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Overview of Green Plants
Chapter 30
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Defining Plants The kingdom Viridiplantae includes land plants and green algae -Red and brown algae are excluded All green plants arose from a single species of freshwater algae The green algae split into two major clades -Chlorophytes – Never made it to land -Charophytes – Did!
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Defining Plants
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Defining Plants Land plants have two major features
1. Protected embryos 2. Multicellular haploid and diploid phases
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Defining Plants Adaptations to terrestrial life
1. Protection from desiccation by a waxy cuticle and stomata 2. Evolution of leaves which increase photosynthetic surface area 3. Shift to a dominant vertical diploid generation
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Plant Life Cycles Humans have a diplontic life cycle
-Only the diploid stage is multicellular Plants have a haplodiplontic life cycle -Multicellular diploid stage = Sporophyte -Multicellular haploid stage = Gametophyte
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Plant Life Cycles Sporophyte produces haploid spores by meiosis
Spores divide mitotically, producing the gametophyte Gametophyte produces gametes by mitosis Gametes fuse to form the diploid sporophyte
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Plant Life Cycles As more complex plants evolved:
1. Diploid stage became the dominant portion of the life cycle 2. Gametophyte became more limited in size 3. Sporophyte became nutritionally independent
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Chlorophytes Green algae have two distinct lineages
-Chlorophytes – Gave rise to aquatic algae -Streptophytes – Gave rise to land plants Chlamydomonas -Unicellular chlorophyte with two flagella -Have eyespots to direct swimming -Reproduces asexually as well as sexually
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Chlorophytes Volvox -Colonial chlorophyte
-Hollow sphere of a single layer of ,000 cells -A few cells are specialized for reproduction
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Chlorophytes Ulva -Multicellular chlorophyte
-Haplodiplontic life cycle -Gametophyte and sporophyte have identical appearance
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Charophytes Charophytes are green algae related to land plants
There are two candidate clades
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Charophytes Charales (300 species) -Macroscopic
-Plant-like plasmodesmata -Sister clade to land plants Choleocaetales (30 species) -Microscopic -Plant-like mitosis -Next closest plant relatives
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Charophytes (Cont.)
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Bryophytes Bryophytes are the closest living descendants of the first land plants -Called nontracheophytes because they lack tracheids (specialized transport cells) Simple, but highly adapted to diverse terrestrial environments Non-photosynthetic sporophyte is nutritionally dependent on the gametophyte
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Bryophytes Liverworts (phylum Hepaticophyta)
-Have flattened gametophytes with liver-like lobes -Form gametangia in umbrella-shaped structures -Also undergo asexual reproduction
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Bryophytes Hornworts (phylum Anthocerotophyta) -Sporophyte has stomata
-Sporophyte is photosynthetic -Cells have a single large chloroplast
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Bryophytes Mosses (phylum Bryophyta)
-Gametophytes consist of small, leaflike structures around a stemlike axis -Anchored to substrate by rhizoids -Multicellular gametangia form at the tips of gametophytes -Archegonia – Female gametangia -Antheridia – Male gametangia Mosses withstand drought, but not air pollution
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Features of Tracheophyte Plants
Cooksonia, the first vascular land plant, appeared about 420 MYA -Only a few centimeters tall -No roots or leaves -Homosporous
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Features of Tracheophyte Plants
Vascular tissues are of two types -Xylem – Conducts water and dissolved minerals upward from the roots -Phloem – Conducts sucrose and hormones throughout the plant -These enable enhanced height and size in the tracheophytes Tracheophytes are also characterized by the presence of a cuticle and stomata
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Features of Tracheophyte Plants
Vascular plants have gametophytes reduced in size and complexity relative to sporophytes Seeds -Highly-resistant structures that protect the plant embryo -Occur only in heterosporous plants Fruits in flowering plants add a layer of protection to seeds -Also attract animals that disperse seeds
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Features of Tracheophyte Plants
Vascular plants include seven extant phyla grouped in three clades 1. Lycophytes (club mosses) 2. Pterophytes (ferns and their relatives) 3. Seed plants
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Lycophytes Club mosses are the earliest vascular plants
-They lack seeds -Superficially resemble true mosses but they are not related -Homosporous or heterosporous
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Pterophytes The phylogenetic relationships among ferns and their relatives is still being sorted out
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Pterophytes Whisk ferns
-Saprophyte consists of evenly forking green stems without leaves or roots -Some gametophytes develop elements of vascular tissue -Only one known to do so
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Pterophytes Horsetails -All 15 living species are homosporous
-Constitute a single species, Equisetum -Consist of ribbed, jointed photosynthetic stems that arise from branching rhizomes
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Pterophytes Ferns -The most abundant group of seedless vascular plants with about 11,000 species -The conspicuous sporophyte and much smaller gametophyte are both photosynthetic
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Pterophytes The fern life cycle differs from that of a moss
