Little Shop of Horrors - Lecture Notes

LITTLE SHOP OF HORRORS - Plants!

CHAPTER 26

Unit 3: Evolution/History of Life

  • The Colonization of Land by Plants

  • Evolution of Plants

  • Plant Characteristics

Early Life

Key Knowledge Areas

  • What do I need to know?

    • Evolutionary history and major characteristics that define plants.

    • Major adaptations among major groups of plant taxa.

  • What do I need to think about?

    • How specific adaptations permitted life on land.

  • What do I need to understand?

    • We live in the era of Angiosperms.

Major Events in the History of Life

  • Earliest cells:

    • Prokaryote microfossils date back to approximately 3.5 billion years ago (b.y.a.).

  • Last common ancestor origin:

    • Eukaryotes emerge around 4.5 b.y.a..

    • 4 b.y.a. - Origin of life sequences including the RNA world.

  • Archean Era:

    • Origin of photosynthesis occurs between 3.5 to 2.8 b.y.a..

  • Proterozoic Era:

    • Oldest eukaryote fossils found, dating back to approximately 1.7 b.y.a..

    • Emergence of multicellular organisms at around 1.2 b.y.a..

  • Paleozoic Era:

    • First land plants appear approximately 470 million years ago (mya).

  • Mesozoic Era:

    • Formation of the Pangaea supercontinent around 250 mya.

    • Dinosaurs go extinct approximately 65 mya.

  • Cenozoic Era:

    • The earliest humans appear around 6 mya.

Phylogeny

  • Plants

    • Groups include flowering plants, conifers, ginkgos, cycads.

  • Fungi

  • Animals

    • Includes arthropods, chordates (sac, club, annelids, fungi, protozoans, roundworms, mollusks, and flatworms).

  • Protists

    • Includes choanoflagellates, ciliates (alveolates), apicomplexans, horse tails, ferns, lycophytes, bryophytes, and fungi.

  • Archaea

  • Bacteria

    • Groups include spirochetes and chlamydias.

Origin of Plants

  • Fossil Evidence:

    • Evidence suggests that plants colonized land more than 470 million years ago.

  • Characteristics Shared with Archaeplastids:

    • Multicellular, photosynthetic, containing chlorophyll a and b.

  • Specific Characteristics Shared with Charophytes:

    • Rings of cellulose-synthesizing complexes.

    • Flagellated sperm structure.

    • Similar nuclear and chloroplast genes.

Why Move to Land?

Benefits

  • Access to sunlight.

  • Availability of carbon dioxide from the atmosphere.

  • Rich nutrient availability in soil.

Costs

  • Availability of water.

  • Structural support issues against gravity.

Derived Traits of Plants

  • Key Derived Traits Appearing in Nearly All Land Plants (absent in charophytes):

    1. Alternation of generations

    2. Multicellular and dependent embryos

    3. Walled spores produced in sporangia

    4. Apical meristems

    5. Cuticles

    6. Stomata

Alternation of Generations

  • Fertilization: Mitosis.

  • Meiosis: Key points leading to the generation changes.

    • Haploid Gametophyte (n):

    • Produces haploid gametes by mitosis.

    • Gametes fertilize to form a diploid zygote.

    • Diploid Sporophyte (2n):

    • Produces haploid spores through meiosis.

    • Spores develop into gametophytes.

Multicellular Dependent Embryos

  • Embryos are retained within the tissue of the female gametophyte.

    • Nutrients are transferred from parent to embryo through placental transfer cells.

Sporangia

  • Multicellular organs that produce spores.

  • Spore Walls: Their walls contain sporopollenin which is critical for protection.

Apical Meristems

  • Specific points of cell division at the very tips of roots and shoots.

  • Apical meristems can divide throughout the plant’s life, usually being the only areas where mitosis occurs.

Cuticles

  • The cuticle is a waxy covering of plants' epidermis.

    • It prevents water loss and protects against microbial attacks.

  • Example: Cuticle present on the surface of a leaf.

Stomata

  • Specialized pores that allow the exchange of carbon dioxide (CO2) and oxygen (O2) between the plant and the air.

  • They open and close during specific times or situations to regulate gas exchange.

Plant Adaptive Radiation

  • Plants are informally grouped based on the presence or absence of vascular tissue.

