Study Notes on Bryophytes

Bryophytes are the earliest lineages to diverge from the common ancestor of land plants.
They are classified as a paraphyletic group, consisting of three phyla of small herbaceous (non-woody), nonvascular plants:
1. Liverworts (phylum Hepatophyta): Approximately 9,000 species.
  - Word breakdown: hepato = liver, phyt = plant, wort = old English for herb.
  - Example: Lunularia cruciata.
2. Mosses (phylum Bryophyta): Approximately 12,000 species.
  - Word breakdown: bryo = moss.
  - Mosses are the most diverse and widely distributed bryophytes.
  - Note: "Bryophyta" refers solely to the mosses, while "bryophytes" refers to all nonvascular plants.
3. Hornworts (phylum Anthocerophyta): Approximately 225 species.
  - Word breakdown: antho = flower, ceros = horn.

Lack of Vascular Tissue:
- Bryophytes do not have true vascular tissue or lignin.
- This absence of vascular transport systems for water and nutrient delivery limits their size.
- Most bryophyte tissues are only one or a few cells thick.
- Lack of structural support tissues restricts growth potential.
Sporophyte Structure:
- Bryophyte sporophytes are unbranched and lack roots and leaves, unlike vascular plants which have branched sporophytes with defined leaves and roots.
- Bryophytes do not form roots; instead, they absorb water through their surfaces and are anchored by structures called rhizoids.
- This limitation confines bryophytes to habitats with abundant water.
- Sporophytes of mosses and hornworts possess stomata for gas exchange, whereas sporophytes of liverworts do not have stomata. Additionally, no extant gametophytes feature stomata.
Gametophyte Structure and Lifespan:
- In all three phyla of bryophytes, gametophytes are larger and longer-living than sporophytes.
- Bryophytes exhibit a free-living haploid (1n) gametophyte, which is the most conspicuous and dominant phase of their life cycle.
- Smaller diploid (2n) sporophytes are present only part of the time and are dependent on the gametophyte.

A spore germinates into a gametophyte (1n), which consists of:
- Protonema: The initial stage of the gametophyte.
- Gametophore: The structure that produces gametes.
Mosses often feature separate male and female gametophytes.
Fertilization Process:
- Requires water: Flagellated sperm swim through a film of water to fertilize eggs.
- Antheridia (male gametangia) produce and release flagellated sperm.
- Archegonia (female gametangia) produce an egg and serve as the fertilization site, where the egg resides within the archegonia.
- Gametophytes produce gametes via mitosis within multicellular structures known as gametangia (archegonia and antheridia are types of gametangia).

Many bryophytes can reproduce asexually. For example:
- Brood bodies: Small gametophyte plantlets that detach from the parent plant and develop into new, genetically identical individuals.
The sporophyte consists of:
- Foot: Base of the sporophyte connected to the gametophyte.
- Seta: Stalk that supports the sporangium.
- Capsule (sporangium): Produces numerous haploid spores (1n) by meiosis.
- These spores are dispersed into the air and have cell walls containing sporopollenin, which offers protection.

Bryophytes typically thrive in moist forests and wetlands, with mosses often dominating the ground cover.
- Example: The moss Sphagnum (commonly referred to as peat moss).
- Plays a vital role in regulating water flow, especially in peat bogs (muskegs) found in Arctic and boreal regions.
- Mosses can withstand drying, entering a dormant state, and then rehydrate when moisture is present.
Peat Formation:
- Sphagnum bogs lead to extensive deposits of undecayed organic matter known as peat.
- Peatlands are characterized by low temperatures, low pH, and low oxygen levels which impede decay processes.
- They serve as significant carbon reservoirs, helping to stabilize atmospheric CO2 levels.