Green Algae and Land Plant Evolution

Phylogenetic Relationships and Evolutionary Origins

Green algae and land plants are photosynthetic eukaryotes that share a common ancestor. Land plants likely evolved from aquatic ancestors, specifically from green algae, which are aquatic protists. The evolutionary tree shows two main lineages of green algae: Chlorophytes and Charophytes. Charophytes are most closely related to land plants, sharing key adaptations such as gametangia, which protect gametes and facilitated the transition from water to land.

Shared Characteristics of Green Algae and Land Plants

• Cell walls made of cellulose

• Storage of sugars as starch

• Photosynthetic pigments: Chlorophyll a and b

These features are fundamental to both groups and are evidence of their shared ancestry.

Charophytes: Spirogyra and Chara

Spirogyra: Structure and Reproduction

Spirogyra is a filamentous, multicellular green alga. Its cells are haploid (n) and contain ribbon-shaped chloroplasts. Spirogyra reproduces both asexually and sexually:

• Asexual reproduction: Occurs under favorable conditions (warm temperatures, sunlight, nutrients). Cells fragment and divide by mitosis, elongating the filaments.

• Sexual reproduction: Occurs under stressful conditions. Involves conjugation between filaments of different mating strains (+ and -), forming diploid (2n) zygospores with thick coats for survival. Zygospores undergo meiosis to produce haploid cells.

Chara: Structure and Adaptations

Chara is a multicellular charophyte with specialized reproductive structures called gametangia. These include:

• Antheridia: Produce sperm

• Archegonia: Produce eggs

• Sterile jacket: Surrounds gametangia, protecting developing gametes—a key adaptation for terrestrial life

Chara's adaptations are similar to those found in land plants, supporting the evolutionary transition from aquatic to terrestrial environments.

Life Cycles: Alternation of Generations

Charophytes vs. Land Plants

• Charophytes: Multicellular haploid (n) bodies; zygote is the only diploid (2n) stage; no alternation of generations.

• Mosses and Ferns (Land Plants): Exhibit alternation of generations, with both multicellular haploid (gametophyte) and diploid (sporophyte) stages.

Alternation of Generations in Land Plants

All land plants have a life cycle with two distinct phases:

• Gametophyte generation (haploid, n): Produces gametes in gametangia (antheridia for sperm, archegonia for eggs).

• Sporophyte generation (diploid, 2n): Produces spores in sporangia via meiosis.

Fertilization produces a diploid zygote, which develops into the sporophyte. Spores produced by meiosis develop into gametophytes.

Life Cycle Equations

Key transitions:

• Fertilization:

• Meiosis:

• Mitosis: or

Adaptations for Terrestrial Life

Major Adaptations

• Prevention of desiccation: Waxy, waterproof covering

• Support: Fibrous cells for structural support

• Resource acquisition: Division into roots and shoots, vascular tissue

• Gas exchange: Pores in leaves

• Fertilization and gamete protection: Gametangia, pollen

Nonvascular Land Plants: Bryophytes (Mosses)

Characteristics and Life Cycle

Bryophytes (including mosses, liverworts, and hornworts) lack vascular tissue, limiting their size. Mosses have a dominant gametophyte (n) stage, with a smaller, dependent sporophyte (2n) attached to the gametophyte. Mosses reproduce via gametangia (antheridia and archegonia), and the sporophyte produces spores in a capsule (sporangium).

Vascular, Seedless Land Plants: Ferns

Characteristics and Life Cycle

Ferns and their relatives are vascular, seedless plants. The sporophyte (2n) is the dominant generation, with vascular tissue (xylem and phloem) allowing for greater height and complexity. The gametophyte (n) is reduced in size and often free-living. Ferns produce clusters of sporangia called sori on the underside of their fronds, where meiosis generates spores.

Fern Life Cycle Table

Summary Table: Plant Groups and Life Cycle Features

Key Terms and Definitions

• Gametophyte: Multicellular haploid (n) stage that produces gametes

• Sporophyte: Multicellular diploid (2n) stage that produces spores

• Gametangia: Structures that produce and protect gametes (antheridia for sperm, archegonia for eggs)

• Sporangium: Structure where spores are produced by meiosis

• Sorus: Cluster of sporangia found on fern fronds

• Vascular tissue: Specialized tissue (xylem and phloem) for transport of water and nutrients

Practice and Study Tips

• Draw and label the alternation of generations for mosses and ferns, including key structures and ploidy levels.

• Make notecards for protists and plant terminology, including descriptions, nutrition, and ploidy.

• Build comparison charts for organisms, noting ploidy levels, dominant stages, and key adaptations.

Additional info:

• Phylum names: Charophytes (green algae), Bryophytes (mosses), Pteridophytes (ferns)

• Alternation of generations is a defining feature of land plants, distinguishing them from their algal ancestors.