BIOL 1030 – TOPIC 4 LECTURE NOTES

Topic 4: Plant Diversity II - Seed Plants (Chs. 30, 38)

  1. Seed plants
  2. all are heterosporous vascular plants
  3. three major reproductive adaptations
  4. gametophyte reduced to dependence on sporophyte; retained in moist reproductive tissue
  5. seed – “baby plant in a lockbox with its lunch”; highly resistant structures that allow for a dormant phase in the life cycle to wait out poor environmental conditions
  6. evolution of pollen as male gametophyte – many seed plants are no longer tied to external water for fertilization
  7. common ancestor with seeds gave rise to all seed plants (gymnosperms and angiosperms); together, gymnosperms and angiosperms are a monophyletic group
  8. fossil evidence indicates origins in progymnosperm group about 360 MYA
  9. gametophytes are completely dependent on parent sporophytes for nutrition and are composed of only a few cells
  10. male gametophytes develop from microspores
  11. become pollen grains
  12. entire male gametophyte moved to the female as pollen grains
  13. cannot perform photosynthesis, depends on nutrients that came from the parent sporophyte
  14. female gametophytes develop from megaspores within ovules
  15. ovule contains female gametophyte surrounded by nucellus(megasporangium)
  16. nucellus is surrounded by 1-2 integuments (cell layers that serve as protective covers)
  17. micropyle – opening in integuments (allows sperm to get in)
  18. cannot perform photosynthesis, depends on nutrients from the parent sporophyte
  19. means of transporting sperm to egg varies, but typically uses a growing pollen tube that does not require outside water
  20. moving pollen to vicinity of ovule called pollination; agents include wind, animals
  21. seed develops from ovule
  22. seeds are highly resistant structures that allow for a dormant phase in the life cycle to wait out poor environmental conditions
  23. embryo protected by a seed coat, an extra layer of hardened tissue derived from sporophyte tissue in the ovule (sporophyte tissue from parent, not from embryo)
  24. enhanced protection from drought, cold, heat
  25. some protection from pathogens and predators
  26. external water only needed at germination
  27. initial food supply for germinating plant is enclosed
  28. seeds replace spores as means of dispersal; can enhance means of dispersal
  29. seeds plants together are a monophyletic group
  30. divided into two “groups” based on whether or not ovule is completely enclosed by sporophyte tissue at time of pollination
  31. gymnosperms – “naked seed”
  32. angiosperms – “covered seed” – covered in Topic 7
  1. gymnosperms
  2. long thought to be a grade, but molecular data shows that living members may actually form a clade
  3. 4 phyla with living members
  4. essentially, all seed plants that are not angiosperms
  5. all lack flowers and fruits that are found in angiosperms
  6. ovule not completely enclosed by sporophyte tissue at time of pollination
  7. instead, ovule sits exposed on a scale (a modified leaf)
  8. 4 phyla
  9. Phylum Coniferophyta (the conifers)
  10. Phylum Cycadophyta (the cycads)
  11. Phylum Ginkgophyta (Ginkgo)
  12. Phylum Gnetophyta (the gnetophytes)
  1. Phylum Coniferophyta (the conifers)
  2. monophyletic group
  3. ~600 living species; worldwide distribution, more common in cold or dry regions
  4. pines, spruces, firs, cedars, junipers, hemlocks, yews, larches, cypresses, redwoods
  5. nearly all are evergreen
  6. many have needle-shaped leaves adapted to dry conditions (resistant to water loss)
  7. thick cuticle
  8. stomata in pits
  9. tallest plant: more than 110 m (Coastal Redwood, Sequoia sempervirens)
  10. oldest tree: Methuselah, estimated more than 4600 years old (Bristlecone Pine, Pinus longaeva)
  11. sources of timber, paper, resin, cancer drug taxol, etc.
