Kingdom Plantae:
Heterospory and
Seed Plants
Setting the stage for pollen/seeds
• Most of plants
discussed so far
do not have
specialized
gametophytes
• They are
homosporous
plants: make one
kind of
meiospore.
Setting the stage for pollen/seeds
• Homosporous
groups
–
–
–
–
–
Bryophyta
Hepaticophyta
Anthocerophyta
Psilophyta
Spenophyta
Setting the stage for pollen/seeds
• Heterosporous plants: make 2 types of
meiospores.
– One becomes male gametophyte and makes sperm
– One becomes female gametophyte and makes eggs
• Thus, make specialized spores and specialized
gametophytes.
Specialization occurs
earlier in life cycle
Setting the stage for pollen/seeds
• Heterosporous groups:
– Some Lycophyta
– Some Pterophyta
– All seed plant phyla (discussed later today!).
Setting the
stage for
pollen/seeds
• Example of
heterosporous
plant life cycle:
Selaginella
(Lycophyta)
• Seen in lab #3.
Setting the
stage for
pollen/seeds
• Heterosporous
plant life cycle:
Selaginella
(Lycophyta)
• Photos of
structures.
Heterospory
• Note deadbeat gametophytes
– Not free-living. Dependent on sporophyte
– Not (very) photosynthetic
– Small: microgametophyte reduced to 1 antheridium!
Megagametophyte develops mostly within
megaspore.
Heterospory
• Spores specialized
– Megaspore makes megagametophyte, which makes
archegonia with eggs
– Microspore makes microgametophyte, which makes
antheridium containing sperm.
Heterospory
• Valuable ideas for life on the land:
– 1) Gametophytes small and protected by spore walls
– 2) Deadbeat gametophytes fed by big strong
sporophyte (gametophytes can focus on reproduction)
– 3) Megaspore: keep it after fertilization and feed
embryo. Embryo starts development inside megaspore
and protected by its walls. Lets new embryo “eat” its
mother (megagametophyte): keeps energy used to
make megagametophyte from being wasted
– 4) Microspore: disperse it through air, instead of
delicate swimming sperm.
Plants
• Nonvascular Plants
– Covered. Reproduce by spores. Gametophytes
independent of sporophyte
• Seedless Vascular Plants
– Covered. Reproduce by spores. Gametophytes
independent of sporophyte
• Seed Plants
– Rest of plant kingdom. Gametophytes dependent on
sporophyte.
Seed plant features
• Have vascular tissue
– Make roots, stems, leaves (megaphylls)
– Some can do secondary growth in stems/roots: make
new phloem and xylem and grow in diameter rather
than just length
• Life Cycle
– Heterosporous, but spores kept by sporophyte
– Gametophytes reduced, dependent on sporophyte
• Male gametophyte (microgametophyte) dispersed as pollen
grain (no free-swimming sperm in water)
• Female gametophyte (megagametophyte) kept by
sporophyte, becomes part of ovule
Seed plant groups
• Gymnosperms (“gymno-” means naked, “sperm”
means seed): do not make make seeds in sealed
container (fruit)
–
–
–
–
Phylum Cycadophyta (cycads)
Phylum Coniferophyta (conifers)
Phylum Ginkgophyta (ginkgo)
Phylum Gnetophyta (gnetophytes)
• Note all gymnosperms do secondary growth
(form wood, are shrubs or trees)
Seed plant groups
• Angiosperms (“angio-” means container, “sperm”
means seed): make seeds in sealed container
(fruit)
– Phylum Anthophyta (flowering plants)
• Many of these are woody, but some are
herbaceous (don’t do secondary growth).
Pine life cycle
• Pine life cycle movie (for overview)
• Example of gymnosperm life cycle
QuickTime™ and a Cinepak decompressor are needed to see this picture.
Pine life cycle
– Female cone: Cone
scales with ovules on
them
Cone scale with ovule
Female
cone at
pollination
stage (top)
Mature female cone
Pine life cycle
• Ovule: integumented megasporangium.
Megaspore mother cell does meiosis, forms
meiospores, one survives & does mitosis to make
female gametophyte.
1
3
2
4
Pine life cycle
• Female gametophyte has several archegonia, each
with an egg.
1
3
2
()
4
()
Pine life cycle
• Male cone: made up of microsporophylls
• Microsporangia on sporophylls
Microsporophyll
Sporangium on underside of sporophyll
Pine life cycle
• Sporangia on microsporophylls have microspore
mother cells (2n), which do meiosis to make
microspores (1n)
• Each microspore does a couple mitotic divisions
to form an immature male gametophyte (called
pollen grain).
Getting male and female
gametophytes together
• Pollen grain flies through air (wind dispersed)
• Arrives at female cone micropyle (gap between
integument ends), and winds up next to female
gametophyte.
Getting male and female
gametophytes together
• Now, pollen grain germinates: makes pollen
tube and spermatogenous cell. Becomes mature
male gametophyte.
Getting male and female
gametophytes together
• Pollen tube penetrates into female gametophyte
to find archegonium, so sperm and egg can fuse
to make zygote.
The new baby sporophyte
• Zygote grows to embryo as ovule becomes seed
– Integument (2n tissue from parent sporophyte)
becomes hard protective seed coat
– Female gametophyte serves as food for growing
embryo.
Remains of seed coat
(most dissected away)
Female gametophyte
Embryo
Seed has wing: wind dispersed
• Winged pine seed helicoptering away
(most pine seeds fly this way)
Seed germinates and new
sporophyte is on its own
• Young pine seedling
Pine life cycle
• Take home lessons:
– 1) Pollen grain is immature male
gametophyte (microgametophyte)
– 2) Male gametophyte matures in
ovule, when pollen tube and sperm
cell made. Entire male gametophyte
is just a few cells.
– 3) Ovule contains megasporangium.
Meiosis produces megagametophyte,
which makes archegonia, still kept in
megasporangium.
Pine pollen grain
Pine life cycle
• Take home lessons:
– 4) Fertilization takes place and embryo digests
female gametophyte.
– 5) Seed is baby plant (embryo), in box (seed coat
from parent sporophyte tissue), with its lunch
(female gametophyte)!.
Pine life cycle
• Advantages of making seeds and pollen:
• 1) Male gametophyte (immature) small, protected by
microspore wall, flies through air to female
gametophyte
• 2) No swimming sperm. Male gametophyte delivers
sperm to female gametophyte protected by ovule in
female cone
• 3) Embryo nourished by megagametophyte and
protected by cone of sporophyte
• 4) Seed has hard protective coat, supply of food
(megagametophyte). Can be dispersed to new location.
• 5) Embryo can be dormant: allows new sporophyte to
wait for good conditions to germinate.
Gymnosperms get their chance to
rule the land....
• The Gymnosperm Revolution and the Permian Period
• Permian Period (290-248 million years ago) followed
Carboniferous: was relatively cool and dry. Most coal
swamps dried up.
Carboniferous coal swamp
Gymnosperms get their chance to
rule the land....
• Many SVPs (especially tree species) went extinct.
Gymnosperm groups prospered!
• Why? Pollen and seeds adapted them to dry conditions.
Carboniferous coal swamp
Gymnosperms
•
•
•
•
Phylum Cycadophyta (cycads)
Phylum Coniferophyta (conifers)
Phylum Ginkgophyta (gingko)
Phylum Gnetophyta (gnetophytes)