Seed Bearing Plants

Alternation of Generations

• All plant life cycles involve alternation of generations between the sporophyte and the gametophyte

• In different types of plants the relationship changes

• Mosses: the gametophye is the longest phase• Flowering plants: the sporophyte is the longest

phase

Seeds

• Recall the definition of the seed• Tougher and more resistant to drying than

spores• Most spores: single cell• Seeds: fully formed plant embryo and

stored food; specialized for distribution• Evolution of the seed uncoupled

reproduction from water

Cones and Flowers

• Contrast with the life cycle of the fern• Tiny gametophytes of the seed bearing

plants do not have independent life of their own

• Gametophytes life inside the sporophyte (in pine cones and flowers)

• Two kinds of spores: pollen and eggs (these are not gametes in plants)

Reproduction in Conifers (gymnosperms)

• Study the picture on page 582: the life cycle of a gymnosperm

• Alternation of generations: gametophyte develops within the sporophyte

• Two kinds of cones: male cones (produce pollen), female cones (produce eggs)

• Typically fertilization and seed formation takes a year

• Seed release is the next year

Reproduction in Angiosperms (flowering plants) 1

• Angiosperms produce seeds encased in a protective tissue of the sporophyte called an ovary

• The combination of seed and ovary is called a fruit

• Specialized reproductive structure called the flower

Reproduction in Angiosperms (flowering plants) 2

• Flower– Typical flowers produce both male and female

gametophytes

– Some have separate male and female flowers on the same plant (corn)

– Some have separate male and female plants (willow)

– Formed from 4 types of leaves (sepals, petals, stamens and carpels)

– See diagram of flower on page 586

Reproduction in Angiosperms (flowering plants) 3

• Flower parts– Sepals

• Leaflike, green, protect the bud

– Petals• Colorful, advertise the flower to pollinators

– Stamens• Male leaves produce pollen (anther and filament make up the

filament)

– Carpels• Female leaves each has an ovary (stigma and style make up the

pistil)

Reproduction in Angiosperms (flowering plants)

• Follows the pattern of alternation of generations

• Production of gametophyte, fertilization of eggs, development of seed takes place within the flower

• See life cycle of an angiosperm on page 587

Reproduction in Angiosperms (flowering plants) 4

• Key difference between gymnosperm life cycle and angiosperm is double fertilization

• A sperm nucleus fuses with one of the 2n embryos to make a 3n cell; which divides to produce the endosperm (food for the embryo)

• In angiosperms, endosperm is produced after the fertilization

• In gymnosperms endosperm is produced before fertilization

Reproduction in Angiosperms (flowering plants) 5

• Fertilization sets into action a number of events• Sepals and petals and stamens fall away, • Carpel thickens, nutrients directed to endosperm

and walls of the ovary• This produces a fruit (an ovary containing seeds)• Review difference between monocots and dicots