Evolution of Land Plants: Bryophytes Notes

Evolution of Land Plants: Bryophytes

Keep track of key terms, such as life stage (whole plant), structures on a plant, and single cells. Use root words for help and the phylogeny of all plants as a roadmap.

Transition to Land

For much of Earth's history, terrestrial environments were lifeless. Protists and cyanobacteria colonized land around 1.2 billion years ago, followed by plants, fungi, and animals around 500 million years ago. Land plants now comprise over 300,000 species and are the foundation of most food chains.

Streptophytes

Streptophytes include plants and charophytes. The boundary dividing plants from algae remains debated; one definition classifies plants as embryophytes (plants with embryos).

Charophytes

Plastid DNA suggests charophytes are the closest living relatives of plants, though not necessarily direct ancestors. Comparing charophytes to distant plant relatives can provide insights into the first land plants. Shared traits include:

• Rings of cellulose-synthesizing proteins.

• Phragmoplast cell wall construction (unlike phycoplasts in chlorophytes).

• Similar flagellated sperm structure.

Adaptations to Land

Moving to land presented challenges and required adaptations:

• Multicellular Sporophyte Embryo Retention: Alternation of generations involves a multicellular sporophyte embryo retained within the "maternal tissue" (gametophyte) for nourishment.

• Gametes in Gametangia: Early plants developed prominent gametangia: antheridia (male) and archegonia (female).

• Walled Spores: Spores are protected from desiccation by sporopollenin within sporangia.

• Cuticle: A cuticle prevents water loss.

• 3D Growth and Apical Meristems: These increase exposure to environmental resources.

Plant Classification and Evolutionary Trends

Plants were traditionally grouped in ways that weren't always clades but still provided information. Land plant evolution reveals trends, one being the evolution of vascular tissue, which is an essential adaptation for water and nutrient transport.

Bryophytes: Non-Vascular Plants

Non-vascular plants, a grade rather than a clade, consist of three clades: liverworts, mosses, and hornworts.

Significance of Bryophytes

• Colonizers of bare, sandy soil.

• Form symbioses with fungi and cyanobacteria, enriching soil with nitrogen.

• Form peat, which consists of partially decayed organic material and is important to humans.

Peat can be used as fuel, and peat moss is used in agriculture. Peatlands, characterized by low temperature, pH, and oxygen levels, inhibit organic decay, cover 3% of Earth's surface, and store 30% of the world's soil carbon.

Bryophyte Morphology

Most bryophyte species are thalloid with undifferentiated bodies. The gametophyte is anchored by rhizoids, which, unlike true roots, do not transport water or minerals. The absence of vasculature limits vertical growth.

Bryophytes possess multicellular gametangia: archegonia (female) and antheridia (male).

Bryophyte Life Cycle

The bryophyte life cycle is gametophyte-dominant, with a nutritionally dependent sporophyte:

1. Haploid spores (n) germinate into protonemata (n).

2. Protonemata develop "buds" that grow into gametophores (n).

3. Gametophores produce antheridia (male) and archegonia (female).

4. Sperm fertilizes the egg within the archegonium.

5. A diploid zygote (2n) forms and develops into an embryo and then a young sporophyte (2n).

6. The sporophyte matures, consisting of a foot, seta, and capsule (sporangium).

7. Meiosis occurs within the sporangium, producing haploid spores (n) for dispersal.

Liverworts (Phylum Hepatophyta)

Liverworts exhibit the most complex gametophytes and the simplest sporophytes. An example of a thalloid liverwort is Marchantia polymorpha.

• Morphology: Thallus (undifferentiated body), Gametophore (specialized structure bearing gametangia).

• Reproduction: Can reproduce asexually via gemmae cups.

Mosses (Phylum Bryophyta)

Mosses have a more complex sporophyte with differentiated parts. In some species, the sporophyte is partially photosynthetic and has evolved stomata for gas exchange and water retention. An example is Polytrichum commune (hairy-cap moss).

Hornworts (Phylum Anthocerophyta)

Hornwort sporophytes are persistently photosynthetic, with stomata, but are not free-living. The increasing prominence, independence, and complexity of the sporophyte stage is a notable evolutionary trend.

Trend: Increasing prominence, independence, and complexity of the sporophyte stage. A clue as to where we are going next…

Bryophytes: Summary

Bryophytes, also called non-vascular plants, are grouped together because they lack homologous vascular tissue. As the earliest diverging group of land plants, they share key homologies with vascular and seed plants but also resemble green algae. Bryophytes provide insights into the evolution of water-conserving adaptations like stomata and the more complex, prominent sporophyte life stage.