  12. “soft” wood (unlike angiosperm trees, no vessels or fibers in xylem)
  13. pines as a representative group
  14. over 100 species
  15. native to Northern hemisphere
  16. typically thick bark (survive fires, drought)
  17. secrete resin from leaves and bark
  18. response to wounding
  19. deters fungal and insect attacks
  20. source of turpentine (volatile liquid, organic solvent) and solid rosin
  21. pine life cycle
  22. pine tree is sporophyte, with sporangia located on cones
  23. gametophyte generations reduced; retained within sporangia
  24. male gametophyte is pollen grain (no antheridium)
  25. female gametophyte produces archegonia within ovule
  26. heterosporous: separate male and female cones
  27. male cones (pollen cones)
  28. clusters of 30-70
  29. usually at tips of lower branches
  30. 1-4 cm long; papery scales in spirals or whorls
  31. pair of microsporangia sacs within each scale
  32. microspore mother cells in microsporangia form haploid microspores
  33. each microspore becomes 4-celled pollen grain
  34. pollen grain carried by wind (pair of air sacs provides buoyancy)
  35. mature pollen grains have a “Mickey Mouse” appearance
  36. one cluster of pollen cones can yield over 1 million pollen grains
  37. female cones (ovulate cones)
  38. typically on upper branches of same tree with pollen cones
  39. larger than pollen cones
  40. scales become woody (highly lignified)
  41. pair of ovules develop at base of each scale
  42. megasporangiumcallednucellusembedded in each ovule
  43. nucellus is nutritive tissue surrounded by thick integument (covering) with hole (micropyle) near one end
  44. one layer of integument later becomes seed coat
  45. single megaspore mother cell in each megasporangium
  46. produces 4 haploid megaspores; 3 break down
  47. surviving megaspore develops over about one year into female gametophyte with sometimes thousands of cells
  48. female gametophyte has 2-6 archegonia, at micropylar end
  49. each archegonium has one large egg (visible without a microscope)
  50. female cones take two or more seasons to mature
  51. reproduction
  52. scales of ovulate cone open
  53. pollen lands near micropyle, caught by sticky fluid
  54. evaporating fluid pulls pollen through micropyle into ovule
  55. scales close (female gametophyte not mature)
  56. pollen grain germinates, forming pollen tube that digests through nucellus (takes about 15 months to reach archegonium)
  57. one of the four pollen grain cells (the generative cell) undergoes mitosis; one of products divides again, making two sperm cells
  58. mature male gametophyte is germinated pollen grain with pollen tube and sperm
  59. when archegonium is reached, one sperm fertilizes egg
  60. zygote develops into embryo within seed; usually only one successful zygote per ovule
  61. embryo (new sporophyte, 2N) has rudimentary root, several embryonic leaves (cotyledons)
  62. food source in seed derives from rest of female gametophyte (1N)
  63. scales of cone open and separate; winged seeds disperse
  64. from initial ovulate cone formation to final seed production 3 years or more
  1. Phylum Cycadophyta (the cycads)
  2. monophyletic group
  3. ~200 living species, tropical and subtropical; many in danger of extinction
  4. along with conifers, dominated Mesozoic era (245 MYA – 65 MYA)
  5. slow-growing; some grow >15 m tall
  6. most resemble palm trees, but produce cones (female cones up to 45 kg, or 100 lbs!)
  7. have life cycle similar to pines
  8. unusual sperm
  9. have thousands of flagella arranged in spirals
  10. swim within ovule to archegonium
  11. largest sperm known
  1. Phylum Ginkgophyta (Ginkgo)
  2. monophyletic group
  3. 1 living species, Ginkgo biloba (also known as the maidenhair tree)
  4. exists only in cultivation (no natural native populations); first cultivated in Japan and China
  5. deciduous – lose leaves
  6. flagellated sperm (similar to cycads)
  7. dioecious
  8. stinky seed coverings (produce butyric and isobutyric acid, making the smell of rancid butter)
  9. males often planted on city streets (do not stink like females; resistant to air pollution)
  1. Phylum Gnetophyta (the gnetophytes)
  2. apparently a monophyletic group, but could be paraphyletic
  3. ~70 living species in 3 genera: Welwitschia, Ephedra, Gnetum
  4. some evidence that they form a clade with angiosperms (if so, then gymnosperms are what?)
