Plant Phylogeny and Diversity

Phylogenetic Relationships & Plant Diversity

• Lecture focuses on phylogenetic relationships and plant diversity

Plant Origins and Evolution

• Land plants originated approximately 475 million years ago (mya).
• Vascular plants evolved around 420 mya.
• Extant seed plants emerged about 305 mya.

Nonvascular Plants (Bryophytes)

• Represented by mosses, liverworts, and hornworts.
• Bryophytes were the first plant lineage to diverge after plants moved onto land.
• They have a small, simple, and tough structure.
• Bryophytes are small due to constraints of fertilization, requiring water for sperm to reach the female gametophyte.
• Typically have flattened photosynthetic structures known as a thallus, only a few cells thick and lack internal air spaces or vascular systems.
• Haploid gametophyte dominates the life cycle, supporting the non-photosynthetic sporophyte.
• Water and CO2 are absorbed through plant surfaces via diffusion.
• They do not possess a waxy cuticle, reducing water loss.
• Bryophytes require a moist environment but can tolerate desiccation.
• They are poor competitors for light and space.
• Peat bogs (Sphagnum) are an exception and play a key ecological role by creating acidic, wet conditions that slow decomposition and store large amounts of organic carbon.
• Peat bogs are found in boreal forests.
• Bryophytes are vulnerable to climate change.

Seedless Vascular Plants

• Differ from nonvascular plants (bryophytes) for a sporophyte dominated diploid lifecycle.
• They possess vascular tissue, xylem for water transport and phloem for sugar transport.
• Seedless vascular plants are less dependent on water for reproduction.
• Represented by lycophytes and ferns/horsetails (pterophytes).
• Pterophytes are a monophyletic group.
• Most ferns have coiled leaves that uncoil during growth.
• Fronds are the distinctive leaves, divided into smaller units called pinnae.
• Stems grow underground and can spread easily.
• Vascular tissue enables plants to grow larger but size is limited by the lack of secondary vascular tissue.
• Sporophyte dominates the life cycle, protecting the small photosynthetic gametophyte.

Seed Plants

• The development of seeds enabled plants to become the dominant producers on land.
• Do not require external water for fertilization.
• Consist of an embryo and nutrients (endosperm) surrounded by a protective coat.
• Pollen facilitates dispersal of male gametes via wind or animals, eliminating the need for water.

Gymnosperms

• Divided into four distinct groups: Cycads, Ginkgos, Conifers, and Gnetophytes.
• Characterized by "naked" seeds not enclosed by ovaries.
• Gymnosperms are better adapted than nonvascular plants to drier conditions.

Gymnosperm Groups

• Cycads: Individuals have large cones and palm-like leaves and can live in nutrient-poor conditions with insect pollination. Most species are tropical and are on endangered species lists.
• Ginkgos: There is only one living species (Ginkgo biloba), which can tolerate air pollution and is a popular ornamental tree with health benefits.
• Conifers: Most conifers are evergreens that carry out photosynthesis all year and tolerate cold, dry conditions. Conifers include Douglas Fir, Wollemi Pine and Sequoia.
  • Conifers can dominate angiosperms in cold/dry climates subject to freeze-thaw cycles.
    • Tallest Tree is the Coast Redwood (Sequoia Sempervirens) which is 115.7 meters tall.
    • Oldest Tree is the Bristlecone Pine (Pinus Longaeva) which is 5064 years old.
  • Three key features of the gymnosperm life cycle: The dominance of the sporophyte generation, the development of seeds from fertilized ovules, and transfer to sperm to ovules by pollen (wind transport).
• Gnetophytes: Comprised of 3 genera with varying appearances and live in either tropical or desert environments with features similar to angiosperms and use double fertilization.
  • Vary in Appearance and consist of (1) Multicellular Xylem Vessels (2) Double Fertilization.

Angiosperms

• Exhibit "protected" seeds enclosed by ovaries.
• Angiosperms consist of six groups.
• Best suited to life on land and are the most widespread diverse group of plants, with diversity possibly attributable to slower rates of species loss than species formation.
• The earliest angiosperms are evolved in the understory of dominating gymnosperm forests,
• Utilize insect pollination, which gave them an advantage in areas where there was limited wind because of this limited species loss and a slower rate of species formation.

Angiosperm Groups

• Amborella, Water Lilies, & Star Anise are the three rarer groups to evolve first.
• Magnoliids include magnolias, laurels, black pepper, and avocados. Common in early angiosperm evolution in tropical areas.
• Monocots comprise about 25% of angiosperms and include food crops such as rice, wheat, corn, sugar cane, bananas, ginger, and pineapple and include economically important plants such as bamboo, orchids, lilies, daffodils, and tulips.
• Eudicots: comprise about 75% of angiosperms, include both economically important and interesting plants, such as carnivorous, parasitic, and epiphytes.