The Evolution of Seed Plants

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Slide 1

Transforming the World

Seeds changed the course of plant evolution, enabling their bearers to become the dominant producers in most terrestrial ecosystems.

A seed consists of an embryo and nutrients surrounded by a protective coat.

The gametophytes of seed plants develop within the walls of spores that are retained within tissues of the parent sporophyte.

Slide 2

What human reproductive organ is functionally similar to this seed?

Slide 3

Seeds and pollen grains are key adaptations for life on land

In addition to seeds, the following are common to all seed plants:

Reduced gametophytes

Heterospory

Ovules

Pollen

Slide 4

Gametophyte / sporophyte relationships in different plant groups

Reduced (usually microscopic), dependent on surrounding sporophyte tissue for nutrition

Reduced, independent (photosynthetic and free-living)

Gametophyte

Sporophyte (2n)

Sporophyte (2n)

Gametophyte (n)

Sporophyte

Example

Gametophyte (n)

Dominant

Dominant

Dominant

Reduced, dependent on gametophyte for nutrition

Mosses and other nonvascular plants

Ferns and other seedless vascular plants

Seed plants (gymnosperms and angiosperms)

PLANT GROUP

Gymnosperm

Angiosperm

Microscopic female gametophytes (n) inside ovulate cone

Microscopic male gametophytes (n) inside pollen cone

Sporophyte (2n)

Sporophyte (2n)

Microscopic female gametophytes (n) inside these parts of flowers

Microscopic male gametophytes (n) inside these parts of flowers

Slide 5

Heterospory: The Rule Among Seed Plants

The ancestors of seed plants were likely homosporous, while seed plants are heterosporous.

Megasporangia produce megaspores that give rise to female gametophytes.

Microsporangia produce microspores that give rise to male gametophytes.

Slide 6

Ovules and Production of Eggs

An ovule consists of a megasporangium, megaspore, and one or more protective integuments.

A fertilized ovule becomes a seed.

Gymnosperm megaspores have one integument.

Angiosperm megaspores usually have two integuments.

Slide 7

From ovule to seed in a gymnosperm

Megasporangium (2n)

Megaspore (n)

(a) Unfertilized ovule

Integument

Spore wall

Immature female cone