Lecture on Bryophytes and Pterophytes

Domain Eukarya

Kingdom Plantae

The Bryophytes

  • Subdivisions:

    • Marchantiophyta (liverworts)

    • Bryophyta (mosses)

    • Anthocerotophyta (hornworts)

Overview of Bryophytes

  • Bryophytes:

    • Earliest diverging groups of plants, similar to the first land plants.

    • Typically found in wet environments due to dependence on standing water.

  • Height:

    • Tallest extant bryophyte: Dawsonia superba Grev., max height of 50 cm.

  • Lack of structural features:

    • No roots or complex vascular tissue, which conveys water and nutrients and provides support for taller growth in other plants.

Life Cycle of Bryophytes

  • Alternation of generations:

    • Cycle consists of a diploid sporophyte (2n) and a haploid gametophyte (n).

    • Dominant generation: Gametophyte, indeterminate; Sporophyte is determinate and nutritionally dependent on gametophyte.

  1. Diploid sporophyte (2n) undergoes meiosis to produce haploid spores (n).

  2. Haploid spores germinate and go through mitosis to develop into haploid thallus (n), the gametophyte.

    • Gametophyte can reproduce asexually using gemmae or sexually via gametangiophores.

  3. Fertilization:

    • Sperm fertilizes egg producing a diploid zygote (2n).

    • Zygote divides by mitosis, forming a new sporophyte.

  • Male gametangiophore:

    • Called antheridiophore; consists of a stalk with a flat-topped head (antheridial head) containing antheridia filled with flagellated sperm.

    • Sperm are washed off by rain to the ground.

  • Female gametangiophore:

    • Called archegoniophore; has a stalk and a head with drooping lobes where archegonia contain a single haploid egg.

    • Rainwater splashes sperm to the archegonia, fostering fertilization.

  • Formation of the sporophyte:

    • The sporophyte features a large sporangium at its apex, where meiosis occurs to produce haploid spores that disperse to germinate the next generation of gametophytes.

Marchantia Thallus

Division Marchantiophyta (Liverworts)

Anatomy

  • Haploid Gametophyte (n):

    • Thallus grows flat on moist ground, multi-cellular thick.

    • Features gas exchange pores on the top surface and air spaces within.

    • Absorbs water throughout its surface and has below-ground structures for anchoring (scales and rhizoids).

Male Reproductive Structures of Marchantia

Antheridiophore

  • Grows from thallus; contains antheridia (sperm chambers) that open at the head.

Female Reproductive Structures of Marchantia

Archegoniophore

  • The structure holds archegonia which are vase-shaped with openings facing down.

  • Each archegonium contains one egg.

Diploid Sporophyte

Development and Structure

  • Diploid sporophyte (2n) is encased by the calyptra (enlarged archegonium).

  • Dependent on gametophyte for nutrients and water.

  • Structure consists of:

    • A foot, attaching to gametophyte.

    • Stalk (seta).

    • Egg-shaped sporangium, where haploid spores are formed through meiosis.

Asexual Reproduction in Marchantia

  • Contains gemmae cups with gemmae (tiny green discs).

  • Gemmae are ejected by raindrops for dispersal and result in genetically identical offspring (clones).

Division Bryophyta - Mosses

Overview of Mosses

  • Archegonia are located at the upper end of the moss with tubular openings facing upwards.

  • Fertilization leads to the development of sporophytes from these structures.

  • Antheridia present on flat tips of male moss stems, surrounded by leaf-like scales.

  • Sporophyte grows directly atop female moss, depending on the gametophyte for survival.

Division Pterophyta - Ferns and Allied Divisions

Comparison with Bryophytes

  • Ferns contain both xylem and phloem, enabling increased height and complexity compared to bryophytes.

  • Gametophyte generation is smaller, but remains dependent on water for fertilization.

  • Ferns are known for thriving in disturbed environments.

Other Divisions of Primitive Vascular Plants

  • Division Sphenophyta: Horsetails (e.g., Equisetum)

  • Division Lycophyta: Club mosses (e.g., Lycopodium, Selaginella)

  • Division Psilophyta: Whisk ferns (e.g., Psilotum)

Alternation of Generations in Pterophytes

  • The diploid sporophyte (2n) represents the fern plant.

  • Structure consists of root, shoot, and fronds (upright leaves).

  • May include a rhizome, a horizontally growing stem that produces roots.

Vascular Tissue Characteristics

  • Xylem: Transports water from the roots.

  • Phloem: Transports nutrients from leaves to other plant parts.

Fern Sporophyte

  • Produces sporangia where meiosis occurs to create haploid spores.

  • Sori (clusters of sporangia) are found on the leaf underside, visually recognized by rusty brown rosettes.

  • Leaf anatomy:

    • Upper and lower epidermis function as protective layers.

    • Mesophyll consists of:

    • Palisade mesophyll: maximizes light absorption.

    • Spongy mesophyll: facilitates gas exchange.

Sorus Structure

  • Each sorus features a central axis where sporangia attach, covered by an indusium.

  • Each sporangium is encircled by an annulus, which aids in spore dispersal when it dries out.

Gametophyte Characteristics

  • Haploid spores develop into a haploid prothallus (n), photosynthetic and heart-shaped.

  • Rhizoids provide anchorage, and antheridia produce flagellated sperm.

  • Upon fertilization, a diploid zygote (2n) is formed that grows into a new sporophyte.

Young Sporophyte Development

  • The young sporophyte has a primary root that grows downward and a primary leaf that points upward.

  • Contains a bifurcating vascular bundle characteristic of sporophytes, distinguishing it from gametophytes.