Unit 4: Bryophytes

Unique characters of archegoniates and alternation of generation

Archegoniates

Archegoniates: A group of plants (Bryophytes, Pteridophytes, and Gymnosperms) that possess a multicellular, flask-shaped female reproductive organ called an archegonium.

Unique Characters of the Archegonium:

• It is a multicellular, flask-shaped structure.
• It consists of a swollen basal venter (which contains the single, non-motile egg cell) and a long, slender neck.
• The venter and neck are surrounded by a sterile jacket of cells. This sterile jacket is a key evolutionary adaptation to protect the gamete from desiccation on land.
• When mature, the neck canal cells degenerate, creating a tube for the male gametes (sperm) to swim down to the egg.

Alternation of Generation

This describes a life cycle in which a plant has two distinct, multicellular "generations" or phases:

1. The Gametophyte Generation:
   • Is haploid (n).
   • Produces gametes (n) (sperm and egg) by mitosis.
   • This is the dominant, photosynthetic, independent phase in Bryophytes.
2. The Sporophyte Generation:
   • Is diploid (2n).
   • Develops from the zygote (2n), which is formed by the fusion of gametes.
   • Produces spores (n) by meiosis.
   • This is the dependent (parasitic) phase in Bryophytes, remaining attached to the gametophyte.

The haploid spores germinate to form a new gametophyte, completing the cycle. This is a heteromorphic alternation of generations because the two generations (gametophyte and sporophyte) look different.

Bryophytes - General characteristics

Bryology: The study of bryophytes.

Bryophytes (e.g., mosses, liverworts, hornworts) are the simplest and most primitive non-vascular land plants.

• "Amphibians of the Plant Kingdom": They are terrestrial plants but require external water (rain, dew) for sexual reproduction, as their male gametes (antherozoids) are flagellated and must swim to the egg.
• Dominant Phase: The gametophyte (n) is the dominant, independent, photosynthetic, and long-lived phase of the life cycle.
• Dependent Phase: The sporophyte (2n) is simple, short-lived, and partially or wholly dependent (parasitic) on the gametophyte for nutrition.
• Vascular Tissue: They lack true vascular tissues (xylem and phloem). Water is conducted by simple parenchymatous cells (or hydroids in some mosses).
• True Roots/Stems/Leaves: They lack true roots, stems, and leaves.
  • Roots: Anchored by unicellular or multicellular hair-like structures called rhizoids.
  • Thallus: The plant body can be a flat thallus (liverworts, hornworts) or a "leafy" axis (mosses).
• Reproduction:
  • Vegetative: Common, by fragmentation or specialized structures called gemmae.
  • Sexual: Oogamous. Reproductive organs (antheridia and archegonia) are multicellular and protected by a sterile jacket.

Adaptation to land habit

Bryophytes are the first land plants and show several key adaptations (though incomplete) to survive a terrestrial environment:

• Multicellular Plant Body: A more complex body than algae.
• Protective Sterile Jacket: The sex organs (antheridia, archegonia) are covered by a layer of sterile cells to protect the gametes from drying out.
• Retention of Zygote: The zygote is retained and develops *inside* the archegonium (on the female gametophyte), where it is protected and nourished. This is the beginning of the "embryo."
• Cuticle (in some): A waxy layer on the epidermis to reduce water loss (though often thin).
• Pores/Stomata: Liverworts have simple pores, while hornworts and mosses have functional stomata on their sporophyte for gas exchange.
• Rhizoids: Developed for anchorage and some water/mineral absorption.
• Spore Dispersal: Spores are produced in a sporangium (capsule) on the sporophyte, are covered in a tough wall, and are dispersed by wind, a key terrestrial adaptation.

Classification (upto orders)

The Division Bryophyta is traditionally divided into three classes:

1. Class 1: Hepaticopsida (Liverworts)
   • Gametophyte is thalloid or foliose (leafy).
   • Rhizoids are unicellular and unbranched.
   • Sporophyte is simple, with no stomata, and short-lived.
   • Orders: Marchantiales, Sphaerocarpales, Jungermanniales.
2. Class 2: Anthocerotopsida (Hornworts)
   • Gametophyte is a simple, thalloid body.
   • Rhizoids are unicellular and unbranched.
   • Sporophyte is a unique, horn-like, long-lived capsule that grows continuously from a basal meristem. It has stomata.
   • Order: Anthocerotales.
3. Class 3: Bryopsida (Mosses)
   • Gametophyte is differentiated into a protonema (juvenile stage) and a "leafy" gametophore (adult stage).
   • Rhizoids are multicellular and branched.
   • Sporophyte is complex, differentiated into foot, seta, and capsule. The capsule has stomata and a sophisticated spore dispersal mechanism (peristome).
   • Orders: Sphagnales, Andreaeales, Polytrichales, Bryales.

