Plant Evolution and Adaptations

• Plants colonized land around 475 million years ago, diversifying into approximately 290,000 living species
• Plants provide oxygen and serve as the primary food source for land animals
• Charophytes, a type of green algae, are the closest relatives to land plants
• Land plants and modern charophytes share a common ancestor, but land plants did not descend from modern charophytes

Land Plant Life Cycle

• Alternation of generations is a derived trait in land plants

Walled Spores in Sporangia

• Sporophytes produce spores in organs called sporangia
• Diploid sporocytes undergo meiosis to produce haploid spores

Multicellular Gametangia

• Gametes are produced in gametangia, which are 'sex organs'
• Female gametangia, called archegonia, produce eggs and serve as the site for fertilization
• Male gametangia, called antheridia, produce and release sperm

Apical Meristems

• Apical meristems are growth regions located at the tips of plants, enabling ongoing growth in length
• Cells from apical meristems differentiate into diverse tissues

Diversity of Modern Plants

• Land plants are informally grouped based on vascular tissue presence
• Nonvascular plants are known as bryophytes
• Most plants, known as vasular plants, possess vascular tissue and are classified as either seedless vascular or seed plants

Seedless Vascular Plants

• Vascular plants without seeds are categorized into two clades:
  • Lycophytes including club mosses and their relatives
  • Pterophytes including ferns and their relatives

Seed Plants

• A seed consists of an embryo and nutrients enclosed in a protective coat
• Seed-bearing plants form a clade that can be further divided:
  • Gymnosperms including "naked seed" plants like conifers, where the cone is the sex organ
  • Angiosperms including flowering plants like monocots and dicots, where the flower is the sex organ

Ten Phyla of Extant Plants

• Nonvascular Plants (Bryophytes):
  • Phylum Hepatophyta: Liverworts, with approximately 9,000 species
  • Phylum Anthocerophyta: Hornworts, with approximately 100 species
  • Phylum Bryophyta: Mosses, with approximately 15,000 species
• Vascular Plants consist of seedless vascular plants
  • Phylum Lycophyta: Lycophytes, with approximately 1,200 species
  • Phylum Pterophyta: Pterophytes, with approximately 12,000 species
• Seed Plants consist of Gymnosperms
  • Phylum Ginkgophyta: Ginkgo, with 1 species
  • Phylum Cycadophyta: Cycads, with 130 species
  • Phylum Gnetophyta: Gnetophytes, with 75 species
  • Phylum Coniferophyta: Conifers, with around 600 species
• Angiosperms consist of:
  • Phylum Anthophyta: Flowering plants, around 250,000 species

Nonvascular Plants

• Bryophytes are nonvascular and are represented by three phyla of small herbaceous plants today:
  • Liverworts are of the phylum Hepatophyta
  • Hornworts are of the phylum Anthocerophyta
  • Mosses are of the phylum Bryophyta
• Mosses are the most closely related to vascular plants
• Gametophytes are dominant as they are larger and live longer than sporophytes. Sporophytes are present only temporarily and are dependent on gametophytes

Ecological and Economic Importance of Mosses

• Mosses thrive in diverse environments, especially moist forests and wetlands
• Some mosses aid in nitrogen retention in soil
• Sphagnum, commonly known as "peat moss," forms substantial deposits of partially decayed organic material
• Sphagnum serves as a significant global reservoir for organic carbon

Ferns and Seedless Vascular Plants

• Bryophytes and bryophyte-like plants dominated vegetation for first 100 million years of plant evolution
• Vascular plants started diversifying during the Devonian and Carboniferous periods
• Vascular tissue enabled vascular plants to attain greater heights
• Seedless vascular plants require moist environments since they have flagellated sperm

Origins and Traits of Vascular Plants

• Vascular plant forerunner fossils have been dated back ~420 million years
• In contrast to bryophytes, sporophytes are the larger generation for seedless vascular plants. While tiny, gametophytes tend to grow on or below the soil surface
• Life cycles are characteristized with dominant sporophytes, vascular tissues like xylem and pholem, and well-developed roots and leaves

Vascular Tissue: Xylem and Phloem

• Vascular plants have two types of vascular tissue: xylem and phloem
• Xylem transports water and minerals, consisting of dead cells called tracheids
• Phloem transports nutrients like sugars and amino acids, consisting of living cells
• Water-conducting cells are reinforced by lignin, which provides structural support
• Increased height was an evolutionary advantage

Evolution of Roots and Leaves

• Roots are organs that anchor vascular plants and absorb water and nutrients from the soil
• Roots may have evolved from subterranean stems
• Leaves increase the surface area of vascular plants to capture more solar energy for photosynthesis

Significance of Seedless Vascular Plants

• Increased photosynthesis may have contributed to the global cooling at the end of the Carboniferous period
• Decaying plants of these Carboniferous forests eventually transformed into coal, resulting in fossil fuels