Seed Plants &
Gymnosperms
Spring 2012
Outline
• Review of land plant phylogeny
• Characters of seed plants
• Gymnosperm phylogeny & diversity
– Cycads
– Gingko
– Conifers
– Gnetophytes
Review of land plant phylogeny
Green plants (viridophytes)
Land plants (embryophytes)
Vascular plants (tracheophytes)
Seed plants (spermatophytes)
Gymnosperms
Angiosperms
Fig. 5.1
Evolution of
wood before
the evolution
of seeds.
Fossil lignophytes
Progymnosperm
(wood, no seeds)
Seed fern
(wood, seeds)
Seed fern fossils from Pella, Iowa
Fig. 5.1
Evolution of
wood (>380
mya) before
the evolution
of seeds
(>360 mya).
Characters of seed plants
• Eustele (ancestral)
• Axillary branching
• Wood (ancestral)
– Cambia (vascular cambium, cork cambium)
• Seed
–
–
–
–
Heterospory
Megaspore reduction/retention
Integument/micropyle
Nutritive tissue
• Male gametophyte
– Pollen grain
– Pollen tube
Characters of seed plants: Eustele
eustele = primary stem vasculature comprising a
single ring of vascular bundles
Characters of seed plants: axillary branching
Bud/branch traces
Leaf traces
Characters of seed plants: cambia
cambia: vascular cambium (wood) & cork
cambium (periderm)
Fig. 5.3
Fig. 5.4
X-section of woody stem
Characters of seed plants: seed
• Heterospory
• Megaspore reduction/retention
• Integument/micropyle
• Nutritive tissue
Life cycle of most seed-free plants
•homospory
Zygote
Multicellular
Sporophyte
[with sporangia]
2n
SYNGAMY
alternation of
generations
MEIOSIS
n
Gametes
[egg + sperm]
Spores
Multicellular
Gametophyte
[with gametangia:
archegonia + antheridia]
Life cycle of seed plants
•heterospory
Multicellular
Sporophyte
Zygote
2n
alternation of
generations
SYNGAMY
MEIOSIS
n
microspores
egg
sperm
Male Gametophyte
[antheridia]
Female Gametophyte
[archegonia]
megaspores
Evolution of the seed
Megaspore reduction:
-reduction to 1 megaspore
Fig. 5.6 #3
reduction to 1 megaspore
Evolution of the seed
Megaspore retention:
-the one megaspore is retained within
megasporangium, not released
Fig. 5.6 #4
Evolution of the seed
Evolution of integument/micropyle from
sterile sporophyte tissue
Fig. 5.6 #5
Evolution of the seed
•pollination droplet:
-secreted by young ovule through micropyle
-water + sugars, amino acids (megasporangium)
-adhering pollen grains pulled inside!
Fig. 5.10
Evolution of the seed
-nutritive tissue from the
female gametophyte
-integument becomes the
seed coat
Fig. 5.10
Characters of seed plants
male gametophyte
•pollen grain = extremely reduced male
gametophyte, a few cells
•pollen tube – formed by the pollen, grows
though sporophytic tissue to deliver sperm
cells to egg (in ovule)
Pine pollen
Characters of seed plants: seed
Adaptive advantages of the seed:
•protection (seed coat)
•dispersal unit of sexual reproduction
•dormancy mechanisms
•nutritive tissue – provides energy for young
seedling, aiding in establishment
Two major groups of seed plants:
• Gymnosperms—not sure of the early
evolutionary history of gymnosperms;
could be monophyletic or could be
paraphyletic
• Angiosperms—monophyly supported
by many characters including the
carpel
Gymnosperm Phylogeny
Cycads
Gingko
Pines
Gnetales
Cypresses et al.
pollen tube
cones (strobili)
simple leaves
1° DNA data
Gymnosperm diversity
-ca. 15 families, 75-80 genera, ca. 1,000 species
-4 or 5 main lineages
-all woody
-mostly without effective vegetative reproduction
-only tracheids in the xylem (except for gnetophytes,
which also have vessels)
-naked seeds
-relatively slow sexual reproduction
-worldwide but dominant in many colder or arctic
regions
-include the tallest, the most massive, and the longest
living individual plants
Gymnosperm wood: tracheids only
Major groups of gymnosperms
• Cycads
• Gingko
• Conifers
• Gnetophytes
Major groups of gymnosperms
Cycadophyta – Cycads
•squat, unbranched trunk (little wood),
usually pinnately compound leaves
•loss of axillary branching
•dioecious: male and female plants
•male and female strobili (cones)
•motile, multiflagellate sperm!