  5. vessels in xylem (common in angiosperms, found only in these gymnosperms)
  6. members of Gnetum have broad leaves similar to angiosperm leaves
  7. some genetic similarity to angiosperms
  8. Welwitschia – bizarre plants of southwest African deserts
  9. stem is shallow cup that tapers into a taproot
  10. two leathery leaves (often split) grow continuously from base
  11. conelike reproductive structures at leaf base
  12. dioecious – separate male and female plants
  13. Ephedra common in Mexico and southwestern US, but found on most continents
  14. shrubby, stems resemble horsetails (jointed, with tiny scale-like leaves at each node)
  15. some species monoecious (male and female parts on same plant), some dioecious
  16. drug ephedrine historically extracted mainly from a Chinese species of Ephedra
  1. Phylum Anthophyta – flowering plants (antho – flower)
  2. also known as angiosperms (angeion – vessel or enclosure; sperma – seed)
  3. ovules enclosed within carpel (parent diploid sporophytic tissue) at pollination
  4. the “vessel” is the carpel, which is a modified leaf
  5. carpels, especially their enlarged basal portion (the ovary), usually develop into fruit, which is unique to angiosperms
  6. about 250,000 known living species (dominant photosynthetic organisms on land)
  7. predominant source of human food
  8. most widespread and diverse plant phylum
  9. range from microscopic to plants with leaves over 6 m long
  10. flowers show incredible variety from species to species
  11. variety of lifestyles includes parasites (ex.: mistletoe, dodder, beechdrops); mycotrophs (derive nutrients from fungi; ex.: Indian Pipe, others); epiphytes (ex.: some orchids); “carnivorous” (ex. pitcher plants, sundews, Venus flytrap)
  12. monophyletic group with seeds, refined xylem, double fertilization, and these synapomorphic characteristics:
  13. seed contains endosperm
  14. presence of flowers (modified stems and leaves)
  15. true fruits
  16. evolutionary history
  17. monophyletic group
  18. origin about 140 MYA
  19. phylogeny
  20. historically divided into two classes, dicots and monocots
  21. recent genetic analysis has shown that the traditional dicots are a paraphyletic group
  22. thus, the old classification scheme is being replaced
  23. no conclusive cladogram has been produced for angiosperms
  24. studies are ongoing
  25. most modern cladograms have Amborella and water lilies as a sister group (or groups) to the rest of the angiosperms
  26. cladogram below from

  • various class-level groupings have been proposed, the overall naming and formal classification within Phylum Anthophyta is still in a state of flux
  • nevertheless, by far most of the living angiosperm species are found within two monophyletic groups, eudicots and monocots
  1. eudicots
  2. most have embryos have two cotyledons (seed leaves)
  3. leaves have netlike veins
  4. flower part typically in multiples of 4 or 5
  5. groups of vascular tissues in a ring
  6. pollen grains mostly with 3 or more apertures
  7. endosperm mostly used up in mature eudicot seeds
  8. about 175,000 living species; includes nearly all flowering trees and shrubs
  9. about a sixth are annuals (entire growth cycle in one year or less)
  10. monocots
  11. embryos have one cotyledon
  12. leaves have essentially parallel veins
  13. flower part typically in multiples of 3
  14. groups of vascular tissues scattered
  15. pollen grains mostly with one aperture
  16. endosperm typically present in mature monocot seeds
  17. about 65,000 living species; no true wood, few annuals
  1. Why were (and are) angiosperms successful?
  2. 130 MYA two major continental masses
  3. Laurasia = North America, Europe, Asia
  4. Gondwanaland = South America, Africa, Australia, Antarctica, India, New Zealand)
  5. angiosperms first appeared in Gondwanaland, in what was likely a drier interior region
  6. advantages of flowering plants
  7. transfer of pollen over great distances promotes outcrossing
  8. efficient seed dispersal via fruit
  9. endosperm gives seedlings a fast start
  10. leaves appropriate for fast growth in hot, dry environment
  11. coevolution with insects
  12. dominant by ~80 MYA, second half of Cretaceous Period
  13. all present angiosperm families represented by that time
  14. many insect orders appeared or became more abundant at that time
  1. Flowers
  2. modified stems with modified leaves
  3. develop as primordium bud at end of stalk called pedicel
  4. pedicel widens at tip to form receptacle
  5. other flower parts attached to receptacle in four whorls; from outside in:
  6. calyx – sepals; usually green, leaf-like, and protect immature flower
  7. corolla – petals; usually colorful, attract pollinators; together with calyx called perianth
  8. androecium – stamens; male reproductive structures
  9. filament + anther
  10. microspores produced within anther, shed as pollen
  11. gynoecium – female reproductive structure
  12. center location is most protected
  13. formed from leaf-like structure with ovules along margin
  14. edges fold inwards around ovules, forming carpels
  15. primitive: many separate carpels
  16. advanced: carpels fused (called pistil)
  17. carpel/pistil segments
  18. ovary – swollen base with 1 to hundreds of ovules; develops into fruit
