Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CHAPTER 30 LECTURE SLIDES

Overview of Green Plants Chapter 30

Defining Plants All green algae and the land plants shared a common ancestor a little over 1 BYA –Kingdom Viridiplantae –Not all photoautotrophs are plants Red and brown algae excluded A single species of freshwater green algae gave rise to the entire terrestrial plant lineage 3

The green algae split into two major clades –Chlorophytes – Never made it to land –Charophytes – Did – sister to all land plants Land plants… –Have multicellular haploid and diploid stages –Trend toward more diploid embryo protection –Trend toward smaller haploid stage 4

5 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Ancestral alga ChlorophytesCharophytesLiverwortsHornwortsMossesLycophytesGymnospermsAngiosperms Seed plants Euphyllophytes Bryophytes Land plants Streptophyta Green plants Green algae Red Algae Tracheophytes Ferns + Allies

Adaptations to terrestrial life –Protection from desiccation Waxy cuticle and stomata –Moving water using tracheids Tracheophytes have tracheids –Xylem and phloem to conduct water and food –Dealing with UV radiation caused mutations Shift to a dominant diploid generation –Haplodiplontic life cycle Mulitcellular haploid and diploid life stages Humans are diplontic 6

Haplodiplontic Life Cycle Multicellular diploid stage – sporophyte –Produces haploid spores by meiosis –Diploid spore mother cells (sporocytes) undergo meiosis in sporangia Produce 4 haploid spores First cells of gametophyte generation Multicellular haploid stage – gametophyte –Spores divide by mitosis –Produces gametes by mitosis –Gametes fuse to form diploid zygote First cell of next sporophyte generation 7

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All land plants are haplodiplontic Relative sizes of generations vary Moss –Large gametophyte –Small, dependent sporophyte Angiosperm –Small, dependent gametophyte –Large sporophyte 9

Green algae Green algae have two distinct lineages –Chlorophytes – Gave rise to aquatic algae –Streptophytes – Gave rise to land plants Modern chlorophytes closely resemble land plants –Chloroplasts are biochemically similar to those of the plants 10 CharophytesLiverwortsChlorophytes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Chlorophytes Early green algae probably resembled Chlamydomonas reinhardtiii –Individuals are microscopic –2 anterior flagella –Most individuals are haploid Always unicellular 11

Volvox –Colonial chlorophyte –Hollow sphere of a single layer of 500– 60,000 cells –Individual cells each have 2 flagella 12

Ulva –Multicellular chlorophyte –Haplodiplontic life cycle Gametophyte and sporophyte have identical appearance No ancestral chlorophytes gave rise to land plants 13 © Dr. Diane S. Littler

Charophytes Also green algae Distinguished from chlorophytes by close phylogenetic relationship to land plants Both charophyte clades form green mats around the edges of freshwater ponds and marshes One species must have successfully inched its way onto land through adaptations to drying 14

Charophytes have haplontic life cycles –Evolution of diplontic embryo and haplodiplontic life cycle occurred after move to land 2 candidate Charophyta clades –Charales –Coleochaetales 15

Bryophytes Closest living descendants of the first land plants Called nontracheophytes because they lack tracheids –Do have other conducting cells Mycorrhizal associations important in enhancing water uptake –Symbiotic relationship between fungi and plants 16

Simple, but highly adapted to diverse terrestrial environments 24,700 species in 3 clades –Liverworts –Mosses –Hornworts Gametophyte – conspicuous and photosynthetic –Sporophytes – small and dependent Require water for sexual reproduction 17

18 Liverworts (phylum Hepaticophyta) Have flattened gametophytes with liverlike lobes –80% look like mosses Form gametangia in umbrella-shaped structures Also undergo asexual reproduction

Mosses (phylum Bryophyta) Gametophytes consist of small, leaflike structures around a stemlike axis –Not true leaves – no vascular tissue Anchored to substrate by rhizoids Multicellular gametangia form at the tips of gametophytes –Archegonia – Female gametangia –Antheridia – Male gametangia Flagellated sperm must swim in water 19

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Hornworts (phylum Anthocerotophyta) Origin is puzzling – no fossils until Cretaceous Sporophyte is photosynthetic Sporophyte embedded in gametophyte tissue Cells have a single large chloroplast 21

