Chapter 22 The Land Plants © Cengage Learning 2016 22.2 Plant Ancestry and Diversity • Plants – Multicelled – Photosynthetic eukaryotes – Adapted to life on land – Close relatives of red algae and green algae – Contain cellulose cell walls and chloroplasts • Chlorophylls a and b – Embryophytes • Embryos form within chamber of parental tissues • Receive nourishment during development © Cengage Learning 2016 Plant Ancestry and Diversity • Adaptive radiation • First plants evolved 500 million years ago – Photosynthetic cells had released oxygen into the atmosphere and ozone layer had formed • Plant spore © Cengage Learning 2016 Plant Ancestry and Diversity • Plant spore – Haploid cell with a cell wall • Some plants have alternating generations – A life cycle in which a diploid generation alternates with a haploid one – Diploid – Haploid © Cengage Learning 2016 multicelled mitosis sporophyte zygote Diploid (2n) Phase of Cycle Fertilization Meiosis Haploid (n) gametes Phase of Cycle spores multicelled gametophyte Stepped Art © Cengage Learning 2016 Major groups of plants • Criteria – Vascular – Seed production – Types of seeds produced – Development and morphology © Cengage Learning 2016 © Cengage Learning 2016 Evolutionary Trends Among Plants • Story of plant evolution – Adapted to life on land – Adapted to increasingly drier habitats • Sporophyte dominates all vascular plant life cycles – Spores more likely to survive in dry conditions • Oak tree – Large and complex sporophyte – Gametophyte: only a few cells © Cengage Learning 2016 Evolutionary Trends Among Plants • Genetic factors – Influenced sporophyte dominance • Structural adaptations – Cuticle • Helps reduce evaporative water loss – Stomata • Open to allow gas exchange for photosynthesis • Close to conserve water © Cengage Learning 2016 Evolutionary Trends Among Plants • Vascular tissues – Internal system of pipelines – Xylem distributes water – Phloem distributes sugars – Lignin provides structural support • Leaves – Contain veins of vascular tissue – Allow plant to capture sunlight and exchange gases © Cengage Learning 2016 Pollen and Seeds • Pollen grains – Walled, immature gametophyte • Gives rise to male gametes – Released by seed-bearing vascular plants – Travel to female gametophytes on the wind or in insect bodies – Allow plants to reproduce even in dry environments © Cengage Learning 2016 Pollen and Seeds • Seeds – Consist of an embryo sporophyte and nutritive tissue enclosed within a waterproof seed coat – Many have features that facilitate their dispersal • Angiosperms disperse seeds inside a fruit © Cengage Learning 2016 Bryophytes • Mosses – Most diverse and familiar group of bryophytes – Example: Sphagnum (peat moss) • 350 species • One of the most economically important bryophytes • Grow in peat bogs in high latitude regions of Europe, Asia, and North America • Blocks are cut, dried, and burned as fuel in Ireland • Helps soil retain moisture © Cengage Learning 2016 7 2 zygote sporophyte (2 n) Diploid (2n) phase Fertilization Meiosis Haploid ( n) 6 phase 3 1 gametophyte (n) ① The leafy green part of a moss is the sperm released from spor e (n) released haploid gametophyte. male gametangium from sporangium ② The diploid sporophyte has a stalk and a male capsule (sporangium). It is not gametophyte photosynthetic. ③ Haploid spores form by meiosis in the capsule, are released, and drift with the 4 winds. sperm ④ Spores germinate and develop into male female or female gametophytes with 5 gametophyte gametangia that produce eggs or sperm by mitosis. ⑤ Sperm swim to eggs. ⑥ Fertilization produces a zygote. egg in female ⑦ The zygote grows and develops into a gametangium new sporophyte while remaining attached to and nourished by the female © 2016 Cengage Learning gametophyte. Jane Burton/ Bruce Coleman Ltd. © Cengage Learning 2016 Bryophytes • Liverworts – Among oldest known fossils of land plants – Some gametophytes look leafy and others are flat sheets – Example: Marchantia • Hornworts – Has a pointy, hornlike sporophyte – Spores form in upright capsule called sporangium – Contain chloroplasts © Cengage Learning 2016 Seedless Vascular Plants • Oldest vascular plant lineages • Have flagellated sperm that swim to eggs • Disperse by releasing spores directly to the environment • Two lineages survive today – Lycophytes – Monilophytes © Cengage Learning 2016 Seedless Vascular Plants • Club mosses – Often grow on floor of temperate forests • Whisk ferns and horsetails – Native to the Southeastern United States – Includes rushes • Ferns – Most diverse and familiar seedless vascular plant – Most live in the tropics © Cengage Learning 2016 1 mature sporophyte 5 young sporophyte 1 The familiar leafy form is the diploid sporophyte. 