-Much greater development, independence and dominance of the fern’s sporophyte Fern morphology -Sporophytes have rhizomes -Fronds (leaves) develop at the tip of the rhizome as tightly rolled-up coils -They unroll and expand
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Pterophytes Fern reproduction -Most fern are homosporous
-Produce distinctive sporangia in clusters called sori on the back of the fronds -Diploid spore mother cells in sporangia produce haploid spores by meiosis -At maturity, the spores are catapulted by snapping action
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The Evolution of Seed Plants
Seed plants first appeared MYA -Evolved from spore-bearing plants known as progymnosperms The seed represents an important advance 1. Protects the embryo 2. Easily dispersed 3. Introduces a dormant phase in the life cycle
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The Evolution of Seed Plants
Seed plants produce 2 kinds of gametophytes -Male gametophytes -Pollen grains -Dispersed by wind or a pollinator -Female gametophytes -Develop within an ovule -Enclosed within diploid sporophyte tissue
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Gymnosperms Gymnosperms are plants with “naked seeds”
-Ovule is exposed on a scale at pollination There are four living groups -Coniferophytes -Cycadophytes -Gnetophytes -Ginkgophytes All lack flowers and fruits of angiosperms
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Gymnosperms Conifers (phylum Coniferophyta) are the largest gymnosperm phylum -Include: -Pines, spruces, firs, cedars and others -Coastal redwood – Tallest tree -Bristlecone pine – Oldest living tree Conifers are sources of important products -Timber, paper, resin and taxol (anti-cancer)
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Gymnosperms -More than 100 species, all in the Northern hemisphere
Pines -More than 100 species, all in the Northern hemisphere -Produce tough needlelike leaves in clusters -Leaves have: 1. Thick cuticle and recessed stomata 2. Canals into which cells secrete resin
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Gymnosperms 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
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Gymnosperms Pine reproduction
-Each ovule contains a megasporangium called the nucellus -Surrounded by the integument -Opening – Micropyle -One layer becomes the seed coat -While scales of female cone are open, pollen grains drift down between them -Are drawn to top of nucellus
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Gymnosperms Pine reproduction
-While female gametophyte is developing, a pollen tube emerges from the pollen grain -It digests its way to the archegonium -Fifteen months after pollination, pollen tube reaches archegonium and delivers its sperm
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Gymnosperms Cycads (phylum Cycadophyta)
-Slow-growing gymnosperms of tropical and subtropical regions -Sporophytes resemble palm trees -Have largest sperm cells of all organisms!
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Gymnosperms Gnetophytes (phylum Gnetophyta)
-Only gymnosperms with vessels in their xylem -Contain three (unusual) genera -Welwitschia -Ephedra -Gnetum
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Gymnosperms Ginkgophytes (phylum Ginkgophyta)
-Only one living species remains -Ginkgo biloba -Dioecious -Male and female reproductive structures form on different trees
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Angiosperms Angiosperms are the flowering plants
-Ovules are enclosed in diploid tissue at the time of pollination -The carpel, a modified leaf that covers seeds, develops into fruit
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Angiosperms Angiosperm origins are a mystery
-The oldest known angiosperm in the fossil record is Archaefructus -The closest living relative to the original angiosperm is Amborella
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Angiosperms
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Angiosperms
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Angiosperms Flower morphology
-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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Angiosperms Flower morphology -Outermost whorl = Sepals
-Second whorl = Petals -Third whorl = Stamens (androecium) -Each stamen has a pollen-bearing anther and a filament (stalk) -Innermost whorl = Gynoecium -Consists of one or more carpels that house the female gametophyte
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Angiosperms
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Angiosperms Carpel structure -Three major regions
-Ovary = Swollen base containing ovules -Later develops into a fruit -Stigma = Tip -Style = Neck or stalk
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Angiosperm Life Cycle The female gametophyte (embryo sac) has 8 haploid nuclei arranged in two groups of four A nucleus from each group migrate toward the ovule’s center and become polar nuclei Cell walls form round remaining three nuclei -At the micropyle end, one cell functions as the egg, and the other two are synergids -At the other end, three cells are antipodals -They eventually break down
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Angiosperm Life Cycle 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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Angiosperm Life Cycle Pollination is the mechanical transfer of pollen from anther to stigma -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
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Angiosperm Life Cycle As the pollen tube enters the embryo sac, a double fertilization occurs -One sperm unites with egg to form the diploid zygote -Other sperm unites with the two polar nuclei to form the triploid endosperm -Provides nutrients to embryo When the seed germinates, a young sporophyte plant emerges
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Angiosperm Life Cycle Angiosperms include:
-Eudicots (about 175,000 species) -Trees, shrubs, snapdragons, peas, other -Use flowers to attract insect pollinators -Monocots (about 65,000 species) -Grasses, lilies, palms, irises, others -Some rely on wind for pollination Note: Self-pollination may also occur
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