Topic 9 - Plant Diversity II

Outcomes:

• compare and contrast derived traits of seed plants

• compare and contrast derived traits of angiosperms

Seed plants have evolved key adaptations that allow them to thrive on land without being surrounded by water.

Truly terrestrial – Unlike their ancestors, seed plants are not dependent on water for reproduction.
Challenges faced on land:
- Reproduction – No longer reliant on water for sperm transport.
- Desiccation (Drying out) – Must retain moisture in reproductive structures.
- Structural support – Must support themselves without buoyancy.
Advantages of seeds:
- Protection – Seed coat shields embryo.
- Nutrient supply – Endosperm provides food for early development.
- Extended lifespan – Seeds can remain dormant until conditions are favorable.
- Two major groups:
  - Gymnosperms – "Naked seeds" not enclosed in fruit.
  - Angiosperms – Seeds enclosed in a "container" (fruit).

Microscopic – Unlike in non-seed plants, the gametophyte stage is tiny.
Dependent on sporophyte – No longer free-living; relies on the parent plant.
Housed within sporangia – Provides protection from desiccation and environmental stress.

Male and female spores develop separately:
- Microsporangium (2n) → Produces microspores (n) → Develops into male gametophyte (sperm in antheridia).
- Megasporangium (2n) → Produces megaspores (n) → Develops into female gametophyte (egg in archegonia).

Ovules contain the megasporangium and house the developing female gametophyte.
Female gametophyte develops from the megaspore and produces eggs for fertilization.

Pollen grain = Male gametophyte.
Sperm develops inside microspores, eliminating the need for water in fertilization.
Pollination = Transfer of pollen from the male part of the plant to the female reproductive structures.

Seeds are exposed on cones (strobili), not enclosed in fruit.
Most gymnosperms are conifers (cone-bearing plants).
Different cones for male and female reproductive structures (heterospory):
- Male cones contain microsporangia, which produce pollen grains (male gametophytes).
- Female cones contain ovules, which develop into seeds after fertilization.
Fertilization process:
- Pollen travels to the female cone (pollination).
- Pollen tube forms and delivers sperm to the egg within the ovule.
- Fertilized ovule develops into a seed containing a zygote (embryo).

Four recognized phyla of seed plants:
1. Ginkgophyta – Includes Ginkgo biloba, a unique broad-leaved gymnosperm.
2. Cycadophyta – Palm-like plants with large cones.
3. Gnetophyta – Includes Ephedra, Gnetum, and Welwitschia.
4. Coniferophyta – The largest phylum, consisting of pines, firs, and redwoods.

Flowers – Composed of modified leaves, sepals, and petals to aid in reproduction.
- Stamen (male part): Produces pollen.
- Carpel (female part): Contains ovaries where seeds develop.
Fruits – Develop from mature ovaries and contain seeds.
- Functions of fruit:
  - Protects the embryo.
  - Aids in seed dispersal, allowing plants to colonize new areas.

Double fertilization – Unique to angiosperms, involving two sperm cells.
- One sperm fertilizes the egg → Forms diploid zygote (2n) → Becomes the embryo.
- One sperm fuses with central cell (n + n) → Forms triploid endosperm (3n) → Provides nutrients to embryo.
Pollination process:
- Pollen lands on another flower’s carpel (cross-pollination).
- A pollen tube grows from the pollen grain to the ovary.
- Two sperm cells travel down the pollen tube to the female gametophyte.
Seed formation:
- 2n zygote → Develops into embryo.
- 3n endosperm → Becomes a nutrient-rich food source.
- Entire ovule → Develops into a seed.