaves.
• The main plant body or gametophyte of mosses is made up
• of stem-like, leaf-like, and rhizoids (root-like). They are attached to the soil through rhizoids. The rhizoids in this class are multicellular, branched, and obliquely septate.
• Vegetative reproduction in mosses is by fragmentation and budding in the secondary protonema.
• In sexual reproduction, the sex organs, antheridia and archegonia are produced at the apex of the leafy shoots. After fertilisation, the zygote develops into a sporophyte, consisting of a foot, a seta, and a capsule. The capsule contains spores. Spores are formed after meiosis.
• The sporophyte in mosses is more elaborated (developed) than that in liverworts.
• The sporophyte of mosses is also partially dependent (semi-parasitic), like that of hornworts. i.e. it is photosynthetic. The Mosses have an elaborate mechanism of spore dispersal.
• Peristomial teeth are present in the moss sporophyte and help with spore dispersal.
• Common examples of mosses are Funaria, Polytrichum and Sphagnum.

2.0Structure of Bryophytes

Bryophytes have simple structures adapted to moist terrestrial habitats:

1. Gametophyte

• Dominant stage in the life cycle
• Green, photosynthetic, and independent
• May be leafy (mosses) or thalloid (liverworts, hornworts)

2. Sporophyte

• Dependent on the gametophyte for nutrients
• Composed of foot, seta, and capsule
• Produces spores via meiosis
• Short-lived in most bryophytes

3. Rhizoids

• Hair-like structures for anchorage
• Unicellular or multicellular
• Do not conduct water like roots

The Life Cycle of Bryophytes

• The main plant body of a bryophyte is haploid. It produces gametes, hence is called a gametophyte.
• Sex organs are formed on the gametophyte. Sex organs are multicellular and jacketed in bryophytes. The male sex organ is called antheridium, and the female sex organ is called archegonium.
• An archegonium is flask-shaped and produces a single egg.
• The male gametes of bryophytes are motile. These motile male gametes are called as antherozoids. Antherozoids are usually comma shaped and biflagellate. A female gamete is called an egg or an ovum.
• In Bryophyta, fertilisation is performed by zoidogamy, i.e., the male gamete swims into water to reach the female gamete and fertilises it.
• As a result of the fertilisation, a diploid zygote is formed. Zygotes do not undergo meiosis (reduction division) immediately.
• A zygote forms an embryo, which then develops into a sporophyte by mitosis. The zygote initiates the sporophyte generation. Sporophyte generation is a diploid stage.
• The sporophyte is not free-living but attached to the photosynthetic gametophyte and derives nourishment from it. The sporophyte of bryophyta is not made of root, stem and leaves, but it is made of foot, seta and capsule, so it is known as sporogonium. Some cells in the sporophyte's capsule function as spore mother cells. Now, meiosis takes place in the spore mother cells. To form haploid spores.
• In bryophyta, the sporophyte is dependent on the gametophyte. This is a unique characteristic of bryophyta.
• The germination of spores is direct or indirect. In liverworts, spore germination is direct; each spore forms a gametophyte or thallus after germination. 
• The germination of moss spores is indirect. In mosses, a multicellular filament is formed after the germination of a spore. This filament is known as protonema.
• Protonema developed from spores is called primary protonema, and that produced from parts other than spores is called secondary protonema.
• Protonema is creeping, green, branched and filamentous.
• Now, lateral buds form on the protonema. Each bud develops and forms a gametophyte plant. Indirect germination is best for survival. Mosses are gregarious because they grow in groups.

3.0Reproduction in Bryophytes

Bryophytes reproduce both asexually and sexually, with water being essential for sexual reproduction.

1. Asexual Reproduction

• Fragmentation: Breaking of the gametophyte into pieces
• Gemmae: Specialised structures in liverworts for vegetative propagation

2. Sexual Reproduction

• Bryophytes are haplodiplontic (alternation of generations)
• Male gametes (antherozoids) swim through water to reach female gametes (archegonia)
• Fertilisation produces a sporophyte, which releases spores

4.0Habitat and Distribution

Bryophytes thrive in moist, shaded, and humid environments, though some can tolerate dry conditions. They are found on:

• Soil, rocks, and tree trunks
• Banks of rivers and streams
• Forest floors and wetlands

Adaptations for Moisture:

• Thin leaves for absorption
• Rhizoids for anchorage and minimal water absorption
• Tolerance to desiccation in some species

Ecological and Economic Importance

Bryophytes play a vital role in various ecosystems, despite their small size.

• Pioneer Species: They are often the first plants to colonise barren rocks and soil, helping to create a suitable environment for other plants to grow. They contribute to soil formation by breaking down rocks and trapping soil particles.
• Soil Erosion Prevention: Bryophyte mats act as a protective cover, absorbing rainwater and reducing soil erosion, particularly in hilly areas.
• Water Retention: They have a remarkable ability to hold water, helping to maintain moisture in their surroundings and providing a microhabitat for other organisms.
• Sphagnum Moss (Peat Moss): A significant genus, Sphagnum, forms dense mats in bogs. The partially decayed remains of this moss accumulate to form peat, which is harvested as fuel, a soil conditioner, and a packing material due to its high water-holding capacity.

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