Classification of Division Bryophyta

Classification of Division Bryophyta

Bryophytes are divided into three subdivisions:

  • Hepaticopsida
  • Bryopsida
  • Anthoceropsida
Hepaticopsida (Liverworts)

Bryophytes belonging to this subdivision are called liverworts. It includes about 900 species. Liverworts are the simplest of all bryophytes. They are normally found on moist rocks and on wet soil. Since they live near water therefore possibilities of drying are significantly reduced.

Hepaticopsida

Characteristics of Hepaticopsida
  • The plant body is a gametophyte. It may be thalloid i.e., flat, or ribbon-like, normally dichotomously branched. It is attached to the soil by means of rhizoids e.g., Marchantia. Other species tend to grow upright and are falsely leafy i.e., differentiated into a false stem, and leaves e.g., Porella. The sporophyte depends on the gametophyte for nutrition and protection.
  • The sex organs establish on the upper surface of the thallus near the tips of the branches. Sometimes they establish unique branches on gametophyte called the antheridiophores and the archegoniophore as in Marchantia.
Bryopsida

Like liverworts, most mosses inhabit damp places. In contrast to other bryophytes, they grow equally well in relatively dry places. However, water is essential in the reproduction of mosses, hence they normally grow to form cushions or mats.

Characteristics of Bryopsida
  • Each adult moss plant, a gametophyte, is always differentiated into structures that look like stems and leaves. Multicellular rhizoids are likewise present. Examples of mosses are Funaria and Polytrichum. Archegonia and antheridia, establish on the tips of different branches on the same plant e.g., Funaria, or on different plants as in Polytrichum. The archegonia and antheridia form clusters and are mixed with sterilized hairs, the paraphyses.
  • Formation of diploid sporophyte and haploid spores follow the very same series of occasions of alternation of generations as in liverworts. However, the spore of a moss, unlike that of liverworts, develops into an alga like structure, the protonema. Haploid moss plants (gametophyte) develop from buds on the protonema and the life cycle is completed.
Anthoceropsida (Horn Worts)

This group of bryophytes differs in lots of respects and is slightly advanced than Bryopsida and Hepaticopsida. The gametophyte is extremely lobed and irregular in an overview. Except for a little early stage of development, the sporophyte is not dependent upon gametophyte for nutrition and protection. Antheridia and archegonia are partly sunken in the gametophytic tissue.

Anthoceropsida

Characteristics of Anthoceropsida

The sporophyte exhibit lots of advanced characters due to which it can flourish better on land as compared to other groups. The sporophyte has stomata and chloroplasts in the epidermis and can thus photosynthesize its own food rather than obtaining it from the gametophyte. It likewise has a waxy cuticle to prevent excessive loss of water (desiccation). Furthermore, at the junction of the foot and spore-producing region, there is a band of meristematic tissue.

This tissue keeps including cells towards the spore-producing region throughout the formation, maturation, and dispersal of spores from the opposite end. Due to the fast growth rate of this meristematic tissue the sporophyte keeps increasing in length for an indefinite amount of time. Due to these characters, the sporophyte continues to make it through as such even after the death and decay of the gametophyte.

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One fine example of Anthoceropsida is Anthoceros which is also found in the hilly areas of Pakistan.

Alternation of generations

In the life history of liverworts, mosses, and hornworts there are two unique multicellular phases or generations. These generations are haploid gametophyte and diploid sporophyte, which regularly alternate with each other. The gametophyte is the dominant generation due to the fact that it is more conspicuous. It produces gametes called spermatozoids or antherozoids and eggs, therefore called gamete-producing generation. A haploid spermatozoid fuses with a haploid egg to produce a diploid oospore.

Alternation-of-genera

The oospore does not produce the gametophyte directly but produces an absolutely different plant called the sporophyte. The sporophyte in bryophytes is a less conspicuous generation, which is normally differentiated into foot, seta, and capsule (likewise called sporogonium). Spores develop within the capsule by reduction division (meiosis) from spore mother cells. The sporophyte produces spores and is, therefore, called spore-producing generation. The spore on germination does not develop into a sporophyte however generates the gametophyte.

Thus, in the life-history of a bryophytic plant, the two generations, the gametophyte, and the sporophyte, regularly alternate with each other. The phenomenon of alternation of gametophyte and sporophyte in the biography of a plant is called alternation of generations. It needs to be kept in mind that the gametophyte or haploid stage begins with spores and ends at gametes, whereas the sporophyte begins with oospore and ends at the spore mother cell.

The significance of alternation of generations

Throughout the formation of spores from spore mother cells by meiotic division reshuffling of genes takes place. As a consequence, a great range of spores with different genes are produced. These spores in turn produce gametophytes with different hereditary combinations.

