gymnosperms

what were gymnosperms 3 key innovations

1. wood

2. seeds

3. pollen

describe the evolution of wood

• main point is to enable plants to get taller

• advantages:

  • access light

  • shade out competitors

  • better spore+seed dispersal

  • modern trees have rings dividing cells beneath the bark into two directions to produce xylem and phloem

• provides more structure, allowing the tree to grow taller

  • increase in girth allows increase in height

• emerald ash borer - eggs feed off of tree’s phloem and kills it

describe xylem and phloem in trees

• xylem provides structure for water and nutrients UPwards

• phloem moves sugar DOWN

• xylem cells accumulate, cells stick around and stockpile

• xylem is lignified - composite of cellulose fibers in lignin matrix

whats a tree

plant with a single stem and a branching canopy that reaches large heights thanks to specialized cells, have vascular tissue

• has wood and increases in girth (width)

what was the first “true tree”

archaepteris

extinct genus of fern-like trees

• shows fern lifestyle→gymnosperm lifestyle

describe the key innovation: heterospory

homospory: spores all the same size (bryophytes, some ferns)

heterospory: spores are of two sizes (some ferns, seed plants)

• megaspore: germinate to make megagametophyte (seeds, female)

• microspore: germinate to make microgametophyte (pollen, male)

explain the evolution of seeds (megagametophyte)

• from many megaspores to 1, megaspores are retained in sporangium while it germinates

• megasporangium becomes surrounded by protective tissue ‘integument’

• ovule: megasporangium enclosed in integument instead of exposed to air

  • desiccation resistant, male gametophyte has to “burrow” to access

• seed: fertilized ovule

Evolution of pollen (microgametophyte)

• microspores evolve into pollen grains (tine male gametophyte) in sporopollenin

• germinates into pollen tube, goes down and to ovule

• pollen grains allow efficient reproduction in dry habitat

  • can endure long period, carried to female gametophyte by wind or animal

describe the gymnosperm reproductive cycle

• male gametophyte found in pollen grain, female gametophyte inside megasporangium

• pollen grain travels through air and lands within the vicinity of the female cone

• pollen grain may stick to pollination drop (excreted by ovule), close to the micropyle (small opening into the ovule)

• pollen grain can germinate and form a pollen tube which will grow into the micropyle and into the ovule to deliver sperm for fertilization

compare the gymnosperm reproductive cycle to others

in common:

• eggs develop within plant

• still sporangia, male and female gametophytes

• alternations of generations

• gametophyte is still multicellular

different

• sporophyte-dominant

• SEEDS!

• sperm doesn’t swim through water - through microspores and pollen tube

• pollination!

• *diploid life stages do dispersal

• gametophyte no longer free-living, germinate on female tissue

gymnosperm basic characteristics

• “naked seed”, unfertilized seed (ovule) open to the air to be directly pollinated

• seeds are either solitary, develop scales or leaves (modified cones)

• include modern all seed-bearing plants that don’t have flowers or ovaries

• “age of gymnosperms” - mesozoic era

list 4 extant groups of gymnosperms

1. cycads

2. ginkgos

3. gnetales

4. conifers

cycads characteristics

• slow-growing but long-lived

• distinguished by large ‘cone’ at apex of trunk

• separate male and female plants: cones either produce pollen or ovules

• motile sperm (40000 tails)

• mostly tropical lineage now

• dominated mesozoic

ginkgos

• “living fossil”

• separate male and female plants, motile sperm

• most closely related to cycads

• unusually resistant to pollution, usually only males planted

• big orange seeds with fleshy coats that have strong odour (not fruit!)

conifers “cone-bearing”

• most diverse group of extant gymnosperms

• woody and cone-bearing, most are trees

• include pines, podocarpus, cypress, araucarias, yews

• dominate largest forest biome: boreal forest

• pines: specialized in hot or cold dry environments

  • winter challenges: weight of snow, dryness, cold

  • thick bark and cones shelter seeds (cold)

  • cone-shaped, down-facing and flexible branches (dry)

  • thin waxy needles, modified leaf, dense (dryness)

whats the urban heat island effect

• dark materials absorb heat, ‘canyons’ from tall buildings trap heat, lack of plant cover

• trees provide shade and cool by evaporation