Plant Sexual Reproduction & Development

Plant Evolution: Appearance of the major plant groups

Reminder: Evolution of plants & plant classification

How does this relate to plant sexual reproduction?

Plant reproduction involves an Alternation of Generations

The seaweed, Ulva, which is

classically called a Protist,

demonstrates this

Alternation of generations

• There are “grown up” (mature) haploid male and female gametophytes that produce haploid gametes.

• The gametes fuse to form a diploid zygote that also grows up into a mature diploid Ulva called a sporophyte.

• The sporophyte cells within the sporangia can undergo meiosis to produce haploid zoospores which are released and then develop into the “grown up” Ulva gametophytes which are haploid!

In mosses, the thing we see most often is the gametophyte (haploid)

In ferns, like most plants, the thing we see most often is the sporophyte (diploid)

Misleading picture…not to scale! The Fern gametophyte is the size of your pinky nail (at most).

Next in evolution, GymnospermsA pine tree is a sporophyte with tiny gametophytes in its cones

The process – in words

• The gymnosperms (like pine trees) you know and love are diploid sporophytes

• They produce male and female cones• The male cone produces haploid spores by meiosis –

called pollen grains (male gametophyte)• The female cone has ovules, where haploid spores are

produced. These haploid spores become the female gametophyte, which makes the egg.

• The male gametophyte (pollen grain) releases the sperm which fuse with the egg and become the diploid zygote.

• The ovule becomes the seed surrounding the zygote, which can then germinate and grow into a new sporophyte!

Angiosperm reproductive organ – male and female reproductive organs all in

the same place… How convenient!

Flower Structure & Function• Sepals: protect bud• Petals: protect reproductive

structures; attract pollinators• Stamen: male reprod. structure

– Filament: raises anther closer to stigma and/or pollinators

– Anther: produces pollen (male gametes)

• Pistil (also called Carpel)– Stigma: “landing pad” & entry point

for male gametes– Style: raises and supports stigma– Ovary: contains ovules; fertilization

& development of seed(s) takes place in here

• Ovule: female gamete

The life of a flowering plant…

Angiosperms – sporophytes with the gametophyte stages in the flowers

Angiosperm Reproduction:What happens here is actually a double fertilization. Each ovule contains several haploid eggs and one diploid cell. Each pollen grain releases two sperm nuclei. One fertilizes the egg to form the diploid zygote (which becomes the embryo).The other sperm fertilizes the diploid cell in the ovule, making it 3N. This triploid cell divides to form the endosperm which will eventually nourish the embryo as it develops (after seed germination).

The ovule develops into a seed

A fruit is a thickened ovary

Seed germination begins the life of a new plant

The Resulting Plant

Seed germination begins the life of a new plant

The Resulting Plant