Plant Biology: Land Plants and Charophyceans

Earth’s Early History and Plant Evolution

• For the first 3 billion years, Earth had no land life; cyanobacteria appeared about 1.2 billion years ago.
• Approximately 500 million years ago, plants, fungi, and animals emerged on land.
• Current plant diversity includes over 290,000 species, primarily in terrestrial environments like deserts, grasslands, and forests.

Land Plants and Their Evolution

• Land plants evolved from green algae, specifically charophyceans, which are their closest relatives.
• Key characteristics such as multicellularity and the presence of chlorophyll a and b are shared with various algae.
• Unique features connecting charophyceans and land plants include specialized cellulose-synthesizing complexes in plasma membranes, specific peroxisome enzymes, structural similarities in sperm cells, and a distinct cytokinesis process.

Adaptations in Land Plants

• Land plants developed adaptations for survival on land, differentiating them from algae.
• Five key derived traits of land plants include:
  • Apical meristems for growth at tips of roots and shoots, maximizing resource exposure.
  • Alternation of generations featuring both gametophyte (haploid) and sporophyte (diploid) life stages.
  • Multicellular embryos dependent on the parent for nutrients.
  • Sporangia that produce hardy, walled spores.
  • Gametangia for gamete production.

Life Cycle of Bryophytes

• Bryophytes consist of liverworts (Hepatophyta), hornworts (Anthocerophyta), and mosses (Bryophyta).
• In bryophytes, the gametophyte is the dominant life stage, while sporophytes are smaller and dependent on gametophytes.
• Bryophytes reproduce via specialized structures called gametophores that produce gametes; fertilization occurs in the presence of water.

Vascular Plant Characteristics

• Ferns and seedless vascular plants thrived during the Carboniferous period, preceded by bryophytes as the dominant flora.
• Vascular plants exhibit several characteristics:
  • Dominant sporophyte lifecycle stage.
  • Vascular tissues (xylem for water/mineral transport; phloem for nutrient transport).
  • Evolution of roots that anchor plants and facilitate nutrient uptake.
  • Emergence of leaves and specialized reproductive structures called sporophylls.

Importance of Bryophytes and Vascular Plants

• Bryophytes contribute significantly to ecosystems by dispersing spores widely and thriving in diverse habitats.
• Peat moss (Sphagnum) forms peat bogs, which play essential roles as carbon reservoirs and in nutrient cycling.
• Vascular plants allowed terrestrial ecosystems to evolve by maximizing light and nutrient absorption due to structural adaptations, leading to taller growth and increased biodiversity in forests.### Evolution of Roots
• Roots may have evolved from stem portions in ancient vascular plants.
• Uncertainty exists whether roots originated from a common ancestor of all vascular plants or developed independently.
• Investigating genes controlling root development can clarify this evolutionary debate.

Evolution of Leaves

• Leaves enhance surface area for photosynthesis by capturing solar energy.
• Classified into microphylls and megaphylls based on size and complexity.
• Microphylls are small leaves in lycophytes with a single unbranched vein, likely evolving from stem outgrowths.
• Megaphylls have a branched vascular system, allowing greater water, mineral transport, and sugar export.
• Fossils suggest megaphylls arose from closely spaced branches on a stem, possibly flattening with webbing tissue.

Sporophylls and Spore Variations

• Sporophylls are modified structures bearing sporangia, varying widely in form.
• Ferns form sori, clusters of sporangia on leaf undersides; gymnosperms produce cone-forming sporophylls.
• Species distinctions include homosporous (one spore type) and heterosporous (two spore types).
• Most seedless vascular plants are homosporous, developing bisexual gametophytes, while seed plants and some others are heterosporous.

Classification of Seedless Vascular Plants

• Major clades: lycophytes (club mosses, spike mosses, quillworts) and pterophytes (ferns, horsetails, whisk ferns).

Phylum Lycophyta:

• Lycophytes are remnant species from the Carboniferous, which had both small and giant tree forms.
• Giant lycophytes, reaching heights of 40 meters, went extinct due to climatic changes.
• Presently, about 1,200 species of small lycophytes exist.

Phylum Pterophyta:

• Ferns diversified significantly from their Devonian origins and thrived in Carboniferous swamp forests, with 12,000 living species today.
• They are most abundant in tropical climates and can adapt to temperate forests and arid regions.
• Historical horsetails reached heights of 15 meters; today, only 15 species exist within genus Equisetum.
• Whisk ferns are unique as they possess neither true roots nor leaves and exhibit dichotomous branching, linking them to early vascular plants.

Significance of Seedless Vascular Plants

• Ancestors of lycophytes and ferns created the first forests during the Carboniferous era.
• Evolution of vascular tissue, roots, and leaves increased photosynthetic rates and CO2 removal from the atmosphere.
• CO2 levels were estimated to drop significantly, leading to global cooling and glacier formation.
• First forests contributed to the formation of modern coal through the conversion of peat under pressure over millions of years.
• Humans continue to utilize coal, burning approximately 6 billion tons annually.

Explore the evolutionary connections between land plants and charophyceans, focusing on the similarities in sperm structure and cell division processes. This quiz dives into the unique features such as phragmoplast formation and cytokinesis in these plant groups.