(ancestral)
•coralloid roots with nitrogen-fixing
cyanobacteria
Zamia female
strobilus
Cycas male
strobilus
Major groups of gymnosperms
Cycadophyta – Cycads
•ca. 11 genera (130 spp.)
•now restricted distribution
•seeds with bright fleshy seed
coat--dispersed by plant-eating
dinos!
A native U.S. cycad:
Zamia floridana
Gymnosperm Phylogeny
Cycads
Gingko
Pines
Gnetales
Cypresses et al.
pollen tube
cones (strobili)
simple leaves
1° DNA data
Major groups of gymnosperms
Ginkgophytes – Ginkgo
extensive fossil record but…only
1 living species: Ginkgo biloba!
•highly branched tree with
well developed wood
•deciduous, fan-shaped leaves with
dichotomous venation
•dioecious: male and female trees
-male: “cone” with lateral stalks bearing
microsporangia
-female: no cone, axis with 2 ovules
(outer integument layer fleshy)
•motile sperm (ancestral)
Major groups of gymnosperms
Ginkgophytes – Ginkgo
Fig. 5.1
Major groups of gymnosperms
Coniferophyta – Conifers
•ca. 700 spp.
•once dominant worldwide, displaced by angios
•shrubs or small trees, highly branched with
well developed wood
•leaves simple, often needle-like or awl-shaped
-pines: in fascicles
•non-motile sperm (pollen tube needed)
•female (seed-bearing) cones in most
•include both traditional conifers but now also
the Gnetales (gnetophytes)
Major groups of gymnosperms
Coniferophyta – Conifers
•pollen cone or male cone
-microsporangia & modified
leaves
male
•seed cone or female cone
-axis with modified leaves
(bracts, usually reduced), each
subtending seed-bearing scale
(modified branch system)
-woody or leathery or fleshy
female
Major groups of gymnosperms
Coniferophyta – Conifers
•seed cone and pine nuts
Stone Pine nuts
[w U.S.]
Korean pine nuts
Evolution of the compound conifer female cone
Fig. 5.19
Gymnosperm Phylogeny
Cycads
Gingko
Pines
Gnetales
Cypresses et al.
pollen tube
cones (strobili)
simple leaves
1° DNA data
Pollen usually with 2 appendages
Leaves linear to needle-like
Resin canals in
wood & leaves
Pinaceae
Winged seeds
Ovules 2, inverted
Pseudotsuga (Douglas fir)
Abies (fir)
Picea (spruce)
Larix (larch)
Pinus
(pines)
-needles in bundles
-cone scales thickened at
the tip and often armed
with a prickle
Gymnosperm Phylogeny
Cycads
Gingko
Pines
Gnetales
Cypresses et al.
pollen tube
cones (strobili)
simple leaves
1° DNA data
Pollen without appendages
Leaves scale-like to linear
Cone scales fused to bracts
Microsporangia 2-10 per microsporophyll &
ovules 1-20 per cone scale
Cupressaceae
Juniperus (juniper)
Taxodium (bald cypress)
Chamaecyparis
Sequoia sempervirens (redwood)
Sequoiadendron giganteum
(giant sequoia)
Taxaceae
Ovules solitary,
cones lacking
Seeds with a fleshy, brightly colored aril
Podocarpus
Araucariaceae
Gymnosperm Phylogeny
Cycads
Gingko
Pines
Gnetales
Cypresses et al.
pollen tube
cones (strobili)
simple leaves
1° DNA data
Major groups of gymnosperms
Gnetophytes or Gnetales
3 extant genera: Ephedra (65 spp.); Gnetum (28
spp.); Welwitschia mirabilis
related to angiosperms?
•recent molecular data: a gymnosperm group
defined by many characters, e.g.:
-opposite leaves, similar pollen
-vessel structure (independent of angiosperms)
-nonmotile sperm (independent)
-double fertilization (independent of angiosperms)
-some with insect pollination
Major groups of gymnosperms
Gnetophyta - Gnetophytes
•Ephedra (65 spp.)
-common desert shrub
-reduced
scale-like
leaves
Major groups of gymnosperms
Gnetophyta – Gnetales
•Gnetum (28 spp.)
•tropical vines, trees, shrubs with opposite
leaves that look like angiosperms!
Major groups of gymnosperms
Gnetales – Gnetophytes
•Welwitschia mirabilis
-a strange plant native to deserts of Namibia,
sw Africa!
-2 big curly leaves!