The Evolution of Seed Plants

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Slide 8

Pollen and Production of Sperm

Microspores develop into pollen grains, which contain the male gametophytes.

Pollination is the transfer of pollen from the male to the female part containing the ovules.

Pollen eliminates the need for a film of water and can be dispersed great distances by air or animals.

If a pollen grain germinates, it gives rise to a pollen tube that discharges two sperm into the female gametophyte within the ovule.

Slide 9

From ovule to seed in a gymnosperm

Male gametophyte (within a germinated pollen grain) (n)

Female gametophyte (n)

(b) Fertilized ovule

Micropyle

Pollen grain (n)

Spore wall

Discharged sperm nucleus (n)

Egg nucleus (n)

Slide 10

The Evolutionary Advantage of Seeds

A seed develops from the whole ovule.

A seed is a sporophyte embryo, along with its food supply, packaged in a protective coat.

Seeds provide some evolutionary advantages over spores:

They may remain dormant for days to years, until conditions are favorable for germination.

They may be transported long distances by wind or animals.

Slide 11

From ovule to seed in a gymnosperm

Seed coat (derived from integument)

(c) Gymnosperm seed

Embryo (2n) (new sporophyte)

Food supply (female gametophyte tissue) (n)

Slide 12

From ovule to seed in a gymnosperm

Seed coat (derived from integument)

(c) Gymnosperm seed

Embryo (2n) (new sporophyte)

Food supply (female gametophyte tissue) (n)

(b) Fertilized ovule

(a) Unfertilized ovule

Integument

Immature female cone

Spore wall

Megasporangium (2n)

Male gametophyte (within a germinated pollen grain) (n)

Megaspore (n)

Micropyle

Pollen grain (n)

Egg nucleus (n)

Discharged sperm nucleus (n)

Female gametophyte (n)

Slide 13

Gymnosperms bear “naked” seeds, typically on cones

The gymnosperms have “naked” seeds not enclosed by ovaries and exposed on modified leaves - cones. There are four phyla:

Cycadophyta (cycads)

Gingkophyta (one living species: Ginkgo biloba)

Gnetophyta (three genera: Gnetum, Ephedra, Welwitschia)

Coniferophyta (conifers, such as pine, fir, and redwood).

Slide 14

Seed plants can be divided into two clades: gymnosperms and angiosperms.

Gymnosperms appear early in the fossil record and dominated the Mesozoic terrestrial ecosystems.