Download Lecture 09, Gymnosperms - Cal State LA

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
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..
Reduction of Gametophyte
Gymnosperm
Bryophyte
Angiosperm
Fern
Gametophyte is visible
Gametophyte is microscopic
-
Homosporous
Sporangium
vs.
Heterosporous
Megasporangium Microsporangium
Megaspores
Spores
Gametophyte
Microspores
Mega-gametophyte
Archegonium
Antheridium
Archegonium
Egg
Sperm
Egg
Zygote
Micro-gametophyte
Antheridium
Sperm
Zygote
Bryophyte life cycle
Gametophyte
is dominant
-
Seedless vascular plant life cycle
Sporophyte
is dominant
- Sperm must still swim to reach egg
- Little protection for the embryo
-
Appeared ~360
million years ago
Seed plants
4 phyla
Gymnosperms -
Gymnosperms
Sporophylls: modified leaves that contain sporangia (spore-producers)
1 of 4
cells
survives
FEMALE
megasporangium
meiosis
2N
megaspore
N
MALE
microsporangium
Gymnosperms
Sporophylls: modified leaves that contain sporangia (spore-producers)
FEMALE
megasporangium
2N
MALE
microsporangium
megaspore
N
microspore
female gametophyte
male gametophyte grows
inside pollen grain
archegonia
sperm
egg
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)
Gymnosperms
Strobilus (= cone) is a branch tip specialized for reproduction
Leaves are modified into..
Mature female
pine cone
Gymnosperms
Strobilus (= cone) is a branch tip specialized for reproduction
Leaves are modified into sporophylls
Megasporangium
Gymnosperm life cycle
Life cycle of Pinus sp.
heterospory
“ovule”
(developing
seed)
“seed”
(after fertilization)
Pine ovule after formation of the megagametophyte, archegonium & egg(s)
Stern 1991
Pine ovule at the fertilization stage
egg
zygote
sperm
second sperm nucleus
Raven et al. 1986
Pine seed showing embryo (baby sporophyte) within nutritive tissue,
surrounded by a seed coat
2n
n
embryo (2n)
Gametophyte (n)
Raven et al. 1986
Phylum Ginkgophyta – Ginkgo biloba
Only 1 living species, almost identical to
fossils 150 million years old
Phylum Cycadophyta – about 160 species
Phylum Gnetophyta
(~ 70 species)
Phylum Coniferophyta
Conifers – cone bearing
entirely wind-pollinated
fertilization can take a year to occur
Cross-section of a pine needle showing adaptations to drought
Dense packing of cells
Sunken stomata
Thick cuticle
Raven et al. 1986
Transport tissues
(secondary xylem &
phloem) in a pine
tree provide
improved longdistance transport
and structural
support
wood
Raven et al. 1986
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
Economic importance of conifers
Pacific Yew (Taxus brevifolia)
# of families
Evolution of Plant Biodiversity
Seedless
vascular
plants
Gymnosperms
dominate
Angiosperms
First
gymnosperms
Mackenzie 2003