Campbell Biology · Chapter 38
Angiosperm Reproduction and Biotechnology
pp. 801–820 · 3 sections
This chapter follows a flowering plant through sexual reproduction: how flowers make pollen and eggs, how pollinators move pollen around, the double fertilization that builds a seed, and how seeds and fruits get spread and sprout. It then weighs sex against cloning and ends with how people reshape crops, from ancient selective breeding to genetic engineering and gene editing. Flower anatomy isn't on the current AP exam, but the chapter keeps using ideas the exam does test: meiosis, genetic variation, artificial selection and biotechnology.
Independent review — not affiliated with or endorsed by the publisher. You'll need your own copy of the book.
38.1 How flowering plants make seeds and fruit
pp. 801–811
Flower parts, double fertilization and seed and fruit types aren't in the current course. What carries over: meiosis making haploid cells (Topic 5.1), seeds responding to cues (Topic 8.1), pollinators as mutualists (Topic 8.5) and water potential (Topic 2.7).
In the course: Topic 5.1 Meiosis, Topic 8.1 Responses to the Environment, Topic 8.5 Community Ecology, Topic 7.2 Natural Selection, Topic 2.7 Tonicity and Osmoregulation (notes, videos and more questions)
Key points
- Plant life cycles swap between a diploid sporophyte and a haploid gametophyte. In flowering plants the sporophyte is the plant you see, and the gametophytes have shrunk to a handful of cells hidden in the flower.
- Flowers are shoots built for reproduction. From the outside in: sepals wrap the bud, petals advertise to pollinators, stamens make pollen in their anthers, and carpels hold the ovules. On a carpel, pollen lands on the sticky stigma, the style connects it downward, and the ovary at the base holds the ovules.
- In the anther, meiosis makes haploid microspores, and each develops into a pollen grain, which is the male gametophyte. In each ovule, meiosis makes four megaspores; usually one survives and divides by mitosis to make the embryo sac, the female gametophyte that holds the egg.
- Pollination means pollen landing on a stigma. It isn't fertilization. Wind-pollinated plants make clouds of light pollen and plain flowers, while animal-pollinated flowers offer nectar or pollen and use colors, scents and shapes that suit their visitors, often shaped by coevolution.
- A pollen tube grows down the style and releases two sperm in the ovule. One fuses with the egg to make a diploid zygote, and the other joins the two polar nuclei to make triploid endosperm, the stored food. This double fertilization means food only goes to ovules that hold a fertilized egg.
- The ovule becomes a seed: an embryo with one cotyledon (monocots) or two (eudicots), a food store in the endosperm or cotyledons, and a tough seed coat. The ovary becomes the fruit, which protects the seeds and helps wind, water or animals carry them away.
- Many seeds stay dormant until a cue such as a long cold spell, heavy rain, fire or light says conditions are good. Germination begins when the dry seed soaks up water (imbibition), and the embryonic root breaks out first.
Key terms (15)
- sporophyte
- The diploid (2n) stage of a plant's life cycle that makes spores. In a flowering plant it's the whole plant you see: roots, stems, leaves and flowers.
- gametophyte
- The haploid (n) stage that makes eggs or sperm by mitosis. In flowering plants it's tiny: a pollen grain on the male side and an embryo sac of a few cells on the female side.
- stamen
- The male part of a flower: an anther, where pollen forms, held up on a slender filament.
- carpel
- The egg-holding part of a flower. Its sticky tip (the stigma) catches pollen, a stalk (the style) leads down, and its swollen base (the ovary) holds the ovules.
- ovule
- A small structure inside the ovary that holds the embryo sac. After fertilization it turns into a seed.
- pollen grain
- A tough-walled package holding the male gametophyte: one cell that grows the pollen tube and one that makes two sperm.
- embryo sac
- The female gametophyte inside an ovule. It's just a few cells, including the egg and a large central cell with two polar nuclei.
- pollination
- Pollen arriving on a stigma, carried by wind, water or animals. It comes before fertilization and doesn't guarantee it.
- double fertilization
- The flowering-plant process in which one sperm fuses with the egg to make the zygote, while a second sperm joins the central cell to start the endosperm.
- endosperm
- A food-storing tissue in seeds, usually triploid (3n), made when a sperm joins the central cell's two polar nuclei. Most of a wheat or rice grain is endosperm.
- cotyledon
- A seed leaf that's part of the embryo. Monocots have one and eudicots have two, and in some seeds the cotyledons hold most of the stored food.