The gametophytes with much better heredity will have a better opportunity for survival in the environment where they take place. On the other hand, the gametophytes with less beneficial attributes will be eliminated. There is no reshuffling of genes during gametogenesis in the gametophyte as gametes are produced after mitosis. The oospore developing after fertilization now has new genetics as compared to the parent.

This hereditary variation passes to the new sporophyte which on maturity once again produces additional genetic recombination which is moved to the gametophyte. In this natural process, the sporophyte, therefore, offers a big amount of genetic variability and nature selects the very best genetic combinations. In the long run, this will enable the populations to end up being significantly much better adapted to their environment.

Multiple Choice Questions (MCQs) with Answers

  1. Which of the following is not a subdivision of Division Bryophyta?
    • A) Hepaticopsida
    • B) Lycopodiopsida
    • C) Bryopsida
    • D) Anthoceropsida
    • Answer: B
  2. Liverworts, belonging to the Hepaticopsida subdivision, are characterized by:
    • A) Highly developed stems and leaves
    • B) Thalloid or ribbon-like gametophyte
    • C) Independence of sporophyte from gametophyte
    • D) Dry habitat preference
    • Answer: B
  3. What is the primary habitat for liverworts (Hepaticopsida)?
    • A) Desert
    • B) Moist rocks and wet soil
    • C) Alpine regions
    • D) Arid plains
    • Answer: B
  4. Mosses, belonging to Bryopsida, differ from other bryophytes in their ability to:
    • A) Survive in arid conditions
    • B) Reproduce without water
    • C) Grow in relatively dry places
    • D) Form extensive root systems
    • Answer: C
  5. Anthoceropsida (Hornworts) exhibit advanced characteristics, including:
    • A) Dependence of sporophyte on gametophyte
    • B) Lack of stomata and chloroplasts in the sporophyte
    • C) Presence of waxy cuticle for water prevention
    • D) Attachment to the soil by rhizoids
    • Answer: C
  6. In Anthoceropsida, the gametophyte is described as:
    • A) Highly lobed and irregular
    • B) Thalloid and ribbon-like
    • C) Dependent on sporophyte for nutrition
    • D) Found in dry places
    • Answer: A
  7. What is the primary significance of alternation of generations in bryophytes?
    • A) Increased competition for resources
    • B) Maintenance of genetic variability
    • C) Accelerated growth of sporophyte
    • D) Prevention of gamete production
    • Answer: B
  8. The dominant generation in the life history of bryophytes is:
    • A) Gametophyte
    • B) Sporophyte
    • C) Zygote
    • D) Gamete
    • Answer: A
  9. The term “antheridiophores” refers to structures in:
    • A) Liverworts
    • B) Mosses
    • C) Hornworts
    • D) Seed plants
    • Answer: A
  10. The structure responsible for reshuffling genes during bryophyte reproduction is:
    • A) Rhizoids
    • B) Sporophyte
    • C) Stomata
    • D) Spore mother cells
    • Answer: D
  11. Which bryophyte subdivision exhibits false stems and leaves?
    • A) Hepaticopsida
    • B) Bryopsida
    • C) Anthoceropsida
    • D) Lycopodiopsida
    • Answer: A
  12. The protonema is associated with the life cycle of:
    • A) Liverworts
    • B) Mosses
    • C) Hornworts
    • D) Ferns
    • Answer: B
  13. Bryophytes are known for their adaptation to:
    • A) Harsh deserts
    • B) Alpine meadows
    • C) Moist environments
    • D) Sandy beaches
    • Answer: C
  14. In Anthoceropsida, what prevents excessive loss of water in the sporophyte?
    • A) Stomata
    • B) Rhizoids
    • C) Waxy cuticle
    • D) Chloroplasts
    • Answer: C
  15. The phenomenon where gametophyte and sporophyte alternate in the life cycle is known as:
    • A) Alternation of generations
    • B) Gametogenesis
    • C) Meiosis
    • D) Mitosis
    • Answer: A
  16. The genetic reshuffling during spore formation leads to:
    • A) Decreased genetic variability
    • B) Increased susceptibility to diseases
    • C) Enhanced genetic variability
    • D) Clonal reproduction
    • Answer: C
  17. Which bryophyte subdivision has a gametophyte that can photosynthesize its own food?
    • A) Hepaticopsida
    • B) Bryopsida
    • C) Anthoceropsida
    • D) Gymnosperms
    • Answer: C
  18. The oospore in bryophytes develops into:
    • A) Gametophyte
    • B) Sporophyte
    • C) Rhizoid
    • D) Antheridium
    • Answer: B
  19. What structure is responsible for the growth of the sporophyte in Anthoceropsida?
    • A) Rhizoids
    • B) Stomata
    • C) Meristematic tissue
    • D) Chloroplasts
    • Answer: C
  20. The term “paraphyses” is associated with:
    • A) Liverworts
    • B) Mosses
    • C) Hornworts
    • D) Vascular plants
    • Answer: B
  21. The survival advantage of genetic variability in bryophytes is related to:
    • A) Gametophyte structure
    • B) Spore dispersal
    • C) Meristematic tissue
    • D) Rhizoid development
    • Answer: B
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Frequently Asked Questions (FAQs) Related to Bryophyte Classification and Alternation of Generations

1. What are the main subdivisions of Division Bryophyta?
  • Answer: Bryophytes are divided into three subdivisions: Hepaticopsida (Liverworts), Bryopsida (Mosses), and Anthoceropsida (Hornworts).

