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 occursearlier 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 aCinepak decompressorare needed to see this picture.

Pine life cycle– Female cone: Cone

scales with ovules on them

Mature female cone

Femalecone atpollinationstage (top)

Cone scale with ovule

Pine life cycle• Ovule: integumented megasporangium.

Megaspore mother cell does meiosis, forms meiospores, one survives & does mitosis to make female gametophyte.

1 2

3 4

Pine life cycle• Female gametophyte has several archegonia, each

with an egg.

1 2

3 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)