Gymnosperm Notes

Fir Trees and Cones

• Fir trees are monoecious, meaning they have both male and female cones on the same tree.
• Dioecious trees have either all male or all female reproductive organs.
• Female cones produce ovules.
• Pollen needs to reach an ovule for reproduction.
• Male cones produce pollen.
• Gymnosperms rely on wind to spread pollen and produce large quantities of pollen to increase the chances of successful pollination.

Progymnosperms

• Progymnosperms are spore-bearing plants and ancestors to all seed plants.
• They first appeared approximately 390 million years ago (MYA).
• Archaeopteris were the first large trees during the Devonian period.
• They possessed secondary vascular tissue (woody tissue) and fern-like fronds.

Evolutionary Tree

• The evolutionary tree includes:
  • Bryophytes: Mosses, liverworts, and hornworts.
  • Pteridophytes: Monilophytes and lycophytes.
  • Spermatophytes: Gymnosperms and angiosperms.
• Gymnosperms include extinct gymnosperms and seed ferns.
• Angiosperms include monocots, eudicots, and other groups.

Seeds

• Seeds are highly resistant structures that protect plant embryos.
• They contain a diploid embryo and food storage within a seed coat.
• Seeds are found in both gymnosperms and angiosperms.

Male Gametophyte

• In seed plants, the male gametophyte is highly reduced and becomes a pollen grain.
• The entire gametophyte moves to the female gametophyte via wind or a pollinator.
• Water is not required for fertilization.

Female Gametophyte

• The female gametophyte has an extra layer of sporophyte tissue around the ovule and contains three parts:
  • Integument: Derived from the sporophyte, producing the seed coat.
  • Megasporangium: Forms the nucellus with an opening for the pollen grain (micropyle).
  • Megaspore: A haploid female spore that develops into the gametophyte.
• The embryo develops within the ovule.

Fertilized Female Gametophyte

• Forms a zygote from the female egg and male sperm.
• The seed enters a dormant stage.
• The embryo survives until environmentally favorable conditions are present.
• The seed coat is a tough, external layer to protect the embryo.
• The cotyledon stores food for embryo development.

Gymnosperms

• Gymnosperms are "naked seeds" that develop on the surface of a scale.
• There are four groups:
  1. Coniferophytes
  2. Cycadophytes
  3. Gnetophytes
  4. Ginkgophytes
• The ovule is not completely encased by sporophyte tissue.

Coniferophytes

• Coniferophytes are the largest group of gymnosperms, making up approximately 40% of forests.
• Pinus is an ecologically and economically important genus.
• Leaves are organized as bundles of needles with thick cuticles and recessed stomata to reduce water loss.
  • These adaptations are often found in arid or frigid environments where the ground freezes part of the year.
  • Cuticle: $(\text{waxy layer})$
  • Xylem: $(\text{vascular tissue})$
  • Phloem: $(\text{vascular tissue})$
  • Stoma: $(\text{pore for gas exchange})$
• Pine trees are native to the northern hemisphere but have been planted throughout the southern hemisphere for lumber.
• They secrete resins to deter insects and fungi growth, which are used by humans to create:
  • Turpentine: A solvent used for thinning paints and as a varnish.
  • Rosin: Used in adhesives, inks, and sealing wax.

Specific Gymnosperm Examples

• Sequoia sempervirens (Coastal Redwood) is the world's tallest species, found only in California.
• Sequoiadendron giganteum (Giant Sequoia) is the world's largest species, found only in California.
• Pinus longaeva (Bristlecone pine) is the world's oldest species, found in California, Nevada, and Utah.
• Pinus radiata: Some trees are fire-adapted and their cones only open up and release seeds at high temperatures.

Araucaria

• Araucaria was widespread until the end of the Cretaceous period (65 MYA).
• Current populations are likely relics of plate tectonics (vicariance).
• An example is the Monkey Puzzle Tree.

Pinus Reproduction

• Male and female gametes are produced on the same tree on cones.
• Male cones produce millions of pollen grains.
• Female cones have seeds located on each scale.
• Pinus has a haplodiplontic lifecycle.
• Female cones contain megasporangia forming the ovule.
• A mother cell undergoes meiosis to produce a megaspore.

Microspores and Pollen

• Male cones contain microsporangia.
• A mother cell within a microsporangium undergoes meiosis to produce microspores.
• Pollen grains are dispersed via wind.
• Pollen grains have air sacs for buoyancy.
• They also have a generative cell.

Pinus Fertilization

• Pollen grain enters the ovule via the micropyle, leading to pollen tube formation, which allows sperm and egg to fuse.
• Archegonia form near the micropyle within the megaspore.
• Sperm are non-motile.

Gnetophytes

• Gnetophytes possess xylem vessels, a cell type for efficient transport of water and nutrients throughout the plant, which is similar to angiosperms but likely the result of homoplasy.
• Examples include Welwitschia sp. and Ephedra sp.

Cycadophytes

• Cycadophytes superficially resemble palm trees but produce cones.
• Cycads are dioecious, with male and female cones produced on separate plants.
• Cycadophyte reproduction system is similar to pines, but sperm are flagellated, which does not require water due to transport via pollen grain.

Ginkgo biloba

• Ginkgo biloba is the only single living species of Ginkgophyte.
• The plant is dioecious.
• Male cones are used as an ornamental plant.
• Female sarcotesta produces butyric acid and smells like rancid butter.
• The fleshy sarcotesta is derived from the seed coat and is not considered a fruit.
• It has flagellated sperm.

Gymnosperm Relationships

1. Gymnosperms form a monophyletic group.
2. Ginkgophytes and Cycadophytes are sister taxa at the base of gymnosperms.
3. Gnetophytes are nested within Coniferophytes.