Tracheophyte Plants Cooksonia, the first vascular land plant –Appeared about 420 MYA –Phylum Rhyniophyta Only a few centimeters tall –No roots or leaves 22

Vascular tissues Xylem –Conducts water and dissolved minerals upward from the roots Phloem –Conducts sucrose and hormones throughout the plant Enable enhanced height and size in the tracheophytes Develops in sporophyte but not gametophyte Cuticle and stomata also found in land plants 23

Tracheophytes Vascular plants include seven extant phyla grouped in three clades 1.Lycophytes (club mosses) 2.Pterophytes (ferns, whisk ferns, and horsetails) 3.Seed plants Gametophyte has been reduced in size relative to the sporophyte during the evolution of tracheophytes Similar reduction in multicellular gametangia has occurred as well 24

Stems –Early fossils reveal stems but no roots or leaves –Lack of roots limited early tracheophytes Roots –Provide transport and support –Lycophytes diverged before true roots appeared Leaves –Increase surface area for photosynthesis –Evolved twice Euphylls (true leaves) found in ferns and seed plants Lycophylls found in seed plants 25

400 million years between appearance of vascular tissue and true leaves Seeds –Highly resistant –Contain food supply for young plant –Lycophytes and pterophytes do not have seeds 26

Fruits in the flowering plants (angiosperms) add a layer of protection to seeds and attract animals that assist in seed dispersal, expanding the potential range of the species 27 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Ancestral alga ChlorophytesCharophytesLiverwortsHornwortsMossesLycophytesGymnospermsAngiosperms Chlorophyll a and b Plasmodesmata Cuticle Antheridia and archegonia Multicellular embryo Stomata Euphylls Seeds Flowers Fruits Dominant sporophyte Stems, roots, leaves Ferns + Allies Vascular tissue

Lycophytes Worldwide distribution – abundant in tropics Lack seeds Superficially resemble true mosses Sporophyte dominant 28 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. HornwortsLycophytesSeed Plants Ferns and Allies

Pterophytes Phylogenetic relationships among ferns and their relatives is still being sorted out Common ancestor gave rise to 2 clades All form antheridia and archegonia All require free water for flagellated sperm 29

Whisk ferns Found in tropics Sporophyte consists of evenly forking green stems without true leaves or roots Some gametophytes develop elements of vascular tissue –Only one known to do so 30

Horsetails 15 living species Constitute a single genus, Equisetum Sporophyte consists of ribbed, jointed photosynthetic stems that arise from branching rhizomes with roots at nodes Silica deposits in cells – scouring rush 31

Ferns Most abundant group of seedless vascular plants –About 11,000 species Coal formed from forests 300 MYA Conspicuous sporophyte and much smaller gametophyte are both photosynthetic 32

Fern life cycle differs from that of a moss Much greater development, independence, and dominance of the fern’s sporophyte Gametophyte lacks vascular tissue 33

Fern morphology –Sporophytes have rhizomes –Fronds (leaves) develop at the tip of the rhizome as tightly rolled-up coils (“fiddleheads”) 34

Fern reproduction Produce distinctive sporangia in clusters called sori on the back of the fronds Diploid spore mother cells in sporangia produce haploid spores by meiosis Spores germinate into gametophyte –Rhizoids but not true roots – no vascular tissue Flagellated sperm 35

The Evolution of Seed Plants Seed plants first appeared 305–465 MYA Success attributed to evolution of seed –Protects and provides food for embryo –Allows the “clock to be stopped” to survive harsh periods before germinating –Later development of fruits enhanced dispersal 36

Seed –Embryo protected by integument An extra layer or 2 of sporophyte tissue Hardens into seed coat –Megasporangium divides meiotically inside ovule to produce haploid megaspore –Megaspore produces egg that combines with sperm to form zygote –Also contain food supply for embryo 37 Stored food Embryo 312  m Integument (seed coat) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. b: © Biology Media/Photo Researchers, Inc.

