PART 3. GAMETOGENESIS AND FERTILIZATION IN FLOWERING PLANTS

Flower structure

As you know from studying plant life cycles, gamete production and fertilization in flowering plants is quite different from those processes in animals. We will begin with a quick look at flowers, the site of plant reproductive organs. Remember that flowers are part of the sporophyte generation and will produce spores by the process of meiosis.

Review the structure of a typical flower and be sure that you know the function of each flower part. Observe the demonstration of a large flower and identify the sepals, petals, stamens, and pistil.

Working in pairs, take an Astromerias flower to your table and note that it has the same parts; they are just smaller. Then carefully remove a stamen and the pistil. Place them under the flex camera and capture an image. Label the anther of the stamen. Label the stigma and ovary of the pistil. Submit to WebAssign for question 15. After you have your picture, use a razor blade to cut open the ovary and observe the tiny ovules within it.

As you examine this flower, note that the tip of the stamens are called anthers (where pollen is produced), and the tip of the pistil is called a stigma (where pollen grains land prior to fertilization). The pollen grains then form tubes that grow down the pistil and enter the ovary. Two haploid (sperm) nuclei migrate down the the tube to reach the ovules within the ovary where fertilization occurs. If fertilized, the ovules develop into seeds.

Pollen grains and fertilization

At the beginning of the class, your TA placed some pollen grains in a sugar solution to see if they would grow pollen tubes. Check the demonstration periodically to see if pollen tubes are growing.

Observe these scanning electron micrographs for a high magnification view of real pollen grains. Note the differences in surface structure which allows the plant species to be identified. Mature pollen grains are released to the surface of the anther where they are picked up by pollinators (mainly insects) or by the wind. If a pollen grain lands on the stigma of a flower, a pollen tube forms, grows down the pistil, and enters an ovule. Observe these steps in the micrographs and video below.

Anthers bearing mature pollen grains	Sticky stigma of a pistil where pollen grains will land	Pollen tubes (aqua) growing into ovaries to reach the ovules within

video - growth of the pollen tube

Now that you have seen examples of pollen grains and pollen tubes, view the following slide. Capture an image at appropriate magnification and label a pollen grain, pollen tube, and sperm nucleus. Submit your labeled image to WebAssign for question 16.

slide - pollen viewed by virtual microscopy

When a pollen tube reaches the ovule, it delivers the two sperm nuclei, one of which will fertilize the egg nucleus of the embryo sac within the ovule. The other sperm nucleus will fuse with the" polar nuclei" of the embryo sac to form the endosperm nucleus which is triploid.  View the following animation which illustrates this process.

animation - double fertilization

After fertilization has occurred, the fertilized egg nucleus (zygote) develops into an embryo, the endosperm grows and stores food for future use by the embryo, and the coverings of the ovule become the seed coat. Thus the ovule develops into a seed after double fertilization occurs.

Each ovule within the ovary of this flower contains a zygote which will develop into an embryo. Endosperm cells within the ovule will accumulate nutrients for use by the embryo as it grows. When nutrient storage is complete, a fibrous coat will form around each ovule. The ovules will then be seeds.

Meanwhile, the surrounding ovary develops into a fruit which aids in seed dispersal. When you understand the entire story of gametogenesis and fertilization in flowering plants, answer question 17.