SECTION 22.2 Reproduction in Flowering Plants Teacher Notes and Answers SECTION 2 Instant Replay 1. They can help to attract animal pollinators. 2. Animal pollination requires less energy output from the plant, since it is more efficient than wind pollination and less pollen needs to be produced. 3. They combine with one sperm and form the endosperm in the seed. Vocabulary Check 1. stamen 2. endosperm 3. carpel 4. sepal The Big Picture 5. anther, stigma, pollen tube, egg 6. Because double fertilization only occurs if an egg has been fertilized, the plant does not expend energy developing a food supply for the embryo unless it is needed. © Houghton Mifflin Harcourt Publishing Company
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SeCTion
22.2 Reproduction in Flowering Plants
Teacher Notes and Answers
SeCtion 2Instant Replay
1.Theycanhelptoattractanimalpollinators. 2.Animalpollinationrequireslessenergyoutput
fromtheplant,sinceitismoreefficientthanwindpollinationandlesspollenneedstobeproduced.
3.Theycombinewithonespermandformtheendospermintheseed.
Vocabulary Check
1.stamen 2.endosperm 3.carpel 4.sepal
The Big Picture
5.anther,stigma,pollentube,egg 6.Becausedoublefertilizationonlyoccursif
anegghasbeenfertilized,theplantdoesnotexpendenergydevelopingafoodsupplyfortheembryounlessitisneeded.
© Houghton Mifflin Harcourt Publishing Company
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SeCTion
22.2 Reproduction in Flowering Plants KEyConCEPT Reproduction of flowering plants takes place within the flowers.
Flowerscontainreproductiveorgansprotectedbyspecializedleaves.
Most people prefer sweet-smelling, colorful flowers for their gardens. These same characteristics attract animal pollinators.
TheoutsideofaFlower You may have noticed that different flower parts are arranged in layers. The outermost layer of a flower is made of sepals. Sepals are modified leaves that protect the flower. They are often green but can be brightly colored. Just inside the sepals is a layer of petals, which are also modified leaves. Brightly colored petals can attract animal pollinators. When sepals and petals are the same color, they are called tepals. This occurs in monocot flowers, such as lilies. Flowering plants that use wind pollination usually have small sepals and petals, or even none at all.
TheinsideofaFlower In some species, flowers have either male or female structures. But most flowers have both. A stamen is the male structure of a flower. It has a stalk, called a filament, that supports an anther. Anthers produce pollen grains. The innermost part of the flower is the female structure, called a carpel. There are often several carpels that join together, forming a pistil. Each carpel has three parts. The stigma is the top part. It’s sticky and holds pollen grains when they land there. The style is a tube that leads from the stigma to the ovary. At the base of the flower is the ovary, where female gametophytes are produced.
How can colorful sepals and petals help a flower reproduce?
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Floweringplantscanbepollinatedbywindoranimals.
When a pollen grain reaches the stigma of the same plant species, that flower has been pollinated. The way a flower looks gives clues about how it is pollinated. Wind-pollinated flowers are usually small and produce large amounts of pollen. This requires a lot of energy.
Insects, birds, and other animals often pollinate flowers that have bright colors or sweet smells. When an animal visits a flower to collect pollen or nectar for food, it is dusted with pollen grains. As the animal looks for food in another flower, pollen from the first flower may brush onto the stigma of the other flower. This method of pollination is more direct and more efficient than wind pollination.
What is the advantage of animal pollination for a flowering plant?
Fertilizationtakesplacewithintheflower.The flower parts you have just learned about are part of the sporophyte. The gametophytes are tiny and enclosed in the flower.
maleGametophytes Anthers produce pollen grains, the male gametophytes of seed plants. Cells within the anthers divide by meiosis to produce four male hap-loid spores. Each spore divides again, by mitosis, to produce two haploid cells. These two cells form a single pollen grain.
FemaleGametophytes One female gametophyte forms in each ovule of an ovary. First, one cell in the ovule divides by meiosis to produce four female spores. The nucleus of one of these spores divides by mitosis three times. This spore now has eight haploid nuclei (1n). Membranes grow between the nuclei to form seven cells. A large cell in the middle has two nuclei, called polar nuclei. One of the other cells develops into an egg. These seven cells form the female gametophyte, also called an embryo sac.
pollengrain
maleandfemalegametophytes Tomato flowers have both male and female structures. Pollen grains, the male gametophytes, are produced in anthers. The flower’s ovary contains many ovules, which can each contain a female gametophyte.
ovule
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DoubleFertilization After pollination, one cell in the pollen grain develops into a pollen tube. The tube extends down the style, which leads to the ovule. The other cell in the pollen grain divides by mitosis to produce two sperm (1n). Both sperm travel down the pollen tube. One sperm fertilizes the egg. The other sperm joins with the two polar nuclei. This cell now has a triploid (3n) nucleus, and will become the endosperm, a food supply for the embryo. The process in which one sperm fertilizes an egg and the other forms a triploid cell is called double fertilization.
With double fertilization, a flowering plant only uses energy to make a food supply if an egg is fertilized. Cone-bearing plants produce a food supply for every egg before fertilization. Double fertilization gives flowering plants an advantage over cone-bearing plants in that energy is not wasted if an egg is not fertilized.
SeedsandFruit The next sporophyte generation begins at fertilization. The ovule becomes a seed. The seed coat protects the embryo and the endosperm inside. The embryo will develop one or two cotyledons, or seed leaves. Cotyledons can also provide food for the new plant until photo-synthesis begins.
pollentube
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Pollination A bee may transfer pollen grains from one flower’s anther to another flower’s stigma. One cell of a pollen grain divides to form two sperm. The other cell forms a tube, down which the sperm travel.
Doublefertilization One sperm fertilizes the egg, which develops into an embryo. The other sperm unites with the polar nuclei to form the endosperm. The outer layer of the ovule becomes a protec-tive seed coat.
VISUALVOCAB
endosperm
embryo
seedcoat
The endosperm nourishes the de-veloping plant embryo inside of the seed coat. The prefix endo- means “inside,” and sperm comes from the Greek word sperma, which means “seed.”
Seedsandfruit Many seeds develop inside the ovary of each tomato flower. While the seeds develop, the ovary tissue develops into the juicy flesh of a tomato. A few seeds will find their way into the soil to grow into new tomato plants.
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The ovary around the seed develops into a fruit. Apples, watermelons, and cherries are fruits. But so are many vegetables, grains, and nuts. Tomatoes and cucumbers are fruits that contain many seeds. A peanut shell is also a fruit. The next section explains how a fruit helps a seed disperse, or move, to a new area.
What happens to the polar nuclei in an embryo sac?
sepal ovary
petal endosperm
stamen double fertilization
carpel
Fill in the blanks with the correct term from the list above.
1. A is the male structure of a flower.
2. The is the food supply for the developing embryo.
3. The female structure of a flower is called a .
4. A is a modified leaf that protects a developing flower.
5. Put the following terms in the correct order to show the journey of a sperm to an egg: pollen tube, egg, anther, stigma
6. What advantage does double fertilization provide to flowering plants?
22.2 VocabularyCheck
Go back and highlight each sentence that has a vocabulary word in bold.
Mark It Up
22.2 TheBigPicture
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