- seed dormancy
- A resting phase in which a living seed won't sprout, even in good conditions, until a specific cue such as cold, heavy rain or light signals that the timing is right.
- imbibition
- The first step of sprouting: a dry seed pulls in water because its water potential is very low. The seed swells, its coat cracks, and the embryo's enzymes switch back on.
- fruit
- A ripened ovary, sometimes with other flower parts attached. It protects the seeds inside and helps get them spread.
- coevolution
- Two species evolving in response to each other, like a flower's shape and its pollinator's mouthparts each being shaped by selection from the other.
Check yourself: 38.1 How flowering plants make seeds and fruit
4 questions on 38.1 How flowering plants make seeds and fruit. Pick an answer to see if you got it, and why.
A squash grower sees plenty of bees visiting her flowers, yet few fruits form. A plant biologist examines the flowers and finds many pollen grains stuck to the stigmas, but no pollen tubes in the styles. Which conclusion do these observations best support?
A wildflower has 2n = 16. In one of its ovules, double fertilization has just taken place. How many chromosomes are in each nucleus of the egg (before fertilization), the zygote, the endosperm and the developing seed coat?
On an island, a vine's flowers stay open for a full day and night. To find out when pollination happens, researchers covered some flowers with fine mesh bags, which keep animals out but let air through, during the day only, during the night only, or the whole time (invented data): Treatment | Flowers that set fruit (%) Uncovered | 68 Covered by day, open at night | 63 Covered at night, open by day | 9 Covered day and night | 2 Which conclusion is best supported?
Suppose that in some ovules only one working sperm arrives, and it fuses with the central cell instead of the egg. What would most likely develop in such an ovule?
0 of 4 answered
38.2 Cloning versus sex in flowering plants
pp. 812–815
The big trade-off is tested: sexual reproduction creates variation (Topic 5.2), and populations with little variation, like clones, are hit hardest by a new disease (Topic 7.11). Plant cloning methods and self-incompatibility genes are background only.
In the course: Topic 5.2 Meiosis and Genetic Diversity, Topic 7.11 Variations in Populations, Topic 6.6 Gene Expression and Cell Specialization, Topic 5.3 Mendelian Genetics (notes, videos and more questions)
Key points
- In asexual reproduction one parent makes offspring without any egg and sperm, so the offspring are clones that share the parent's genes, apart from rare new mutations.
- Plants clone easily because meristems never stop dividing and many mature cells can change type again. A broken-off piece, a shoot sprouting from a spreading root, or a plantlet on a runner can each grow into a full plant.
- Some plants make seeds with no fertilization at all (apomixis). A diploid ovule cell becomes the embryo, so the plant gets the travel benefits of seeds without mixing genes.
- Cloning needs no mate or pollinator, passes on a well-suited genotype whole, and gives offspring a strong start from the parent's resources. But a group of clones has almost no variation, so one new disease or big change can wipe it out.
- Sexual reproduction shuffles alleles every generation and makes seeds that disperse and can stay dormant, which helps in changing conditions. It's costly, though: flowers, fruits and huge numbers of seeds, most of whose seedlings die.
- Many plants block self-fertilization. Some species have separate male and female plants, some flowers ripen their stamens and stigmas at different times or keep them apart, and many use self-incompatibility, in which the stigma or style turns away pollen with matching S alleles.
- People clone useful plants on purpose. They root cuttings, graft a shoot (the scion) onto a rooted base (the stock), and grow tissue culture, where hormones coax a lump of unspecialized cells (callus) into making shoots and roots. Single engineered cells can be grown into whole plants this way.
Key terms (14)
- asexual reproduction
- Reproduction by one parent through mitosis alone, with no gametes fusing, so the offspring inherit the parent's genes.
- clone
- An organism, or a group of organisms, genetically identical to the one it came from.
- vegetative reproduction
- Asexual reproduction from a plant's non-flower parts, such as stems, roots or leaves.
- fragmentation
- A form of plant cloning in which a broken-off piece, such as a stem section or bud, roots and grows into a new plant.
- apomixis
- Making seeds without fertilization. An embryo grows from a diploid ovule cell, so the seed is a clone of the parent.
- self-fertilization
- A plant's own pollen fertilizing its own eggs, also called selfing. It still involves meiosis, so offspring of a heterozygous plant aren't all identical.
- dioecious
- Describes a species whose individuals carry only male flowers or only female flowers, so no single plant can fertilize itself.