    • Evolutionary Groups:

    • Origin of land plants.

    • Origin of vascular plants.

    • Origin of extant seed plants.

Bryophytes

  • Three Clades of Small Herbaceous (Non-Woody) Plants:

    • Mosses:

    • Lack vascular tissue.

    • Probably exhibit the earliest lineages diverged from the common ancestor of land plants.

  • Anchoring Mechanism:

    • Bryophytes are anchored to substrates by rhizoids, not true roots.

  • Reproductive Mechanism:

    • Flagellated sperm must swim through water to reach the egg, while gametophytes are larger and often longer-living than sporophytes.

Seedless Vascular Plants

  • Timeline: Emerged about 425-420 mya.

  • Advantage: Vascular tissue allowed these plants to grow tall but lacked seeds.

  • Key Structures:

    • Monilophytes (ferns): exhibit strobili (cone-like structures containing spores).

Vascular Tissue in Seedless Vascular Plants

  • There are two types of vascular tissues:

    • Xylem:

    • Transports water and minerals, including supportive tube-shaped cells fortified by lignin.

    • Phloem:

    • Transports sugars, amino acids, and other organic products arranged into tube structures.

Evolution of Roots and Leaves

  • Roots:

    • Organs that anchor vascular plants and enable absorption of water and nutrients from the soil.

  • Leaves:

    • Organs designed to increase surface area for photosynthesis and respiration.

Dominant Generation of Seedless Vascular Plants

  • In seedless vascular plants, the sporophyte is now the dominant generation, contrasted with small gametophytes that thrive on or beneath the soil’s surface.

Seeded Plants

  • Emerged approximately 360 mya.

    • Seeds allowed them to expand into diverse terrestrial habitats.

  • Seed Structure:

    • Consists of an embryo, a food supply, and is surrounded by a protective coat.

  • Two Clades of Extant Seed Plants:

    • Gymnosperms:

    • Characterized by naked seeds not enclosed in chambers.

    • Angiosperms:

    • Seeds develop inside protective chambers (ovaries).

Ovules and Pollen in Seed Plants

  • Ovule:

    • Contains the female gametophyte surrounded by a protective layer of sporophyte tissue.

    • Includes parts like style, stigma, and ovary.

  • Pollen:

    • Contains the male gametophyte within a protective pollen wall.

    • Pollination:

    • The transfer of pollen to parts of seed plants containing ovules, eliminating the need for water for sperm mobility, allowing for greater distance dispersal.

Advantages of Seeds

  • Seeds develop from the entire ovule, which consists of a sporophyte embryo and a food supply, packaged within a protective coat.

  • Advantages over spores:

    • Can remain dormant for days to years.

    • Built-in food supply enhances survival during adverse conditions.

Rise of the Gymnosperms

  • Approximately 305 mya:

    • Gymnosperms replaced most other plants due to drier climatic conditions at the end of the Carboniferous period.

    • Adaptations for dry conditions include the development of seeds and pollen, thick cuticles on leaves, and leaves with small surface areas.

Origin and Diversification of Angiosperms

  • Angiosperms:

    • Seed plants characterized by flowers and fruits.

    • Primarily pollinated by insects or animals; some are wind-pollinated.

    • They are the most widespread and diverse group among plants with around 250,000 species.

Flower Structure in Angiosperms

  • The flower is a specialized shoot with up to four distinct types of modified leaves (floral organs):

    • Sepals:

    • Enclose the flower.

    • Petals:

    • Brightly colored to attract pollinators.

    • Stamens:

    • Produce pollen.

    • Carpels:

    • Produce ovules.

Fruit and Dispersal Mechanisms

  • Dispersal by Animals:

    • Example: Fruits of puncture vine (Tribulus terrestris) and seeds dispersed in bear feces.

    • Example: Squirrels hoarding seeds underground.

  • Dispersal by Wind:

    • Example: Dandelion seeds (actually one-seeded fruits) and winged fruits of maple.

  • Dispersal by Water:

    • Example: Coconut seeds with buoyant husks aiding in dispersal.

Timeline of Angiosperm Evolution

  • Emergence: Approximately 140 mya.

  • Dominance: Angiosperms have dominated terrestrial landscapes since about 65 mya.