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Freeman,Biological Science, 4e, Chapter 30 Chapter 30 - Green Algae and Land Plants Learning Objectives: Students should be able to ... • Explain the ecological importance of green algae and land plants. • Describe the evolutionary adaptations that allowed plants to survive and reproduce on land. • Compare the traits of green algae, bryophytes, seedless vascular plants, gymnosperms, and angiosperms, and give a few examples from each group. Lecture Outline I. Why Do Biologists Study the Green Algae and Land Plants? A. Plants provide ecosystem services. 1. Plants improve the atmosphere, surface water, and soil in ways that benefit other organisms. 2. Plants produce oxygen. 3. Plants build and hold soil. (Fig. 30.1) 4. Plants hold water and moderate climate. 5. Plants are primary producers. a. Plants are the primary food producers on Earth. b. Plants produce sugars and oils, via photosynthesis, that provide the foundation for virtually all food chains. c. Herbivores eat plants, carnivores eat herbivores, and omnivores eat many sources of food. d. Plants are the key to the carbon cycle on land. (1) Plants take CO from the atmosphere and use it to make sugar. 2 (2) Plants fix more CO 2han they generate. B. Plants provide humans with food, fuel, fiber, building materials, and medicines. 1. Food a. Crop plants were domesticated at several different locations between 10,000 and 2000 years ago. (Fig. 30.2a) b. Humans gradually changed the characteristics of food plants via artificial selection. Example: kernel size in maize. (Fig. 30.2b) 2. Fuel a. For thousands of years, wood was the primary source of fuel used by humans. b. In recent centuries, coal, formed by plants that lived hundreds of millions of years ago, has replaced wood as our primary energy source. (Fig. 30.3) 3. Fiber and building materials a. Cotton and other plant fibers are used in clothing and other cloth- based household items. b. Woody plants are used to make lumber and paper. © 2011 Pearson Education, Inc. Freeman,Biological Science, 4e, Chapter 30 4. Pharmaceuticals a. Plants contain many compounds helpful in fighting disease. (Table 30.1) II. How Do Biologists Study Green Algae and Land Plants? A. Analyzing morphological traits 1. Similarities between green algae and land plants a. Both have chloroplasts with chlorophyll a and b and ˜-carotene. b. They have similar structure and composition of their thylakoids, cell walls, sperm, and peroxisomes. c. They both synthesize starch as a storage product. d. The ancestor of land plants was probably a multicellular green alga that lived in freshwater, similar to stoneworts. (Fig. 30.4) 2. Morphological differences among land plants a. Non-vascular plants (bryophytes) lack vascular tissue. (Fig. 30.5a) b. Seedless vascular plants have vascular tissue but do not make seeds. (Fig. 30.5b) c. Seed plants have vascular tissue and make seeds. (Fig. 30.5c) B. Using the fossil record (Fig. 30.6) 1. The first known green algae lived 700−725 million years ago (mya), when oxygen levels began to rise. 2. Origin of land plants a. The first known land plants occurred 475 mya. b. These fossils resemble today’s land plants because they have sheets of waxy cuticle, spores with sporopollenin-containing walls, and spores contained within specialized structures called sporangia. 3. Silur ian-Devonian explosion a. Macroscopic fossils from all major lineages of land plants are found from about 416−359 mya. b. Most structural adaptations to terrestrial life appeared: vascular tissue and roots. c. Plants probably colonized land with the assistance of fungi. 4. The Carboniferous period a. Extensive coal deposits formed 359−299 mya. b. Most of the coal was formed from club mosses, horsetails, and ferns. c. Coal formation requires water, so swamps were probably widespread. 5. Diversification of gymnosperms a. Gymnosperms dominated 251−145 mya. b. Gymnosperms grow well in dry habitats, so plants probably colonized drier environments during this time. 6. Diversification of flowering plants a. Fossil angiosperms appeared about 150 mya and are still dominant today. 7. Summary of the fossil record: The green algae appear first, followed by © 2011 Pearson Education, Inc. Freeman,Biological Science, 4e, Chapter 30 the non-vascular plants, seedless vascular plants, and seed plants. Organisms that appear late in the fossil record are much less dependent on water than are groups that appear earlier. C. Evaluating molecular phylogenies (Fig. 30.7) 1. The current molecular phylogenetic tree suggests these interpretations: a. The green algal group Charophyceae (stoneworts) is the sister group to land plants. b. Land plants, vascular plants, seed plants, gymnosperms, and angiosperms all are monophyletic taxa. c. However, non-vascular plants and seedless vascular plants are grades; they are paraphyletic. 2. Students should be able to explain how morphological data, the fossil record, and molecular phylogenies all support the hypothesis that land plants evolved from green algae. III. What Themes Occur in the Diversification of Green Plants? A. The transition to land, I: How did plants adapt to dry conditions? 1. Terrestrial habitats have more sunlight and more CO than2aquatic habitats. 2. Preventing water loss: cuticle and stomata a. The cuticle is a waxy sealant that prevents the loss of H O2but also inhibits the uptake of CO 2 (Fig. 30.8a) b. Stomata are pores bounded by guard cells that allow the uptake of CO while controlling water loss. (Fig. 30.8b) 2 c. Liverworts have simple pores; all other land plants today have stomata. 3. The importance of upright growth a. Early plants probably had a low, sprawling growth habit. b. Plants capable of vertical growth would have an advantage in competing for space and light. c. Two problems had to be overcome: transporting water up the plant from the soil and becoming rigid enough to withstand wind and gravity. 