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CHAPTER 30 LECTURE SLIDES
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Overview of Green Plants
Chapter 30
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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
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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
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Green plants Streptophyta Land plants Bryophytes Tracheophytes
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Green plants Streptophyta Land plants Bryophytes Tracheophytes Euphyllophytes Green algae Green algae Seed plants Red Algae Chlorophytes Charophytes Liverworts Mosses Hornworts Lycophytes Ferns + Allies Gymnosperms Angiosperms Ancestral alga
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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
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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
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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
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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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chlorophytes Charophytes Liverworts
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Chlorophytes Early green algae probably resembled Chlamydomonas reinhardtiii Individuals are microscopic 2 anterior flagella Most individuals are haploid Reproduces asexually and sexually Not haplodiplontic Always unicellular
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Volvox Colonial chlorophyte
Hollow sphere of a single layer of 500–60,000 cells Individual cells each have 2 flagella Few cells are specialized for reproduction Asexual or sexual
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No ancestral chlorophytes gave rise to land plants
Ulva Multicellular chlorophyte Haplodiplontic life cycle Gametophyte and sporophyte have identical appearance No ancestral chlorophytes gave rise to land plants © Dr. Diane S. Littler
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Charophytes Clade of streptophytes Also green algae
Distinguished from chlorophytes by close phylogenetic relationship to land plants
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Charophytes have haplontic life cycles
Evolution of diplontic embryo and haplodiplontic life cycle occurred after move to land 2 candidate Charophyta clades Charales Coleochaetales 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
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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
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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
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Liverworts (phylum Hepaticophyta)
Have flattened gametophytes with liverlike lobes 80% look like mosses Form gametangia in umbrella-shaped structures Also undergo asexual reproduction
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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
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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
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Tracheophyte Plants Cooksonia, the first vascular land plant
Appeared about 420 MYA Phylum Rhyniophyta Only a few centimeters tall No roots or leaves Homosporous – only 1 type of spore
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Vascular tissues Xylem Phloem
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
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Tracheophytes Vascular plants include seven extant phyla grouped in three clades Lycophytes (club mosses) Pterophytes (ferns, whisk ferns, and horsetails) 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
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Stems Roots Leaves 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
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without vascular tissue Photosynthetic tissue
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Lycophyll Origins Single vascular strand (vein) Stem with vascular tissue Stem, leafy tissue without vascular tissue Stem, leafy tissue with vascular tissue Euphyll Origins Branched vascular strands (veins) Branching stems with vascular tissue Unequal branching Branches in single planes Photosynthetic tissue “webs” branches
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400 million years between appearance of vascular tissue and true leaves
Natural selection favored plants with higher stomatal densities in low-CO2 atmosphere Higher stomatal densities favored larger leaves with a photosynthetic advantage that did not overheat Seeds Highly resistant Contain food supply for young plant Lycophytes and pterophytes do not have seeds
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Copyright © The McGraw-Hill Companies, Inc
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chlorophytes Charophytes Liverworts Mosses Hornworts Lycophytes Ferns + Allies Gymnosperms Angiosperms Flowers Fruits Seeds Euphylls Stems, roots, leaves Dominant sporophyte Vascular tissue Stomata Multicellular embryo Antheridia and archegonia Cuticle Plasmodesmata Chlorophyll a and b Ancestral alga 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
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Lycophytes Worldwide distribution – abundant in tropics Lack seeds
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Lycophytes Ferns and Allies Seed Plants Hornworts Lycophytes Worldwide distribution – abundant in tropics Lack seeds Superficially resemble true mosses Sporophyte dominant
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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
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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
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Horsetails All 15 living species are homosporous
Constitute a single species, Equisetum Sporophyte consists of ribbed, jointed photosynthetic stems that arise from branching rhizomes with roots at nodes Silica deposits in cells – scouring rush
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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
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Fern life cycle differs from that of a moss
Much greater development, independence, and dominance of the fern’s sporophyte Gametophyte lacks vascular tissue
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Fern morphology Sporophytes have rhizomes
Fronds (leaves) develop at the tip of the rhizome as tightly rolled-up coils (“fiddleheads”)
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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
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The Evolution of Seed Plants
Seed plants first appeared 305–465 MYA Evolved from spore-bearing plants known as progymnosperms 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
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b: © Biology Media/Photo Researchers, Inc.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Stored food Integument (seed coat) Embryo 312 m b: © Biology Media/Photo Researchers, Inc. 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
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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
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Gymnosperms Plants with “naked seeds” There are four living groups
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Gymnosperms Ferns and Allies 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 Gymnosperms Angiosperms
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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)
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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
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Male gametophytes (pollen grains)
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
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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
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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!
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Gnetophytes (phylum Gnetophyta)
Only gymnosperms with vessels in their xylem Contain three (unusual) genera Welwitschia Ephedra Gnetum
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Ginkgophytes (phylum Ginkgophyta)
Only one living species remains Ginkgo biloba Flagellated sperm Dioecious Male and female reproductive structures form on different trees
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Angiosperms Flowering plants
Ovules are enclosed in diploid tissue at the time of pollination Carpel, a modified leaf that covers seeds, develops into fruit
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(bottom right): © Goodshoot/Alamy RF
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Ovules (seeds) Carpel (fruit) Ovules Cross section Modified leaf with ovules Folding of leaf protects ovules Fusion of leaf margins (bottom right): © Goodshoot/Alamy RF
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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
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Flower morphology Modified stems bearing modified leaves
Primordium develops into a bud at the end of a stalk called the pedicel Pedicel expands at the tip to form a receptacle, to which other parts attach Flower parts are organized in circles called whorls
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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
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Carpel has 3 major regions
Ovary – swollen base containing ovules Later develops into a fruit Stigma – tip where pollen lands Style – neck or stalk
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Single megaspore mother cell in ovule undergoes meiosis
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Nucellus Megaspore mother cell Integuments Micropyle Stalk of ovule (funiculus) b. Single megaspore mother cell in ovule undergoes meiosis Produces 4 megaspores 3 disappear Nucleus of remaining megaspore divides mitotically Daughter nuclei divide to produce 8 haploid nuclei 2 groups of 4 Integuments become seed coat Form micropyle
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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
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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
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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
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Seed may remain dormant for many years
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
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