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.