2. How many species are included in the subdivision Hepaticopsida (Liverworts)?

  • Answer: Hepaticopsida includes about 900 species of liverworts.

3. Where are liverworts commonly found?

  • Answer: Liverworts are normally found on moist rocks and wet soil, often in close proximity to water bodies.

4. What are the characteristics of Hepaticopsida?

  • Answer: Hepaticopsida has a gametophyte body, which may be thalloid or ribbon-like, and is attached to the soil by rhizoids. Sex organs are located on the upper surface of the thallus.

5. How do mosses (Bryopsida) differ from other bryophytes?

  • Answer: Mosses can grow equally well in relatively dry places, unlike other bryophytes. However, water is essential for their reproductive processes.

6. Describe the characteristics of Bryopsida.

  • Answer: Bryopsida (Mosses) have a differentiated gametophyte with structures resembling stems and leaves. They form multicellular rhizoids, and archegonia and antheridia are located on the tips of branches.

7. What is the unique structure formed by the spore of a moss during its development?

  • Answer: The spore of a moss develops into an alga-like structure called the protonema.

8. How does Anthoceropsida (Hornworts) differ from other bryophytes?

  • Answer: Anthoceropsida has an irregularly lobed gametophyte, and the sporophyte is not dependent on the gametophyte for nutrition and protection.
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9. What advanced characteristics does the sporophyte of Anthoceropsida exhibit?

  • Answer: The sporophyte of Anthoceropsida has stomata, chloroplasts for photosynthesis, a waxy cuticle to prevent water loss, and a band of meristematic tissue for continuous growth.

10. How does alternation of generations occur in bryophytes?

  • Answer: Bryophytes exhibit two multicellular phases or generations – haploid gametophyte and diploid sporophyte, which alternate in their life cycle.

11. What is the significance of alternation of generations in bryophytes?

  • Answer: Alternation of generations results in genetic reshuffling during spore formation, leading to a wide range of genetic combinations. This genetic variability enhances adaptation to the environment.

12. How do the gametophytes contribute to genetic diversity in bryophyte populations?

  • Answer: Gametophytes with better heredity have a higher chance of survival, leading to the elimination of less favorable attributes. This natural selection process ensures genetic diversity.

13. What happens during the formation of spores in bryophytes?

  • Answer: During spore formation, reshuffling of genes occurs through meiotic division, producing spores with different genetic combinations.

14. How does the genetic variation pass from one generation to another in bryophytes?

  • Answer: Genetic variation passes from the sporophyte to the gametophyte, providing a large amount of genetic variability that nature selects for better adaptation over time.

Summary

The tutorial on the Classification of Division Bryophyta covers the three main subdivisions: Hepaticopsida (Liverworts), Bryopsida (Mosses), and Anthoceropsida (Hornworts). Liverworts, the simplest of bryophytes, thrive in moist environments, while mosses exhibit unique characteristics such as cushions or mats. Anthoceropsida, more advanced, features a sporophyte with stomata and chloroplasts.

Key Points:

  1. Hepaticopsida (Liverworts):
    • Liverworts, with about 900 species, have thalloid or ribbon-like gametophytes.
    • Characteristics include rhizoids, antheridiophores, and archegoniophores.
  2. Bryopsida (Mosses):
    • Mosses adapt to both damp and dry places, forming differentiated gametophytes with stems and leaves.
    • Moss spores develop into protonema, leading to the completion of the life cycle.
  3. Anthoceropsida (Hornworts):
    • Anthoceropsida, slightly advanced, has an irregular gametophyte and a sporophyte with advanced features.
    • Characteristics include stomata, chloroplasts, a waxy cuticle, and a meristematic tissue band.
  4. Alternation of Generations:
    • Bryophytes exhibit alternation of haploid gametophyte and diploid sporophyte generations.
    • The dominant gametophyte produces gametes, leading to the formation of a diploid oospore.
    • The oospore develops into a sporophyte, completing the life cycle.
  5. Significance of Alternation of Generations:
    • Genetic reshuffling during spore formation results in diverse genetic combinations.
    • Gametophytes with favorable traits survive, contributing to population adaptation.
    • Sporophytes provide genetic variability, enhancing adaptation to the environment.

The tutorial emphasizes the importance of alternation of generations in the evolutionary success of bryophytes, showcasing their unique life cycle and genetic diversity. Further readings are recommended for a deeper understanding of related topics.