Angiosperms and course
summary
Lecture #18
Seed evolution and changing life cycles
Homospory,
heterospory and
the seed habit
Homospory
Land plant life cycles
Produce 1 size of spore
Exosporic gametophyte development
Free living gametophytes with multicelluar sex organs-antheridia and
archegonia
Free sporing plants (=shed their spores from the sporangia)
Need water for spore germination, gametophyte development and
fertilization to take place
Heterospory
Produce 2 sizes of spores (megaspores and microspores)
Endosporic gametophyte development (inside of spore wall)
Free sporing plants
Only need water for fertilization as gametophytes develop inside the
spore wall
Seed habit - gymnosperms
• Plants produce 2 sizes of spores
• Endosporic gametophytes
• Megasporangium never sheds its megaspore –
i.e., it is not free sporing
• Has eliminated the need for water in the
completion of the life cycle
• New to seed plants-the integument, pollen
tubes that deliver the pollen to the archegonia
or close to it.
Cycads-swimming sperm
Conifers-non-flagellate sperm
Nucellar apex
Cycads & Ginkgo have
multiflagellate sperm
Pinus microgametophyte development
Seed habit- angiospermy
• Plants produce 2 sizes of spores
• Endosporic gametophytes
• Megasporangium never sheds its megaspore –
i.e., it is not free sporing
• Seeds – double integumented
megasporangium enclosed in a carpel
• The megagametophyte has no archegonia and
only one egg
• The microgametophyte is only 3 cells total (vs.
6 cells in pines and 5 cells in cycads, 32 cells in
heterosporous plants like Selaginella)
Characters of angiosperms
• Carpel encloses the seeds
• Double integument for seed
• Flowers – determinate stem tip with leaves and
modified leaves that bear the pollen and seeds
• 7-celled, 8 nucleate megagametophyte (= embryo
sac)
3 antipodals, 2 synergids, 1 egg, large central cell with
2 nuclei
• 3-celled microgametophyte
tube cell (pollen tube) and 2 sperm-each sperm used
• Double fertilization
egg + sperm = zygote polar nuclei + sperm=
endosperm
Angiosperm
Ovule
Central cell
Nucellus
7 celled embryo sac = megagametophyte
Angiosperm
Ovule
Double
fertilization
Central cell
2 sperm
Nucellus
Pollen tube
7 celled embryo sac = megagametophyte
Primary endosperm
nucleus
3n
(degenerate)
xx x
(degenerate)
x x
Zygote 2n
2n
2n
3n
Capsella mature embryo l.s.
Pollination in Angiosperms
.
The transfer of pollen from the anther
to the stigma
.
Pollination droplet
Integument
Megaspore
Ovule
enclosed
in
carpel
Pollination in gymnosperms
Nucellus
The transfer of pollen from the pollen sac to
the micropyle of the seed/ovule
Parts of a typical gymnosperm seed
micropyle
outer layer of the integument=
sarcotesta
stony layer=sclerotesta
inner layer=endotesta
nucellar beak
pollen chamber
nucellus=megasporangium
archegonial chamber
archegonium with egg
Cycad ovule
megagametophyte
Angiosperm
Ovule
Central cell
Nucellus
7 celled embryo sac = megagametophyte
Angiosperm life cycle
Text book, Pages 472-473; Also, see self-test exercise on course web site
Today’s Lab a series of demonstration slides on
the angiosperm life cycle
18 stations (17 from Lilium life cycle
–a monocot
and 1 from Capsella life cycle that we saw last
lab – a dicot)
Use this as a review, and to see these stages
on slides
No drawings today
Final Exam: March 19, 8:00 a.m.
1070 Cordley Hall
• Exam will cover all material taught in the course
• Exam will emphasize material not covered in exams 1 and 2
• Exam will be of the same structure as previous exams
• Exam will cover Labs 13-17 in detail (seed plants)
• Exam will cover lectures 13-18 in detail (seed plants)-
i.e., Feb. 17- Mar 12 lectures on course website
Please use the extra material available on the website links:
Hydrasperman Reproduction
Life Cycle of an Angiosperm
(also, reviews of fern life cycle and Equisetum (homospory)
are available there)