4. The origin of vascular tissue a. Fossilized plants dated to 400 mya had elongated cells, containing lignin, that were probably vascular tissue. (Fig. 30.9a, b) 5. Elaboration of vascular tissue: tracheids and vessels a. Tracheids have long, thin tapered ends, with lignin rings in secondary cell walls for support. Water flows through pits in secondary cell walls. (Fig. 30.9c) (1) Tracheids first appear in fossils dated to 380 mya. b. Vessel elements are shorter and stacked end to end like a pipe. Water flows through perforations that completely lack cell-wall material. (Fig. 30.9d) (1) Vessels first appear in fossils dated to 250−270 mya. © 2011 Pearson Education, Inc. Freeman,Biological Science, 4e, Chapter 30 c. Both tracheids and vessel elements are dead and lack cytoplasm at maturity. B. Mapping evolutionary changes on the phylogenetic tree (Fig. 30.10) 1. Fundament ally important adaptations to dry conditions—such as the cuticle, pores, stomata, vascular tissue, and tracheids—evolved only once. 2. Water-conducting cells evolved independently in mosses and vascular plants. 3. Vessels evolved independently in gnetophytes and angiosperms. C. The transition to land, II: How do plants reproduce in dry conditions? 1. Sporopollenin-encased spores were one of the first innovations making the colonization of land possible. 2. Producing gametes in protected structures a. The evolution of an elaborate gametangium protected gametes from drying and from mechanical damage. b. There are two types of gametangia: Antheridia produce sperm, and archegonia produce eggs. (Fig. 30.11) 3. Retaining and nourishing offspring: land plants as embryophytes a. Land plants retain their eggs in the archegonia. Once fertilized, the zygote begins to develop on the parent plant and receives nutrients from it. b. For this reason, plants are called embryophytes. (Fig. 30.12) 4. Alternation of generations a. In green algae, coleochaetes, and stoneworts, the multicellular form is haploid; the only diploid form is the zygote. (Fig. 30.13) b. Plants alternate between a multicellular haploid stage and a multicellular diploid stage. (Fig. 30.14) (1) A haploid, multicellular form called the gametophyte produces haploid gametes via mitosis. (2) Two gametes fuse to form a diploid zygote. (3) The zygote grows via mitosis to become the diploid multicellular form, called the sporophyte. (4) The diploid sporophyte produces haploid spores by meiosis. (5) A spore grows via mitosis to become a new gametophyte. c. Students should be able to draw a life-cycle diagram illustrating alternation of generations. They should be able to label the gametophyte, gametes, zygote, sporophyte, and spores, and indicate where meiosis or mitosis takes place. 5. The gametophyte-dominant to sporophyte-dominant trend in life cycles a. In non-vascular plants, the dominant form is the haploid gametophyte. (Fig. 30.15a) b. In ferns, the diploid sporophyte is larger. (Fig. 30.15b) c. In gymnosperms and angiosperms, the sporophyte is dominant and the microscopic gametophyte is retained within the sporophyte. © 2011 Pearson Education, Inc. Freeman,Biological Science, 4e, Chapter 30 d. The transition from gametophyte-dominated life cycles to sporophyte-dominated life cycles is one of the most striking trends in land plant evolution. (Fig. 30.16) e. Students should be able to examine the photos of hornworts (a non-vascular plant) and horsetails (a vascular plant) in Figure 30.16 and identify which is the gametophyte and which is the sporophyte. 6. Heterospory a. All non-vascular plants and most seedless vascular plants are homosporous, meaning that they produce a single type of spore. (Fig. 30.17a) b. Seed plants are heterosporous, with two distinct spore-producing structures that produce two types of spores. (Fig. 30.17b) (1) Microsporangia make microspores. Microspores grow to become male gametophytes, which produce the sperm. (2) Megasporangia make megaspores. Megaspores grow to become female gametophytes, which produce the eggs. (3) The gametophytes of seed plants are either male or female, but never both. 7. Pollen a. A pollen grain is a tiny male gametophyte surrounded by a tough coat of sporopollenin. b. When pollen evolved, heterosporous plants no longer needed water to accomplish fertilization. Instead of swimming to the egg as a naked sperm cell, the tiny male gametophytes took to the skies. 8. Seeds a. A seed includes an embryo and a food supply surrounded by a tough coat. b. Seeds enable embryos to be dispersed to new habitats. (Fig. 30.18) c. A summary of the traits of seed plants can be seen in the life cycle of a pine tree. (Fig. 30.19) 9. Flowers a. Flower ing plants (angiosperms) are the most successful land plants. b. Flowers contain two reproductive structures: stamens and carpels. (Fig. 30.20) (1) Stamens have anthers, which contain the microsporangia. (2) Carpels have ovaries, containing ovules, which contain the megasporangia. (i) The evolution of the ovary was a key innovation that protects female gametophytes from insects and other predators. c. In double fertilization, a pollen grain produces two sperm cells. One sperm cell fuses with the egg to form an embryo. The other sperm cell fuses with two nuclei in the female gametophyte to form triploid endosperm. © 2011 Pearson Education, Inc. Freeman,Biological Science, 4e, Chapter (1) The adaptive significance of double fertilization is still not understood. 10.Pollination by insects and other animals a. Sepals and petals enclose the ovary and give flowers a wide variety of colors, shapes, and smells. b. Directed-pollination hypothesis: Flowers are an adaptation to attract specific pollinators (rather than leaving pollination to the wind), thus increasing the probability that pollination will occur. c. Evidence for this hypothesis: The characteristics of a flower (scent, flower shape, color, etc.) correlate with the characteristics of its pollinator. (1) Examples: carrion flowers, hummingbird-pollinated flowers, bee- pollinated flowers. (Fig. 30.21) d. Experimental evidence: the spur length of theDisa draconis orchid (1) Flowers with artificially shortened spurs received less pollen
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