Mosses and Bryophyte Life Cycle

Overview

• Topic: Bryophytes and their life cycle, focusing on moss as an example.
• Purpose: Explain defining characteristics, alternation of generations, and reproductive structures.
• Scope: Characteristics, key processes (meiosis/mitosis), gametophyte vs sporophyte, moss life cycle.

Bryophyte Key Characteristics

• Lack vascular tissue (no xylem or phloem), so cannot grow tall.
• No cuticle, so cannot prevent drying out; prefer moist, shaded habitats.
• Rely on water for reproduction; sperm must reach eggs via water movement.
• Lack true roots, stems, and leaves; possess rhizoids that anchor the plant.

Alternation Of Generations (Basic Concepts)

• Plants alternate between two multicellular generations: sporophyte (2n) and gametophyte (n).
• Sporophyte: diploid, produces spores via meiosis.
• Gametophyte: haploid, produces gametes (sperm and eggs) via mitosis.
• Mitosis: cell division that preserves chromosome number; used for growth.
• Meiosis: reduction division that halves chromosome number; produces spores/gametes.
• Zygote: result of sperm (n) + egg (n) → diploid (2n); develops into sporophyte.

Moss Life Cycle (Step-by-Step)

• Start with haploid spore released from sporangium on sporophyte.
• Spore germinates to form a protonema, a juvenile vegetative stage.
• Protonema produces rhizoids for anchoring and develops gametophytes.
• Gametophytes are haploid and often the dominant visible stage in moss.
• Separate male and female gametophytes grow near each other.
  • Male gametophyte contains antheridia (produce sperm).
  • Female gametophyte contains archegonia (produce eggs).
• Water (rain or surface water) allows sperm to swim to archegonia for fertilization.
• Fertilization produces a diploid zygote within the archegonium.
• Zygote develops into a sporophyte that grows out of the female gametophyte.
• Sporophyte consists of a stalk and terminal capsule (sporangium).
• Inside sporangium, sporocytes undergo meiosis to form haploid spores.
• Spores are released and dispersed (wind or water) to begin a new cycle.

Reproductive Structures And Definitions

• Sporangium: capsule at sporophyte tip where spores are produced.
• Sporocyte: diploid cell in sporangium that undergoes meiosis to form spores.
• Spore: haploid reproductive cell that develops into gametophyte.
• Protonema: early filamentous/growing stage from spore germination.
• Rhizoid: anchoring filamentous structure (not true root).
• Antheridium: male gametangium producing sperm.
• Archegonium: female gametangium producing egg (analogous to an ovary).
• Zygote: diploid cell formed by fusion of sperm and egg.

Comparison: Gametophyte vs Sporophyte (Moss Emphasis)

Important Terms And Processes (Summary)

• Vascular Tissue: xylem and phloem; absent in bryophytes.
• Cuticle: waxy protective layer; absent in bryophytes.
• Mitosis: growth division, maintains chromosome number.
• Meiosis: reduction division, halves chromosome number to form spores/gametes.
• Haploid (n): one set of chromosomes; gametophyte phase.
• Diploid (2n): two sets of chromosomes; sporophyte phase.
• Fertilization: union of sperm and egg to form zygote.
• Spore Dispersal: spores released from sporangium; dispersed by wind or water.

Notes And Observations Specific To Moss

• Mosses commonly found in moist, shaded areas due to lack of cuticle and water-needed reproduction.
• Unique trait: sporophyte grows physically out of and depends on the gametophyte.
• Moss life cycle emphasizes the visible gametophyte stage, unlike many other plant groups.

Action Items / Next Steps (If Studying)

• Review definitions of meiosis vs mitosis and haploid vs diploid states.
• Draw a labeled moss life cycle showing gametophyte and sporophyte phases.
• Identify antheridium and archegonium on a moss specimen or diagram.
• Compare bryophyte life cycle with that of vascular plants (to note differences).