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Unformatted text preview: The Evolution of Plants
Chapter 30- pp. 626-661 Life in water is easy. Why? Domain: Eukarya Recall- green algae Plant Origins
s Closest relatives of modern plants are green algae.
x Chlorophyll a, b x Cell walls primarily cellulose x Starch as primary reserve s E.g. Charophytes, Chlamydomonas, Volvox,
x p. 631 Plants vs. Green Algae
Charophyceans are closest relative to plants1. Same proteins in plasma membrane for synthesizing cellulose. 2. Share enzymes in organelles called peroxisomes (p. 126) Fig. 30.6 Plants are Monophyletic Life in water is easy
s Bathed in nutrients s Supported against gravity s Extensive transport not necessary s Gametes, offspring can be transmitted by water s No problem of desiccation/ dehydration Why leave the water? s More direct sun
x Enough ozone 425 million years ago to filter harmful UV radiation s Nutrients and minerals on land s Abundant CO2 for photosynthesis s Initially, an absence of herbivores Relative to water, land is harsh
s Land plants must:
x Conserve water3 Cuticle x Transport nutrients x Withstand gravity x Transfer gametes x Protect or reduce vulnerable life stages
3 sporopollenin s Much of land plant evolution can be understood as a response to these challenges! To appreciate plant evolution, you must understand Alternation of Generations:
s Sporophyte Start here produces spores
x By meiosis; 2n 1n
s Gametophyte produces gametes
x By mitosis; 1n 1n 30.16 Alternation of generations
s Sporophyte and gametophyte may look:
x Identical, e.g. Ulva x Similar in size, e.g. Bryophytes x Very different, e.g. Conifer Ulva- no difference Gametophyte (1n) Barely visible Sporophyte (2n) 80 meters Origin of Alternation of Generations? Charophyte Coleochaete Ancestor? Ancestor? p. 640 There have been four successive radiations of land plants
s Bryophytes s Ferns s Gymnosperms s Angiosperms See Fig. 30.9 Each of these radiations has left living members and is associated with adaptations to land 2 - Ferns and their "allies" 1 - Bryophytes 3 - Gymnosperms 4 - Flowering plants = Angiosperms 1. Bryophytes
s Mosses, Liverworts, Hornworts s Can be abundant
x Moist habitats, tundra Bryophytes
s Sporophyte, Gametophyte similar in size Sporophyte Gametophyte Bryophytes Gametophyte stage is dominant
See 30.17a Bryophytes
s Two major breakthroughs
x x 1. Cuticle 2. Protected Embryo Bryophytes
s Limited by
3 Lack of vascular tissues and support 3 Reliance on water for reproduction 3 Lack of a durable dispersal phase 2. Ferns First vascular plants (horsetails, club mosses & ferns) Ferns can grow quite large, and once formed huge forests Seedless vascular plants Seedless vascular plants
s Principle breakthrough was vascular tissue. This allowed:
xLarge size - lignin xEfficient distribution of water and nutrients by xylem & phloem
Vascular tissue of an advanced tracheophyte Seedless vascular plants such as ferns, still limited by...
s Presence of a fragile gametophyte stage x Although it as been reduced & is independent See 30.17b Early Pteridophytes, such as ferns, are still limited by... s Reliance on water for reproduction Fern life cycle
Sporophyte is the dominant generation 30.17b Homosporous plant = Sporophyte produces 1 type spore with both male/female sex organs. 3. Gymnosperms and the origin of seed plants "naked seed" = No ovaries for development of seeds. Gymnosperms
s Once the dominant type of land plant x Dinosaurs strolled through gymnosperm forests- Mesozoic Era x About 720 species remain Gymnosperms show two principle breakthroughs 30.20
s 1. The evolution of seeds
x Embryo packed in with food supply x Can wait for favorable conditions x Can be carried by wind or animals s 2. Reduction of the Gymnosperms show two principle breakthroughs gametophyte
x Male gametophyte can be carried by wind to female gametophyte x Now free from water for reproduction! Wind pollination has some drawbacks, though ...
s Inefficient ... must produce lots of pollen 4. Angiosperms = Flowering Plants Like gymnosperms, angiosperms have large sporophytes and tiny gametophytes Angiosperm success
s Extremely successful x> 250,000 species
3 Compare to only about 720 species of gymnosperms xRapid diversification about 130 mya xDarwin's "abominable mystery" Angiosperm success - Why?
s Many possible reasons for success, including ...
x Flowers! x Fruits Flowers and Success s More efficient pollination
xDon't need to rely on wind xEasier to avoid inbreeding xCan reproduce even when in low abundance Pollination relationships
s May be very specific, or very general s Flower structure relates to type(s) of pollinators. x Coevolution Fruits
s Fruits are typically derived from the ovary of the flower (and frequently other structures) s Like flowers, fruits are costly to produce. Why bother? Fruits
s Like flowers, fruits are costly to produce. Why bother? x They promote dispersal s Land plants must
x Conserve water
3 Cuticle, Summary protected embryo, reduced gametophyte tissue tissue with lignin pollination, fruit x Transport nutrients
3 Vascular x Withstand gravity
3 Vascular x Transfer gametes or otherwise disperse genes
3 Gametophyte->pollen, 3 Protected x Protect or reduce vulnerable life stages embryo, reduced gametophyte, seed s 4 radiations of land plants
x Examples and characteristics of each ...
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