2 Meiosis in cells on the underside of fronds (leaves) produces haploid spores. zygote 3 After spores are released, they germinate and grow into tiny gametophytes that sori (clusters of sporangia) produce eggs and sperm. rhizome on underside of frond 2 Sperm swim to eggs and 4 Diploid (2n) phase fertilize them, forming a Fertilization Meiosis zygote. Haploid ( n) phase egg 5 The sporophyte begins its 4 development attached to the gametophyte, but it continues to grow and live independently female mature spore after the gametophyte dies. gametangium gametophyte (underside) male gametangium 3 sperm photo, A. & E. Bomford/ Ardea, London; art, © 2016 Cengage Learning © Cengage Learning 2016 Rise of the Seed Plants • Advantages of seed-bearing plants – Reproductive traits – Structural traits © Cengage Learning 2016 pollen sac (2n) ovule (2n) meiosis meiosis microspores (n) in pollen sac megaspores (n) in ovule develop into develop into sperm-producing male gametophytes (pollen grains) egg-bearing female release and pollination gametophytes in ovule delivers sperm to fertilization zygote (2n) in ovule develop into seed (embryo sporophyte in ovule) Stepped Art © Cengage Learning 2016 Figure 22.21 p365 Gymnosperms • Vascular seed plants – Produce seeds on the surface of ovules – Some seeds enclosed in a fleshy or papery covering • Types – Conifers – Cycads – Ginkgos – Gnetophytes © Cengage Learning 2016 Gymnosperms • Conifers – 600 species – Trees and shrubs with woody cones – Needle-like or scale-like leaves with a thick cuticle – More resistant to drought and cold than flowering plants – Evergreen – Include the longest lived plants • Some bristlecone pines are 4,000 years old © Cengage Learning 2016 One scale sectioned One scale sectioned through ovule through pollen sac 1 Scales of pollen 4 Scales of cones hold pollen ovulate cones sacs. contain ovules . pollen cone with ovulate cone with many scales many scales sporophyte (2n) Meiosis Meiosis 9 The seed seed coat germinates and embryo grows into a new seed sporophyte. stored food 2 Meiosis of cells in 5 Meiosis of cells pollen sacs yields in ovules yields microspores ( n). megaspores ( n). 8 Fertilization produces a zygote zygote (2 n) that develops into a seed. Fer tilization Inside 3 Microspores 6 Megaspores ovule develop into pollen develop into egg- grains (male game- bearing female tophytes) that are pollen tube eggs (n) gametophyte s released and travel inside the ovule. on the wind. female sperm (n) gametophyte 7 A pollen grain alights on a scale of an ovulate cone. It germinates and a pollen tube grows toward the ovule. Sperm form as the tube grows. © 2016 Cengage Learning; photos, Robert & Linda Mitchell Photography; Robert Potts, California Academy of Sciences; R. J. Erwin/Science Source © Cengage Learning 2016 Gymnosperms • Cycads – Native to dry tropics and subtropics • Ginkgos – One species remains today: Ginkgo biloba • Native to China – Deciduous • Gnetophytes – Tropical trees – Desert shrubs – Leathery vines © Cengage Learning 2016 Angiosperms – The Flowering Plants • Vascular seed plants that make flowers and fruits • Flower – Specialized reproductive shoot – Contain sepals, petals, and stamen – Innermost part of the flower is the ovary – After fertilization, ovule matures into seed • Ovary becomes the fruit © Cengage Learning 2016 1 An anther has two pollen sacs pollen sac with diploid cells of anther that give rise to (cutaway ovule microspores. view) 4 An ovule forms on the ovary wall. It contains a diploid 2n cell that will undergo meiosis. 9 The ovule seed coat (2n) develops into seed Meiosis embryo (2 n) Meiosis a seed. endosperm (3 n) Double Fertilization 2 Meiosis produces microspores (n) microspores. 5 Meiosis megaspores (n) yields four megaspores. Microspores 3 pollen grain(n) Three will develop into pollen degenerate. grains (male gametophytes). mature female gametophyte in ovule pollination pollen tube 6 The remaining delivers megaspore devel- 2 sperm to cell with ops into a female 7 Pollinatio n ovule 2 nuclei gametophyte tha t occurs, and the pollen consists of an egg grain germinates. pollen and six other cells, mature male A pollen tube grows to tube egg including a cell wit h and through ovary gametophyte sperm 8 During double fertilization, one two nuclei. tissue to the ovule, sperm fertilizes the egg to form a where it releases two zygote; the other fertilizes the cell with © 2016 Cengage Learning sperm. two nuclei to form a triploid (3n) cell. © Cengage Learning 2016 Angiosperms – The Flowering Plants • 90 percent of all species today are angiosperms • Angiosperm characteristics that give them a selective advantage over gymnosperms – Shorter life cycle – Animal-pollinated flowers – Enhanced seed dispersal © Cengage Learning 2016 Angiosperm Diversity and Importance © Cengage Learning 2016.