Seed plants produce 2 kinds of gametophytes Male gametophytes –Pollen grains –Dispersed by wind or a pollinator –No need for water Female gametophytes –Develop within an ovule –Enclosed within diploid sporophyte tissue in angiosperms 38

Gymnosperms Plants with “naked seeds” There are four living groups –Coniferophytes –Cycadophytes –Gnetophytes –Ginkgophytes All lack flowers and fruits of angiosperms All have ovule exposed on a scale 39 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Angiosperms GymnospermsFerns and Allies

Conifers (phylum Coniferophyta) Most familiar gymnosperm phylum Pines, spruces, firs, cedars, and others –Coastal redwood – Tallest living vascular plant –Bristlecone pine – Oldest living tree Found in colder and sometimes drier regions of the world Conifers are sources of important products –Timber, paper, resin, and taxol (anti-cancer) 40

Pines –More than 100 species, all in the Northern hemisphere –Produce tough needlelike leaves in clusters –Leaves have thick cuticle and recessed stomata to retard water loss –Leaves have canals with resin to deter insect and fungal attacks 41

Pine reproduction Male gametophytes (pollen grains) –Develop from microspores in male cones by meiosis Female pine cones form on the upper branches of the same tree –Female cones are larger, and have woody scales –Two ovules develop on each scale –Each contains a megasporangium Each will become a female gametophyte 42

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Female cones usually take 2 or more seasons to mature During the first spring, pollen grains drift down between open scales –Pollen grains drawn down into micropyle –Scales close A year later, female gametophyte matures –Pollen tube is digesting its way through –Mature male gametophyte has 2 sperm 15 months after pollination, pollen tube reaches archegonium and discharges contents –One sperm unites with egg = zygote –Other sperm degenerates 44

Cycads (phylum Cycadophyta) Slow-growing gymnosperms of tropical and subtropical regions Sporophytes resemble palm trees Female cones can weigh 45 kg Have largest sperm cells of all organisms! 45

Gnetophytes (phylum Gnetophyta) Contain three (unusual) genera –Welwitschia –Ephedra –Gnetum 46

Ginkgophytes (phylum Ginkgophyta) Only one living species remains –Ginkgo biloba Flagellated sperm Dioecious –Male and female reproductive structures form on different trees 47

Angiosperms Flowering plants Ovules are enclosed in diploid tissue at the time of pollination Carpel, a modified leaf that covers seeds, develops into fruit 48

49 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Ovules Cross section Ovules (seeds) Carpel (fruit) Fusion of leaf margins Folding of leaf protects ovules Modified leaf with ovules (bottom right): © Goodshoot/Alamy RF

Angiosperm origins are a mystery –Origins as early as 145–208 MYA –Oldest known angiosperm in the fossil record is Archaefructus –Closest living relative to the original angiosperm is Amborella 50

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Flower whorls –Outermost whorl – sepals –Second whorl – petals –Third whorl – stamens (androecium) Pollen is the male gametophyte Each stamen has a pollen-bearing anther and a filament (stalk) –Innermost whorl – gynoecium Consists of one or more carpels House the female gametophyte 52

Carpel has 3 major regions –Ovary – swollen base containing ovules Later develops into a fruit –Stigma – tip where pollen lands –Style – neck or stalk 53

Embryo sac = female gametophyte –8 nuclei in 7 cells –8 haploid daughter nuclei (2 groups of 4) 1 from each group of 4 migrates toward center –Functions as polar nuclei – may fuse Egg –1 cell in group closest to micropyle –Other 2 are synergids Antipodals –3 cells at other end – no function 54

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Pollen production occurs in the anthers –It is similar but less complex than female gametophyte formation –Diploid microspore mother cells undergo meiosis to produce four haploid microspores –Binucleate microspores become pollen grains 56

Pollination –Mechanical transfer of pollen from anther to stigma –May or may not be followed by fertilization –Pollen grains develop a pollen tube that is guided to the embryo sac –One of the two pollen grain cells lags behind This generative cell divides to produce two sperm cells No flagella on sperm 57

Double fertilization –One sperm unites with egg to form the diploid zygote New sporophyte –Other sperm unites with the two polar nuclei to form the triploid endosperm Provides nutrients to embryo Seed may remain dormant for many years –Germinate when conditions are favorable 58