1. Gymnosperms: Evolution of the Seed
Major Themes in Land Plant Evolution
Progressive reduction of the gametophyte stage
Independent sporophyte
Evolution of pollen (replaces swimming sperm)
Evolution of the seed (replaces spores as dispersal agent)
Homospory  Heterospory (separate male and female spores)
also... Improved vascular tissues
Increased height and structural strength
Anatomical + physiological adaptations to drier environments

2. Adaptations to Dry Living: Stomata and Cuticle
Cuticle = waxy protective layer on the outside of cells that prevents
water loss
In stomata (singular = stoma), guard cells open to allow CO2 to enter
open pores..

3. Reduction of Gametophyte
Gymnosperm Angiosperm
Bryophyte Fern
Gametophyte is visible
Gametophyte is microscopic -

4. Homosporous vs. Heterosporous
Sporangium
Megasporangium Microsporangium
Megaspores Microspores
Spores
Gametophyte
Mega-gametophyte Micro-gametophyte
Archegonium Antheridium
Archegonium Antheridium
Egg
Sperm
Egg
Sperm
Zygote
Zygote

5. Bryophyte life cycle
Gametophyte
is dominant -

6. Seedless vascular plant life cycle
Sporophyte
is dominant
- Sperm must still swim to reach egg
- Little protection for the embryo -

7. Appeared ~360
million years ago

8. Seed plants
Gymnosperms - -
4 phyla

9. Gymnosperms
Sporophylls: modified leaves that contain sporangia (spore-producers)
FEMALE
megasporangium
megaspore
MALE
microsporangium 2N
meiosis
1 of 4
cells
survives N

10. male gametophyte grows
Gymnosperms
Sporophylls: modified leaves that contain sporangia (spore-producers)
FEMALE
megasporangium
megaspore
female gametophyte
archegonia
egg
MALE
microsporangium
microspore
male gametophyte grows
inside pollen grain
sperm 2N N

11. Development of the gymnosperm seed from an ovule in Pinus sp.
Gymnosperm ovules have 1 protective outer layer, or integument,
composed of tissue from sporophyte (mom’s tissue)
Pollen grain grows a pollen tube of haploid cells to penetrate the
micropyle, the opening through the integument; releases sperm
Fertilized egg grows into embryo, surrounded by female gametophyte
tissue (its food) and the sporophyte’s integument (its protection)

12. Gymnosperms
Strobilus (= cone) is a branch tip specialized for reproduction
Leaves are modified into..
Mature female
pine cone

13. Gymnosperms
Strobilus (= cone) is a branch tip specialized for reproduction
Leaves are modified into sporophylls
Megasporangium

14. Gymnosperm life cycle

15. Life cycle of Pinus sp. heterospory “ovule” (developing seed) “seed”
(after fertilization)

17. Pine ovule after formation of the megagametophyte, archegonium & egg(s)
Stern 1991

18. Pine ovule at the fertilization stage
egg
zygote
sperm
second sperm nucleus
Raven et al. 1986

19. 2n n embryo (2n) Gametophyte (n)
Pine seed showing embryo (baby sporophyte) within nutritive tissue,
surrounded by a seed coat 2n n
embryo (2n)
Gametophyte (n)
Raven et al. 1986

20. Phylum Ginkgophyta – Ginkgo biloba
Only 1 living species, almost identical to
fossils 150 million years old

21. Phylum Cycadophyta – about 160 species

22. Phylum Gnetophyta
(~ 70 species)

23. Phylum Coniferophyta Conifers – cone bearing entirely wind-pollinated
fertilization can take a year to occur

24. Cross-section of a pine needle showing adaptations to drought
Dense packing of cells
Sunken stomata
Thick cuticle
Raven et al. 1986

25. Transport tissues (secondary xylem & phloem) in a pine tree provide
improved long-
distance transport
and structural
support
wood
Raven et al. 1986

26. Biggest + oldest
Height: coast redwoods (Sequoia sempervirens) >100 m tall
Mass: giant redwoods (Sequoiadendron gigantea) > 26 m. circumference
Age: bristlecone pine (Pinus aristata)
4900 years old

27. Economic importance of conifers
Pacific Yew (Taxus brevifolia)

28. Evolution of Plant Biodiversity
Gymnosperms
dominate
Seedless
vascular
plants
# of families
Angiosperms
First
gymnosperms
Mackenzie 2003