  19. stigma – tip; sticky and/or feathery to catch pollen
  20. style – usually present; separates stigma from ovary
  21. nectaries may be present at base of pistil; secrete sugar, amino acids, and other compounds to attract pollinators
  22. know the structures of a flower [Figure 38.2] and their functions

  1. typical Angiosperm life cycle
  2. female gametophyte
  3. single diploid megaspore mother cell in ovule undergoes meiosis while flower develops
  4. of 4 haploid megaspores produced, usually 3 break down
  5. remaining megaspore expands and replicates and divides until there are 8 haploid nuclei in two groups of 4
  6. one nuclei from each group migrates toward center; these are polar nuclei
  7. polar nuclei usually fuse to make a diploid nucleus, but may remain separate – in either case, they wind up in a single cell
  8. cell walls form around other nuclei, creating the 7-celled, 8-nucleate embryo sac or megagametophyte (female gametophyte)
  9. meanwhile, two layers (integuments) of ovule develop into seed coat with micropyle (small opening)
  10. in the megagametophyte, one of the cells closest to the micropyle becomes the egg; the other two there are synergids
  11. the three cells on the other end (the antipodals) eventually break down
  12. male gametophyte
  13. anthers with patches of tissue that become chambers lined with nutritive cells
  14. each patch has many diploid microspore mother cells
  15. microspore mother cell undergoes meiosis, making 4 haploid microspores that typically remain grouped in a tetrad
  16. each microspore nucleus replicates and divides once (via mitosis) without cytokinesis (meaning they remain as one cell with two nuclei, a binucleate microspore)
  17. usually, tetrad then breaks up
  18. two-layered wall develops around each binucleate microspore, now called a pollen grain
  19. outer wall – sculptured, appearance usually species-specific, often has chemicals that can react with an appropriate stigma to stimulate pollen tube formation
  20. apertures in outer wall – where pollen tube may grow out; eudicots – usually 3; monocots – usually 1
  21. pollination – transfer of pollen to a stigma
  22. usually between flowers of separate plants
  23. agents include wind, water, gravity, mammals, birds, insects
  24. various reward systems for animal agents (pollen, nectar, etc.)
  25. evolution of floral characteristics associated with pollination
  26. some plants self-pollinate (inbreeding) – pollen to same plant
  27. pollination followed by fertilization only if chemical signals are right
  28. fertilization
  29. pollen grain cytoplasm absorbs substances from stigma
  30. bulge forms through an aperture in pollen grain; becomes pollen tube
  31. pollen tube follows chemical gradient through style to micropyle
  32. chemicals diffuse from embryo sac
  33. micropyle usually reached within a few days (up to a year in some species)
  34. pollen grain has two nuclei; one, the generative nucleus, lags behind
  35. generative nucleus undergoes mitosis to make two non-flagellated sperm; this may occur in pollen grain or in pollen tube (male gametophyte now mature)
  36. pollen tube enters embryo sac, destroying a synergid
  37. double fertilization – essentially unique to angiosperms
  38. one sperm unites with egg, forming zygote
  39. other sperm unites with polar nuclei, forming 3N primary endosperm
  40. primary endosperm rapidly undergoes many cycles of mitosis, forming endosperm
  41. endosperm provides nutrients for embryo; in many seeds, it is gone by the time the seed is mature
  42. seed coat hardens
  43. remaining haploid cells degenerate
  44. now have seed with 2N embryo, 3N endosperm, and 2N seed coat (seed coat from parent female tissue)
  1. Seeds
  2. embryo – quickly forms all systems, then growth arrested (dormancy) – mature seed about 10% water, very low metabolic activity
  3. typically, dormancy occurs just after first leaves (cotyledons, or seed leaves) form
  4. stored food (in angiosperms, 3N endosperm and/or cotyledons)
  5. seed coat – tough, relatively impermeable
  6. protection from predators, pathogens
  7. protection from desiccation, harsh conditions (crucial on land)
  8. may allow seed to last hundreds of years
  9. dormancy broken only when conditions are right (seed bank in soil)
  10. germination = breaking dormancy = resuming metabolic activity, growing out of seed coat; occurs after water penetrates seed coat to embryo, bringing oxygen
  1. Fruits – mature ovaries
  2. fleshy – pomes (apples), drupes (peaches), true berries (blueberries, peppers), hesperidiums (oranges), pepos (melons, gourds), aggregate fruits (strawberries, raspberries), multiple fruits (pineapple, fig)
  3. dry – follicles (milkweed, magnolia), legumes (peas, beans), siliques and silicles (mustards), capsules (irises, lilies, orchids), caryopses (grasses), nuts (chestnuts, hazelnuts, acorns), achenes (sunflowers), samaras (maples, elms, ashes), schizocarps (parsleys)
  4. dispersal
  5. by wind
  6. wings – maples
  7. parachutes – dandelions, milkweeds
  8. dust-like seeds – orchids
  9. by water – coconuts
  10. by vertebrates
  11. fleshy, edible fruits (blue, black, red) – seeds often deposited in feces
  12. dry, edible – nuts, others – squirrels bury and forget about them
  13. dry, inedible – hooks to grasp hair, feathers (cockleburs, etc.)
