Bryophyta

Introduction

The phylum Bryophyta is a group of ~12,000-14,000 species of non-vascular plants represented by the mosses, the liverworts, and the hornworts, belonging to the sub-kingdom Embryophyta. These are non-woody herbaceous plants with a relatively small and simple body structure. Famously referred to as the “amphibians of the plant kingdom”, these plants hold significant ecological and economic importance.

General features of Bryophytes

• Bryophytes are the “amphibians of the plant kingdom” because they require a lot of moisture and humidity for their growth

• Body organization may be leafy and thalloid.

• The leafy forms have simple leaf-like “phyllids” and stems, without any vascular tissues. They represent the gametophyte.

• The thalloid forms grow prostrate on the ground. The thalli may be just one cell thick in some species or several layers thick in others.

• Plant bodies are attached to the substratum via rhizoids.

• Vascular tissues are absent except in some mosses

• Water absorption occurs via capillary action

• They lack true leaves and roots. However, structures that are functionally equivalent to leaves are present

• The dominant phase of bryophytes’ life cycle is the gametophyte

• The sporophyte has a foot, a seta and a capsule that contains spores. The sporophyte is short-lived and partially/ fully dependent on the gametophyte.

• Spores may contain elaters, which help in dispersal

• The female sex organ is the flask-shaped archegonium. Bryophytes were evolutionarily the first plants to develop the archegonium.

• The male sex organ is called the antheridium, an ellipsoidal structure. Sperms are biflagellate and whiplash type

• Fertilisation of the egg occurs in water, resulting in a zygote which undergoes successive divisions to form the embryo

Classification of Bryophyta

Liverworts (Hepatophyta/ Marchantiophyta)

• About 9,000 species

• The name refers to the liver-like shape of the thallus

• Gametophytes are leafy (e.g., Schistocila balfouriana)or thalloid (e.g., Marchantia)

• Thallus often differentiated into photosynthetic cells, storage cells and air chambers

• The upper surface of the thallus comprises photosynthetic cells, while the lower surface harbours unicellular rhizoids that help in anchorage to substratum

• The sporophyte consists of a capsule that contains the spores. The leafy liverworts contain a stalk called seta, the terminal end of which carries the capsule. Elaters are present.

• Sexual reporduction involves the archegonia and the antheridia. Fertilization occurs in water.

• Asexual reproduction occus via fragmentation and gemmae in gemmae cups

  Examples − Marchantia, Riccia, Scapania, Pellia, etc

Hornworts (Anthocerotophyta)

• About 350 species known

• Name refers to horn-like shape of the sporophytic generation

• Comprises only one order, namely Anthocerotales.

• Gametophyte is dorsiventral and thalloid, often rosette-like

• Smooth-walled rhizoids aid in anchorage to substrata.

• Plant body is undifferentiated, and does not contain air chambers

• Antheridia and archegonia are sunken on the dorsal side of the thallus

• The sporophyte is a linear sporogonium that arises at the dorsal side of tahllus, from a basal meristem. Pseudoelaters are present.

• Asexual reproduction occurs via fragmentation, gemmae, tubers, and apospory and persistent growing apices.

• Sexual reproduction occurs via fertilization of sperms released by antheridia and eggs released by archegonia. The thallus may be dioecious or monoecious, depending on species.

  Examples − Anthoceros. Dendroceros, Megaceros, Phaeoceros and Notothylas

True Mosses (Bryopsida)

• About 15,000 species

• Gametophytes consist of spirally arranged leaves which are unistratose. The protonema stage precedes the leafy stage.

• Rhizoids are multicellular, tuberculate and branched

• The gametophyte may be monoecious or dioecious

• Sporophyte consists of foot, seta and capsule that has an operculum

• Elaters are absent

  Examples − Funaria, Sphagnum, Polytrichum, etc

Ecological Importance of Bryophyta

• The dense growth of bryophytes helps prevent soil erosion

• Mosses have been found to increase the content of nitrogen in the soil. For example, Pleurozium forms a symbiotic association with nitrogen-fixing cyanobacteria, which help increase the concentration of nitrogen in the soil.

• Peatlands harbour up to one-third of the world's total carbon. This implies that the rising use of peat as fuel also contributes to global warming.

• Given their water-holding capacities, peat mosses are used in maintaining soil moisture levels as well.

• Mosses are the pioneer species during ecological succession- they are the first land plants that colonized rocks and barren land.

Economic Importance of Bryophyta

• Peat moss, i.e., Sphagnum is an economically important species of Bryophyta. This moss, along with liverworts, is used as a source of fuel in many European and Asian countries.

• Peat moss is also the source of the organic matter known as peat, which is widely used in agriculture as a soil conditioner and organic fertilizer.

• Peat moss has excellent water holding capacity, allowing its use as a packaging material for the shipment of living material.

• Some bryophyte species such as Marchantia, Riccia, Anthoceros, etc are widely used in the treatment of diseases such as hepatic disorders, pulmonary tuberculosis, kidney and gall bladder stones, and also as antimicrobials and antipyretics.

Life Cycle of Bryophytes

Like all other plants, the sexual life cycle of bryophytes exhibits the phenomenon of alternation of generation, whereby the plant body alternates between the gametophytic form (haploid), which produces gametes and the sporophytic form (diploid) which produces spores via meiosis.

The haploid spores thus formed begin the gametophytic generation. The spore germinates into a gametophyte, which bears both the sex organs, i.e., the antheridia (produces sperm) and the archegonia (produces eggs).

The sperms are ultimately released from the antheridia into the surrounding water, wherein they swim to get to the archegonia. This movement is facilitated by the chemotactic substances released by the archegonium.

Fertilisation results in the formation of a diploid zygote, which marks the beginning of the gametophytic generation. The zygote undergoes multiple mitotic divisions to form the multicellular embryo.

Conclusion

• Bryophytes are also referred to as the amphibians of the plant kingdom.

• They are nontracheophytes that comprise the liverworts, hornworts and the mosses

• They exhibit alternation of generation, with the gametophyte being the dominant phase of the life cycle

• They may undergo asexual reproduction via fragmentation, gemmae, etc

• Sexual reproduction occurs by fertilisation of the archegonia with sperm released from the antheridia. Fertilisation is external and requires water

• Bryophytes hold ecological and economic importance, as they are used as fuels and for improving soil fertility.

FAQs

Q1. Why bryophytes are called nontracheotype plants?

Ans. Some bryophytes possess a strand of cells for conducting water and nutrients. Hence, nontracheotype instead of nonvascular is a more suitable term.

Q2. Humans also possess a haploid stage (gametes). Is it correct to say that humans exhibit alternation of generations as well?

Ans. No. This is because the term alternation of generation applies only to those life cycles in which both, the haploid and the diploid stages are multicellular. The haploid stage of humans is represented only by single-celled gametes.

Q3. Which bryophyte does not contain chlorophyll? How does it get its nutrition?

Ans. Cryptothallus lacks chlorophyll and depends on a fungal species for its food requirements.

Q4. Which group of plants lack stomata?

Ans. Liverworts

Q5. Anthoceros is known to form a symbiotic association with which microbe?

Ans. Nostoc, a blue-green alga, is found in the mucilage cavities of the thallus.