Evolution of sporophyte

The sporophyte (2n) in bryophytes shows a clear evolutionary trend of increasing complexity and sterilization, from a simple spore-producing sac (like in *Marchantia*) to a complex structure (like in *Polytrichum*).

The theory is that as the sporophyte evolved, more of its tissue became sterile (non-spore-producing) and was dedicated to other functions, such as photosynthesis, support (seta), and complex spore dispersal (peristome).

1. Simple Sporophyte (e.g., *Marchantia*):
   • A simple capsule with a small foot and seta.
   • Almost the entire internal mass is fertile (produces spores).
   • A few sterile cells are present as elaters (which help in spore dispersal).
2. Intermediate Sporophyte (e.g., *Anthoceros*):
   • More complex. It is a long, horn-like structure with a foot.
   • Has a basal meristem, allowing it to grow indefinitely.
   • Has a large, sterile central column called the columella.
   • Has a photosynthetic epidermis with stomata. It is semi-independent.
   • Fertile (spore) tissue is a thin layer between the columella and the wall.
3. Complex Sporophyte (e.g., *Polytrichum*):
   • Highly complex, differentiated into foot, a long seta, and a capsule.
   • The capsule has a complex internal structure with a large columella, photosynthetic tissue, and an air space.
   • It has a highly specialized spore dispersal mechanism called the peristome (a ring of "teeth").
   • A large portion of the sporophyte is sterile and dedicated to support, photosynthesis, and dispersal.

Structure, morphology and life cycle of:

Marchantia (Class: Hepaticopsida)

• Gametophyte (n):
  • Morphology: A flat, prostrate, dichotomously branched, dorsiventral thallus.
  • Dorsal Side: Has a central midrib and diamond-shaped air chambers, each opening by a single air pore.
  • Ventral Side: Bears unicellular rhizoids (for anchorage) and multicellular scales (for protection).
  • Reproduction (Vegetative): By fragmentation or by gemmae – small, asexual buds produced in gemma cups on the dorsal surface.
  • Reproduction (Sexual): Plants are dioecious (separate male and female).
    • Male Plant: Bears sex organs on a stalked, star-shaped disc called an antheridiophore. Antheridia are embedded in the disc.
    • Female Plant: Bears sex organs on a stalked, umbrella-shaped disc called an archegoniophore. Archegonia hang down from the disc.
• Sporophyte (2n):
  • Fertilization: Raindrops splash sperm from the antheridiophore to the archegoniophore. Sperm swim to the archegonia, and one fertilizes the egg, forming a diploid (2n) zygote.
  • Structure: The zygote develops into a simple sporophyte, which remains attached to the female gametophyte. It is differentiated into a foot (anchorage), a short seta (stalk), and a globular capsule.
  • Spore Production: Inside the capsule, spore mother cells (2n) undergo meiosis to produce haploid (n) spores. Sterile cells develop into elaters (long, spiral-thickened cells).
  • Dispersal: The capsule ruptures, and the elaters twist with changes in humidity, flicking the spores out. The spores germinate to form a new gametophyte (n).

Anthoceros (Class: Anthocerotopsida)

• Gametophyte (n):
  • Morphology: A simple, prostrate, dorsiventral thallus (rosette-like). It is not dichotomously branched.
  • Structure: Thallus is simple, with no internal air chambers. Each cell has a single, large chloroplast with a pyrenoid (an algal character). Often has internal cavities filled with *Nostoc* (for nitrogen fixation).
  • Reproduction (Sexual): Plants are typically monoecious. The sex organs (antheridia and archegonia) are embedded (sunken) within the dorsal surface of the thallus.
• Sporophyte (2n):
  • Structure: After fertilization, the zygote (2n) develops into a unique, horn-like sporophyte. It consists of a bulbous foot (embedded in the thallus) and a long, cylindrical capsule. There is no seta.
  • Growth: A basal meristem (between the foot and capsule) allows the sporophyte to grow continuously for a long time.
  • Internal Structure: The capsule has a central sterile columella, a photosynthetic epidermis with functional stomata, and a surrounding layer of fertile tissue that produces spores (n) (from meiosis) and pseudo-elaters (simple, multicellular sterile cells).
  • Dispersal: The capsule matures from the tip downwards, splitting open to release spores.

Polytrichum (Class: Bryopsida)

• Gametophyte (n):
  • Morphology: This is an advanced "leafy" moss. It consists of an erect, central "stem" (axis) bearing spirally arranged, "leafy" structures (phyllids).
  • Structure: The "leaves" are complex, with a broad midrib that has vertical plates of photosynthetic cells (lamellae) on its upper surface. The "stem" has some internal differentiation (central conducting strand). Anchored by multicellular, branched rhizoids.
  • Reproduction (Sexual): Plants are dioecious. Sex organs are borne at the tips of the main shoots.
    • Male Plant: Antheridia are in a "splash cup" formed by a rosette of modified leaves.
    • Female Plant: Archegonia are in a cluster at the tip.
• Sporophyte (2n):
  • Stru