Gymnosperm Notes

Gymnosperms

Seed plants that do not form flowers but cones.

Seeds are ‘naked,’ not protected by ovary or fruit; exposed on megasporophylls arranged in female cones.

Seeds contain a haploid endosperm, nutritive remains of female gametophyte.

Characteristics

Possess vascular cambium for secondary growth.

Xylem consists of tracheids (except Gnetophyta) with much lignin, enabling water conduction to great heights and providing structural support.

Adaptations to cold and dry habitats include soft wood for furniture etc.

Heterosporous: microspores in microsporangia on microsporophylls (male cones), megaspores in megasporangia on megasporophylls (female cones).

Gametophytes are very reduced; wind-pollinated due to lack of flowers.

Classification

Four groups:

  • Cycadophyta (cycads)
  • Ginkgophyta (Ginkgo)
  • Gnetophyta (gnetophytes)
  • Coniferophyta (conifers)

Coniferophyta (Conifers)

Most diverse group of evergreen plants forming coniferous forests.

Tall, needle-leaved trees in cold temperate, subtropical, and highland habitats, adapted to cold, dry, and windy conditions.

Adaptations:

  • Conical shapes and flexible branches to shed snow.
  • Wide tracheids for water transport in cold/dry conditions.
  • Anti-freeze in cells.
  • Needle-like leaves, sunken stomata, and thick cuticles for water conservation.
  • Winged pollen for wind pollination and winged seeds for dispersal.
  • Mostly monoecious with male (staminate) and female (ovulate) cones.

Economically important for paper making, soft timber, and furniture manufacturing (e.g., Pinus, Cupressus, Juniperus).

Pinus sp. (Pine) Reproduction

Sporophytes have male and female cones.

Male cones: microsporophylls with microsporangia (pollen sacs) produce microspores.

Pollen grain (n): outer wall expanded to form wings for floatation; male gametophyte develops inside; 2 sperm cells formed.

Female cone: ovuliferous scale (megasporophyll) with 2 megasporangia; female gametophyte develops inside the ovule from megaspore division; 1-2 archegonia produced.

Gamete Formation

Male: Microspore nucleus undergoes mitosis to form 2 prothallial cells. One prothallial cell undergoes mitosis to form a prothallial cell and an antheridial initial. The antheridial initial then undegoes mitosis to form a tube nucleus and a generative nucleus (pollination occurs). The generative nucleus undergoes mitosis and forms a stalk cell and a body cell. The body cell undergoes mitosis and forms 2 sperm cells. Male gametophyte ((pollen) = 6 cells (nuclei)

Female: In the ovule (megasporangium), one cell megasporocyte undergoes meiosis to produce 4 megaspores (only one develops). The megaspore undergoes mitosis to create the female gametophyte which has 2 archegonia with eggs. This fertilizes into a zygote.

Fertilization and Seed Development

Pollen tube carrying sperm cells grows towards archegonium after landing on ovule.

One male gamete fuses with egg to produce a zygote; second male gamete dies.

Zygote develops into embryo; usually only one archegonium is fertilized.

Fertilized ovule develops into seed: embryo + endosperm (female gametophyte (n)) + integuments.

Seed germination leads to sporophyte (tree).

Cycadophyta (Cycads)

Large, palm-like plants with frond-like leaves; primitive gymnosperms in tropical/subtropical regions.

Mostly dioecious; dominated Earth alongside dinosaurs ~200 million years ago.

Slow-growing; cultivated as ornamentals.

Endangered species; commercial trade regulated by CITES (e.g., Cycas and Zamia).

Produce male (pollen) and female (seed) cones; attract beetles for pollination via smelly chemicals.

Retain primitive condition of swimming sperm with flagella.

Ginkgophyta (Ginkgo)

One living member, Ginkgo biloba: deciduous tree with bilobed, fan-shaped leaves.

No female cones; ovules/seeds produced naked on branches.

Cultivated as ornamental; tolerate heat, cold, and pollution; can live >1000 years.

Gnetophyta (Gnetophytes)

Angiosperm-like features: vessels in xylem (unlike other gymnosperms with tracheids).

Pollen-producing structures resemble stamens; display adaptations for water conservation.

Examples: Welwitschia (desert octopus).

  • Welwitschia mirabilis: xerophyte in Namib desert with two long, strap-like leaves.
  • Leaves tough and leathery with sunken stomata.
  • Large storage tap roots; well-adapted to dry habitats; live 1000-2000 years.