Seed Plants & Gymnosperms

Spring 2014

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

Evolution ofwood before the evolutionof seeds.

Fig. 5.1

Seed fern(wood, seeds)

Progymnosperm(wood, no seeds)

Fossil lignophytes

Seed fern fossils from Pella, Iowa

Evolution ofwood (>380 mya) before the evolutionof seeds (>360 mya).

Fig. 5.1

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

eustele = primary stem vasculature comprising a single ring of vascular bundles

Characters of seed plants: Eustele

Characters of seed plants: axillary branching

Leaf traces

Bud/branch traces

cambia: vascular cambium (wood) & cork cambium (periderm)

Characters of seed plants: cambia

Fig. 5.3

X-section of woody stemFig. 5.4

Characters of seed plants: seed

• Heterospory

• Megaspore reduction/retention

• Integument/micropyle

• Nutritive tissue

MulticellularSporophyte

MulticellularGametophyte

Gametes[egg + sperm]

Zygote

MEIOSIS

Spores

2n

n

SYNGAMY<<

<

<<

<<<

alternation of generations

[with sporangia]

[with gametangia: archegonia + antheridia]

Life cycle of most seed-free plants

embryo cells in sporangium

• homospory

MulticellularSporophyte

egg

Zygote

MEIOSIS

2n

n

SYNGAMY<<

<

<

<<

alternation of generations

megasporangia

[archegonia]

embryo

Male Gametophytesperm

<

<Female Gametophyte

<microspores

megaspores[antheridia]

<

microsporangia

Life cycle of seed plants• heterospory

Megaspore reduction:-reduction to 1 megaspore

Evolution of the seed

reduction to 1 megasporeFig. 5.6 #3

Megaspore retention:-the one megaspore is retained within megasporangium, not released

Evolution of the seed

Fig. 5.6 #4

Evolution of the seedEvolution of integument/micropyle fromsterile sporophyte tissue

Fig. 5.6 #5

• pollination droplet: -secreted by young ovule through micropyle -water + sugars, amino acids (megasporangium) -adhering pollen grains pulled inside!

Evolution of the seed

Fig. 5.10

-nutritive tissue from the female gametophyte-integument becomes the seed coat

Evolution of the seed

Fig. 5.10

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)

Characters of seed plants

Pine pollen

Adaptive advantages of the seed:

Characters of seed plants: 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 PhylogenyCycads Gingko Pines Gnetales Cypresses et al.

1° DNA data

pollen tube

simple leaves

compound ♀cones (strobili)

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

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!

Major groups of gymnosperms

A native U.S. cycad:Zamia floridana

Gymnosperm PhylogenyCycads Gingko Pines Gnetales Cypresses et al.

1° DNA data

pollen tube

simple leaves

compound ♀cones (strobili)

Ginkgophytes – Ginkgoextensive fossil record but…only

1 living species: Ginkgo biloba!

• highly branched tree withwell 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 – GinkgoMajor groups of gymnosperms

Fig. 5.1

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• 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

Major groups of gymnosperms

male

female

Coniferophyta – Conifers• seed cone and pine nuts

Major groups of gymnosperms

Korean pine nutsStone Pine nuts[w U.S.]

Fig. 5.19

Evolution of the compound conifer female cone

Gymnosperm PhylogenyCycads Gingko Pines Gnetales Cypresses et al.

1° DNA data

pollen tube

simple leaves

compound ♀cones (strobili)

Pinaceae

Leaves linear to needle-like

Ovules 2, invertedWinged seeds

Pollen usually with 2 appendages

Resin canals inwood & leaves

Abies (fir)

Pseudotsuga (Douglas fir)

Larix (larch)Picea (spruce)

Pinus(pines)

-needles in bundles-cone scales thickened atthe tip and often armed with a prickle

Gymnosperm PhylogenyCycads Gingko Pines Gnetales Cypresses et al.

1° DNA data

pollen tube

simple leaves

compound ♀cones (strobili)

Cupressaceae

Leaves scale-like to linearPollen without appendages

Microsporangia 2-10 per microsporophyll &ovules 1-20 per cone scale

Cone scales fused to bracts

Juniperus (juniper)

Chamaecyparis

Taxodium (bald cypress)

Sequoia sempervirens (redwood) Sequoiadendron giganteum(giant sequoia)

Taxaceae

Seeds with a fleshy, brightly colored aril

Ovules solitary,cones lacking

Podocarpus

Araucariaceae

Gymnosperm PhylogenyCycads Gingko Pines Gnetales Cypresses et al.

1° DNA data

pollen tube

simple leaves

compound ♀cones (strobili)

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!

Major groups of gymnosperms