chapter 30 Flashcards
Sporangium—structure in which spores are
produced by meiosis
Gametangium—structure in which gametes
are produced by mitosis
male gametangium—antheridium sperm
female gametagium—archegonium eggsHomosporous—produce one type of spore
Heterosporous—produce 2 types of spores
If a plant has no vascular tissue, it can’t form true
roots, stems or leaves
Haplontic life cycle—most of the life cycle is haploid
Diplontic life cycle—most of the life cycle is diploid
Haplodiplontic life cycle—life cycle has both mature diploid and haploid generations
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
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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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—so if one
gene on a homologous chromosome mutated, it
still had another nonmutated gene
Haplodiplontic life cycle
Mulitcellular haploid and diploid life stages
Humans are diplontic
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Haplodiplontic Life Cycle
Multicellular diploid stage – sporophyte (2n)
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 and become the gametophyte generation
Produces gametes by mitosis
Gametes fuse to form diploid zygote
First cell of next sporophyte generation
All land plants are haplodiplontic Relative sizes of generations vary Moss Large gametophyte—dominant generation 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
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—haplontic life cycle
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
Ulva
Multicellular chlorophyte
Haplodiplontic life cycle
Gametophyte and sporophyte have identical appearance
No ancestral chlorophytes gave rise to land plants
Charophytes
Clade of streptophytes
Also green algae
Distinguished from chlorophytes by close phylogenetic relationship to land plants
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
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—swimming sperm
Homosporous
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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
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
Hornworts (phylum Anthocerotophyta)
Origin is puzzling – no fossils until Cretaceous
Sporophyte is photosynthetic
Sporophyte embedded in gametophyte tissue
Cells have a single large chloroplast. Stomata.
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 Vascular tissue in stems
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
Sporophyte is the dominant generation
Cuticle and stomata also found in land plants
Tracheophytes
Vascular plants include seven extant phyla grouped in three clades
1Lycophytes (club mosses)
2Pterophytes (ferns, whisk ferns, and horsetails)
3Seed 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
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 Lycophytes
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
Lycophytes
Early vascular plants Worldwide distribution – abundant in tropics Lack seeds Superficially resemble true mosses Sporophyte dominant Lycophylls
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
Whisk ferns (Pterophytes)
Found in tropics
Sporophyte consists of evenly forking green stems without true leaves or roots
True stems—have vascular
tissue but no roots or leaves
Some gametophytes develop elements of vascular tissue
Only one known to do so
Horsetails
All 15 living species are homosporous
Constitute a single genus, Equisetum
Sporophyte consists of ribbed, jointed photosynthetic stems that arise from branching rhizomes with roots at nodes
Scale-like leaves arise from nodes and are nonphotosynthetic
Silica deposits in cells – scouring rush
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
True roots, stems and leaves
Fern morphology
Sporophytes have rhizomes (underground, horizontal
roots
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
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
Seed
Embryo protected by integument
An extra layer or 2 of sporophyte tissue
Hardens into seed coat
Also contain food supply for embryo
Heterosporous—2 kinds of spores megaspores and microspores Seed plants produce 2 kinds of gametophytes Male gametophytes--microgametophytes Pollen grains Dispersed by wind or a pollinator No need for water Female gametophytes--megagametopytes Develop within an ovule Enclosed within diploid sporophyte tissue in angiosperms
Female reproductive structure—cone or flower
Inside ovule of female—megasporangium (2n) divides meiotically to produce megaspores (n)germinates
(mitosis) into megagametophyte (n)egg (n)
Male reproductive structure—cone or flower
Microsporangium (2n) meiosismicrospores (n) germinate into microgametophytes (n) (pollen grains)mitosisproduce sperm (n)
Pollen flies or is carried to female part of plantreleases
spermfertilizes eggzygote
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
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)
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
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
Pine reproduction
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
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!
Flagellated sperm carried in pollen grain but swims to egg
Gnetophytes (phylum Gnetophyta)
Only gymnosperms with vessels in their xylem Contain three (unusual) genera Welwitschia--Africa Ephedra--ephedrine Gnetum
Ginkgophytes (phylum Ginkgophyta)
Only one living species remains Ginkgo biloba Flagellated sperm Dioecious Male and female reproductive structures form on different trees Resists pollution
Angiosperms
Flowering plants
Ovules are enclosed in diploid tissue at the time of pollination
Carpel, a modified leaf that covers seeds, develops into fruit
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
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
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
Carpel has 3 major regions Ovary – swollen base containing ovules Later develops into a fruit Stigma – tip where pollen lands Style – neck or stalk
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 = female gametophyte
Integuments become seed coat
Form micropyle
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 fusesingle 2n nucleus or single cell
with 2 haploid nuclei
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
Nucleus in microspores divide once
generative cellmitosis2 sperm
tube nucleus pollen tube
Now it is a mature microgametophyte
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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
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
