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Chapter 30

• 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
4

Red Algae
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Green plants
Streptophyta
Land plants
Bryophytes
Green algae Green algae
Chlorophytes Charophytes Liverworts Mosses Hornworts
Tracheophytes
Lycophytes
Euphyllophytes
Ferns + Allies
Seed plants
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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• 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 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
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• Early green algae probably resembled
Chlamydomonas reinhardtiii
– Individuals are microscopic
– 2 anterior flagella
– Most individuals are haploid
• 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
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• 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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• 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
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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
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• 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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• Have flattened gametophytes with liverlike lobes
– 80% look like mosses
• Form gametangia in umbrella-shaped structures
• Also undergo asexual reproduction
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• 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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• 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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• Cooksonia , the first vascular land plant
– Appeared about 420 MYA
– Phylum Rhyniophyta
• Only a few centimeters tall
– No roots or leaves
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• 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
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• 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
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• 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
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• 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
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Chlorophytes Charophytes Liverworts Mosses Hornworts Lycophytes Ferns + Allies Gymnosperms Angiosperms
Seeds
Flowers
Fruits
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
27

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• Worldwide distribution – abundant in tropics
• Lack seeds
• Superficially resemble true mosses
• Sporophyte dominant
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• 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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• 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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• 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
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• 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 rolledup coils (“fiddleheads”)
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• 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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• 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

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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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• 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
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• 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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• 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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• 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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• Contain three
(unusual) genera
– Welwitschia
– Ephedra
– Gnetum
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• Only one living species remains
– Ginkgo biloba
• Flagellated sperm
• Dioecious
– Male and female reproductive structures form on different trees
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• 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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Ovules
(seeds)
Cross section
Ovules
Modified leaf with ovules
Folding of leaf protects ovules
Fusion of leaf margins
(bottom right): © Goodshoot/Alamy RF
Carpel
(fruit)
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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 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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• 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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• 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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