Detailed Notes – 2- Chapter3-Plant Kingdom

3.2 BRYOPHYTES

1. Description:

   • Bryophytes comprise various mosses and liverworts, commonly found growing in moist shaded areas, particularly in hills.
   • They are often referred to as the amphibians of the plant kingdom because they can live in soil but rely on water for sexual reproduction.
   • Bryophytes typically thrive in damp, humid, and shaded environments, playing a significant role in plant succession on bare rocks and soil.

2. Structure:

   • The plant body of bryophytes is more differentiated than that of algae, appearing thallus-like and either prostrate or erect.
   • They lack true roots, stems, or leaves but may possess root-like, leaf-like, or stem-like structures.
   • Bryophytes are attached to the substrate by unicellular or multicellular rhizoids.

3. Life Cycle:

   • The main plant body of bryophytes is haploid and called a gametophyte, which produces gametes.
   • Sex organs in bryophytes are multicellular, with the male sex organ (antheridium) producing biflagellate antherozoids, and the female sex organ (archegonium) producing a single egg.
   • Fertilization occurs when an antherozoid fuses with the egg, forming a zygote. The zygote develops into a multicellular sporophyte, which remains attached to the gametophyte and derives nourishment from it.
   • Some cells of the sporophyte undergo reduction division (meiosis) to produce haploid spores, which germinate to produce new gametophytes.

4. Ecological and Economic Importance:

   • Bryophytes have limited economic importance, although some mosses provide food for herbaceous mammals, birds, and other animals.
   • Species like Sphagnum moss provide peat, historically used as fuel and packing material for trans-shipment due to their water-holding capacity.
   • Bryophytes, along with lichens, are among the first organisms to colonize rocks, playing a crucial ecological role in decomposing rocks and facilitating the growth of higher plants.
   • Mosses form dense mats on the soil, reducing the impact of falling rain and preventing soil erosion.

5. Classification:

   • Bryophytes are divided into two main groups: liverworts and mosses

3.2.1 Liverworts

1. Habitat:

   • Liverworts typically grow in moist and shady habitats, such as the banks of streams, marshy ground, damp soil, bark of trees, and deep in the woods.

2. Plant Body:

   • The plant body of a liverwort is thalloid, as seen in species like Marchantia.
   • The thallus is dorsiventral, meaning it has distinct upper and lower surfaces, and it closely adheres to the substrate.
   • Leafy liverworts have tiny leaf-like appendages arranged in two rows on stem-like structures.

3. Asexual Reproduction:

   • Asexual reproduction in liverworts occurs through fragmentation of thalli or the formation of specialized structures called gemmae (singular: gemma).
   • Gemmae are green, multicellular, asexual buds that develop in small receptacles called gemma cups located on the thalli.
   • These gemmae become detached from the parent body and germinate to form new individuals.

4. Sexual Reproduction:

   • During sexual reproduction, liverworts produce male and female sex organs, which may occur on the same or different thalli.

5. Sporophyte Development:

   • The sporophyte, the diploid phase of the life cycle, is differentiated into a foot, seta, and capsule.
   • After meiosis, spores are produced within the capsule.
   • These spores germinate to form free-living gametophytes, completing the life cycle of liverworts.

3.2.2 Mosses

1. Life Cycle:

   • The predominant stage of the moss life cycle is the gametophyte, which consists of two stages.
   • The first stage is the protonema stage, developing directly from a spore. It is a creeping, green, branched, and often filamentous stage.
   • The second stage is the leafy stage, which develops from the secondary protonema as a lateral bud. It consists of upright, slender axes bearing spirally arranged leaves.

2. Structure:

   • Mosses are attached to the soil through multicellular and branched rhizoids.
   • The leafy stage of mosses bears the sex organs, namely antheridia and archegonia.

3. Reproduction:

   • Vegetative reproduction in mosses occurs through fragmentation and budding in the secondary protonema.
   • Sexual reproduction involves the production of sex organs (antheridia and archegonia) at the apex of the leafy shoots.
   • After fertilization, the zygote develops into a sporophyte, which consists of a foot, seta, and capsule.
   • The sporophyte in mosses is more elaborate than that in liverworts and contains spores. Spores are formed after meiosis.

4. Spore Dispersal:

   • Mosses have an elaborate mechanism for spore dispersal, aiding in their reproduction and spread.

5. Common Examples:

   • Common examples of mosses include Funaria, Polytrichum, and Sphagnum.