Life on Earth
Kingdom Plantae
Part IV
The Seed Plants:
Gymnosperms
Origin of the Seed
Female Gametophyte Scenario
• Megasporangium produces only one
functional megaspore
• Megaspores are not released from the
plant
• Megaspore develops into the female
gametophyte within the old
megasporangium
Origin of the Seed (cont.)
• Archegonia are formed with egg
cells and fertilized in situ
• Embryo (new sporophyte)
develops surrounded by female
gametophyte tissue and old
megasporangium wall (nucellus)
Origin of the Seed (cont.)
• Megasporophyll surrounds the
structure and acts as integument
(seed coat)
• The mature ovule (seed) is released
from the plant (old sporophyte)
Origin of Pollen
Male Gametophyte Scenario
• Micorsporophylls produce microsporangia
• Microspores (N) are not released from the
microsporangium but divide to form a 4celled microgametophyte (pollen grain)
• Four cells are 2 prothallial cells, 1
generative cell and 1 tube cell
Pine Pollen
Origin of Pollen (cont.)
• Pollen grains (male gametophytes) are
released from the “strobilus” or “cone”
• The pollen grains germinate forming a
pollen tube
• The generative cell divides to form
sperm nuclei which travel down the
pollen tube to fertilize the egg
Why evolve seeds and pollen?
• Embryo is well protected in the
seed coat
• Seeds can be “dormant” and wait
for suitable growth conditions
• Transfer of pollen (male to female)
does not require water like sperm
Why evolve seeds and pollen? (cont.)
• Embryo has a ready supply of
nutrients for early development
(female gametophyte tissue)
• A seed is an excellent dispersal
unit (wind, animals)
Seed Plant Divisions
• Gymnosperms
– plants with “naked” seeds not found in a fruit
– includes 4 Divisions
• Angiosperms
– seed plants with “hidden” seeds, i.e. formed
inside a fruit
– only one Division
Gymnosperms
•
•
•
•
Coniferophyta
Cycadophyta
Ginkgophyta
Gnetophyta
Division Coniferophyta
• Most species are
monoecious, woody
perennials producing
cones
• Some are deciduous, most
are evergreen with needlelike leaves
• Extended life cycle (3
years)
Primary vs. Secondary Growth
• Primary tissues are derived from
meristems
– Zones of cell division in root tips, shoot
tips, axillary buds etc.
– Primary growth generally creates
elongation of stems, branches, and
roots or creates reproductive organs
Primary vs. Secondary Growth (cont.)
• Secondary tissues are derived from
cambia
– Cambial activity generally causes
increase in girth of the plant (root, stem)
– Woody plants develop 2 cambia:
• vascular cambium (producing
secondary xylem and phloem)
• cork cambium (producing “bark”
layers)
PITH
Pine Anatomy (stem)
RESIN DUCT
VASCULAR
CAMBIUM
SECONDARY
XYLEM
SECONDARY
PHLOEM
RADIAL
Wood Sections
TRANSVERSE
TANGENTIAL
Wood Rays
Pine Needles
Pine Life Cycle
• Trees produce both male (staminate)
and female (ovulate) cones
• Male cones are usually lower on the tree
to assist with out-crossing
• Pollination takes place in the spring
when female cones are small (about 1/2
inch)
Pine Life Cycle (cont.)
• Development of the mature ovule with
archegonia takes more than a year
• Pollen tube digests its way slowly through
the nucellus to the archegonium for
fertilization
• Nearly another year is required before the
embryo is mature and the female cone
opens to release the seeds
Pine Life Cycle
Pinus (male and female cones)
Male Cone (longitudinal Section)
Conifer Diversity
• Highly diverse in both
Northern and Southern
Hemispheres
– Includes pine, spruce,
hemlock, fir, Douglas fir,
yew, juniper, cypress,
sequoia, larch, podocarps,
araucarias and others
Taxodium
(bald cypress)
Giant Sequoia
Taxus (yew) with female cones
Sequoia and Sequoiadendron
Araucaria family –Wollemi “pine,” discovered living in
Australia, 1994, known from Jurassic fossils
Aucariaceae (Southern Hemisphere)
Norfolk Island Pine
Division Cycadophyta
• Perennial, dioecious plants
with coarse palm-like
leaves
• Female cones are often
very large
• Only 10 genera known many with highly restricted
ranges, primarily tropical
• Produce flagellated sperm
Cycas revoluta and C. media
With “female cone” and seeds
Dioon edule
Encephalartos
Division Ginkgophyta
• Ginkgo biloba only
species
• Probably extinct in the
wild (originally from
China)
• Separate male and
female trees (dioecious)
Division Ginkgophyta (cont.)
• Female ovules produce
butyric acid (plant males!)
• Male trees produce small,
fleshy pollen cones;
motile sperm are
produced
• Tolerates air pollution
well
Ginkgo biloba
Ginkgo Leaves and Ripe Ovules
Division Gnetophyta
• Gnetum, Ephedra
(Mormon Tea) and
Welwitschia very different
organisms
• Have vessels in the xylem
• Do not produce
multicellular archegonia
• Have double fertilization
Ephedra male cones
Gnetophyta (cont.)
• Gnetum is a genus of tropical vines
with flowering plant-like leaves
• Ephedra is a desert shrub with several
species in the SW United States (called
Mormon Tea)
• Welwitschia mirabilis is a bizarre, taprooted, perennial plant of the desert
areas of SW Africa
Ephedra nevadensis
Welwitschia mirabilis
Habitat view in SW Africa
Welwitschia mirabilis in Namib
Desert, Namibia
Welwitschia mirabilis -female
(with ovulate cones)
Welwitschia mirabilis - male
(with staminate cones)
Gnetum
With Ovules
Importance of Gymnosperms
•
•
•
•
Major source of lumber for construction
Primary source of pulp for paper manufacture
Many ornamentals
Source of resins for turpentine, rosin (“naval
stores”)
• Juniper (“berries”) used to flavor gin
• Source of amber and even lemon flavoring
• Anticancer drug taxol from Pacific yew bark/
English yew leaves
Importance of Gymnosperms
Originally took 4 trees to
produce a single human dose!
Now, using leaves of common
English yew, a synthetic
process makes the drug.
TAXOL:
31 years from “bark” to “business”