Lecture 22 - Plant Diversity (part 2) Flashcards
Further life on land is possible due to…
seeds & pollen
Seed plants have a set of adaptations further than those of bryophytes & seedless vascular plants. What are they?
- Seeds
- Reduced gametophytes
- Heterospory
- Ovules and pollen
Describe the reduced gametophytes in seed plants.
• Microscopic
• Tiny gametophytes can develop directly in the sporangium
- Protection from the elements such as UV radiation, desiccation
• Gametophytes nutritionally dependent on the sporophyte now
Describe heterospory
- Recall most seedless plants were homosporous
* Most seed plants are heterosporous
Describe the cycle starting from female spore leaves –> female gametophyte (n)
MegasporoPHYLLS (2n) (Female spore LEAVES) ->
MegaSPORANGIA (2n) (Female spore bearing STRUCTURES) ->
MegasporoCYTES (2n) (Female sporeproducing CELLS) ->
MegaSPORES (n) (Female spore) ->
Female gametophyte (n)
Describe the cycle starting from male spore leaves –> male gametophyte (n)
MicrosporoPHYLLS (2n) (Male spore LEAVES) ->
MicroSPORANGIA (2n) (Male spore bearing STRUCTURES) ->
MicrosporoCYSTS (2n) (Male spore producing CELLS ) ->
MicroSPORES (n) (Male SPORE) ->
Male gametophyte (n)
Seed plants retain the megasporangium within the…
parent sporophyte
Integument
is a 2n sporophytic tissue that envelopes and protects the megasporanium
How many integuments does Gymnosperms have?
1 integument
How many integuments does Angiosperms have?
2 integuments
Ovule
-> megasporangium (2n) + megaspore (n) + integuments (2n)
Where will the female gametophyte develop?
within the ovule
Pollen grain (n)
develops from a microspore and contains the male gametophyte
Pollen grain doesn’t equal…
male gametophyte
What is inside & outside of the pollen grain?
Inside = male gametophyte (n)
Outside pollen wall = secrete by microsporocytes (2n), contains sporopollenin
What can the shape of pollen grains & outer wall spikes do?
can ID the plant species it came from
Pollination
transfer of pollen to the ovule through a pollen tube in the micropyle of the ovule
What is an advantage of pollen grains?
can travel large distances without needing to swim in water
What are evolutionary advantages of seeds?
- If a sperm fertilises an egg, zygote will grow into a sporophyte
- In non-vascular plants, spores could be released in a dormant state that germinates when conditions are good; their small size allows for this
- In seed plants, the ovule develops into a multicellular seed, which can remain dormant for longer periods of time
- Seed plants share characteristics that arose during the Devonian period (380 million years ago)
- First evidence of seed plants themselves arose around 360 million years ago based on fossil record
Gymnosperms
“naked seed”
What is apart of the Gymnosperms?
- Cycads
- Gnetophytes
- Ginkgos
- Conifers
Cycads
palm like leaves, large cones
- part of gymnosperms
Gnetophytes
Gnetum, Ephedra, Welwitchia
- part of gymnosperms
Ginkgos
Ginkgo biloba, “maiden-hair” tree
- part of gymnosperms
Conifers
Pines, Spruce, Fir, junipers, larch, sequoia
- part of gymnosperms
Earliest Gymnosperm fossils are approx. 305 million years old. Where did they live and where did they survive better in?
• Lived in moist Carboniferous ecosystems still dominated by lycophytes and
other seedless vascular plants
• Survived better in drier climates due to seeds and pollen, thicker cuticles and
small surface area of their needles (leaves)
What are some things to note amongst all gymnosperms?
- Increasing dominance of the sporophyte
- Seed as a resistant, dispersible stage in the life cycle
- Pollen as an airborne agent brining sperm to the female gametophyte
- All are heterosporous
- Most produce cones: ovulate cones (female) and pollen cones (male)
* The scales on the cones are specialised sporophylls - Wind is the primary mode of dispersal
Are gymnosperms heterosporous or homosporous?
HETEROsporous
What is the primary mode of dispersal?
wind
Describe the Pine Life Cycle as an example of Gymnosperm cycles
1) In most conifer species, each tree has both ovulate (female) and pollen (male)
cones
2) Microsporocytes divide by meiosis to produce microspores (n). The microspore develops into a pollen grain (the male gametophyte (n) + surrounding wall (2n))
3) An ovulate cone scale has 2 ovules, each containing a megasporangium.
- Only 1 ovule is shown here
4) Pollination occurs when the pollen grain reaches the ovule, germinating to form a pollen tube that grows through the megasporangium
5) While the pollen tube develops, the megasporocyte (2n) undergoes meiosis. - Producing 4 haploid cells.
- Only 1 survives as the megaspore (n)
6) The megaspore develops into a female gametophyte that contains 2-3 archegonia, each of which will form an egg
7) By the time eggs are mature, sperm cells are fully developed and travel down the pollen tube for fertilisation
8) Fertilisation usually occurs 1 year after pollination. All the eggs may be fertilised, but only 1 zygote develops into an embryo (2n).
- The ovule becomes a seed (embryo + food supply + seed coat)