  14. by explosive dehiscence (jewelweed, others)
  1. many angiosperms also have asexual (or vegetative) reproduction
  2. stolons – runners – long slender stems that grow along soil (ex.: strawberry)
  3. rhizomes – underground stems – common in grasses; bulbs and tubers are rhizomes specialized for storage (ex.: potato)
  4. suckers – roots produce sprouts that grow into new plants (ex.: apple, raspberry, banana)
  5. adventitious leaves – numerous plantlets develop from tissue in notches along leaves
  6. apomixes – embryos in seeds may be produced asexually
  7. artificial: cuttings (for some species, can get roots to grow with appropriate environment)
  1. Floral Evolution
  2. first flowers
  3. numerous spirally arranged sepals, petals, stamens, and carpels
  4. petals and sepals similar in color and form
  5. all parts free (not fused)
  6. parts
  7. complete – calyx + corolla + androecium + gynoecium
  8. incomplete – one or more whorls absent
  9. perfect – has both androecium and gynoecium
  10. imperfect – missing either androecium or gynoecium
  11. complete flowers are always perfect; incomplete flowers can be either perfect or imperfect
  12. trends
  13. separate floral parts grouped together or fused
  14. connation – fusing within a whorl
  15. adnation – fusing between whorls (for example, sepals and petals fused together)
  16. reduction or loss of floral parts
  17. bilateral symmetry instead of radial symmetry
  18. ancestral type: radial symmetry; example: buttercups
  19. derived type: bilateral symmetry; examples: snapdragons, orchids
  20. bilateral symmetry in some cases has arisen independently in different groups
  1. Pollination mechanisms (pollination syndromes)
  2. wind – passive, primitive (oaks, cottonwoods, birches, grasses)
  3. copious amounts of pollen
  4. most pollen travels no more than 100 m
  5. flowers small, greenish, odorless
  6. corollas reduced or absent
  7. often grouped in large numbers, may hang down with tassels that wave in wind and shed pollen freely
  8. male and female parts often well-separated on plant to reduce chance of self-pollination
  9. often flower before leaves grow – keeps leaves out of the way
  10. animals – some cycads and gnetophytes also have this, so symplesiomorphic trait
  11. bees – most numerous of insect-pollinated plants use bees
  12. find via odor
  13. orient via shape, color, and texture
  14. usually blue or yellow flowers, bee sees in ultraviolet
  15. many have stripes or lines of dots to indicate nectaries (nectar guides)
  16. nectar offered as food for bees (pollen also)
  17. often close association between a bee species and a plant species
  18. flower only open when bees are active
  19. pollen collecting apparatus specific for particular plant
  20. other insects
  21. butterflies – flower usually has flat landing platform and long, slender floral tubes for long proboscis
  22. moths – flower usually white, yellow, or other pale color, heavily scented, typically need to be found at night
  23. flies – flower usually smells and somewhat appears like feces or rotting meat
  24. beetle – large flowers, copious pollen; beetle may eat other flower parts
  25. birds
  26. large amounts of nectar
  27. red – bees can’t see red, less likely to feed on the copious nectar
  28. usually odorless – birds have a poor sense of smell
  29. often in long, thick tube
  30. mammals (bats especially) – uncommon, but for some species is the only means of pollination; variety of appearances
  31. self-pollination
  32. small, inconspicuous flowers
  33. shed pollen directly onto stigma (or falls there by gravity); often before bud opens
  34. advantageous occasionally because no other plant is needed and no vector is needed – good when pollinators aren’t around (Artic, mountains)
  35. if you are well-adapted, might as well produce clones
  36. disadvantage of genetic load of bad mutations
  1. Promoting outcrossing
  2. staminate and pistillate flowers
  3. dioecious – separate sexes
  4. monoecious
  5. dichogamous – stamens and pistils reach maturity at different times
  6. stigma and stamens don’t touch (includes heterostyle)
  7. genetic self-incompatibility – pollen tube arrested or never germinates

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