- self-incompatibility
- A genetic system in which a flower's stigma or style stops pollen carrying a matching S allele, which keeps the plant from fertilizing itself (and often its close kin).
- S allele
- One of the many versions of the self-incompatibility gene in a population. Pollen is blocked when its S allele, or its parent's, matches one in the stigma's plant.
- cutting
- A piece of stem, leaf or root cut from a plant and grown into a new plant, which is a clone of the first.
- grafting
- Joining a shoot or bud (the scion) from one plant onto the rooted base (the stock) of a related plant so they grow as one.
- callus
- A clump of unspecialized cells that keep dividing, which forms at a cut surface or in a culture dish. The right hormone mix can turn it into shoots and roots.
- plant tissue culture
- Growing plant cells or small bits of tissue on a sterile medium with nutrients and hormones, often to make many clones or to grow engineered plants.
- protoplast
- A plant cell with its wall digested away by enzymes, leaving just the membrane-bound cell. Protoplasts from two species can be fused into one hybrid cell.
Check yourself: 38.2 Cloning versus sex in flowering plants
4 questions on 38.2 Cloning versus sex in flowering plants. Pick an answer to see if you got it, and why.
Which pair of offspring would you expect to be genetically identical to each other?
A plant biologist grows a wild grass in a greenhouse with no other grasses nearby and bags its flower heads. It still sets plenty of seed. She checks 6 DNA markers, each on a different chromosome, at which the mother plant is heterozygous: all 40 seedlings have exactly the mother's genotype at every marker. What best explains this?
Under which conditions would a plant that reproduces mostly by cloning most likely outdo a close relative that reproduces mostly by seed?
In a species with sporophytic self-incompatibility, a pollen grain fails on a stigma if either S allele of the plant that made the pollen matches either S allele of the plant the stigma belongs to. Pollen comes from a plant with genotype S₄S₇. On which plant's stigmas would all of this pollen be accepted?
0 of 4 answered
38.3 Breeding and engineering crops
pp. 815–819
Crop breeding is a classic case of artificial selection (Topic 7.3), and genes passed through chloroplasts come up in Topic 5.4. Topic 6.8 covers biotechnology tools, but GM crop methods and the GMO debate themselves aren't tested.
In the course: Topic 7.3 Artificial Selection, Topic 6.8 Biotechnology, Topic 5.4 Non-Mendelian Genetics, Topic 7.2 Natural Selection, Topic 8.7 Disruptions in Ecosystems (notes, videos and more questions)
Key points
- People have reshaped crops for about 10,000 years by artificial selection, keeping seed from plants with bigger grains, seeds that stay put, or better taste. Many crops now rely on farmers to plant them, since they've lost the ability to scatter their own seeds.
- Breeders bring in useful alleles by crossing a crop with a wild relative, then crossing the offspring back to the crop for several generations, keeping the new trait while losing the wild plant's unwanted traits. Treating seeds with radiation or chemicals can add new mutations to choose from.
- Even different species can sometimes be crossed. Hybrid embryos that would starve in the seed can be rescued and grown in culture, and a hybrid that keeps full chromosome sets from both parents can be a fertile new crop.
- Genetic engineering moves a chosen gene from any organism into a plant, often using the bacterium Agrobacterium to deliver it. Newer gene-editing tools such as CRISPR change a plant's own genes. Both are faster and more precise than breeding and aren't limited to species that can cross.
- Engineered traits include insect resistance, herbicide tolerance, virus resistance and extra vitamins or protein. Fast-growing plants are also being developed for biofuels, which release CO₂ the plants recently took in, though the fuel used for farming and processing cuts into that benefit.
- Major scientific reviews have found no good evidence that approved GM foods are less safe to eat than ordinary ones, but each new crop is judged case by case. Ecological worries include harm to non-target species and transgenes spreading to wild relatives by pollen.
- Ways to keep transgenes from escaping include plants that make no working pollen, seeds made by apomixis, genes placed in chloroplast DNA (passed only through the egg in many crops), and flowers that pollinate themselves without opening.
Key terms (14)
- artificial selection
- Humans choosing which plants or animals get to breed, so the traits people want become more common over the generations.
- domestication
- The long process of turning a wild species into a crop or farm animal through artificial selection.
- hybridization
- Crossing two different varieties, or even species, to bring their traits together in the offspring.
- backcross
- Crossing a hybrid back to one of its parent lines, often again and again, to keep one new trait while getting back that parent's other traits.
- induced mutation
- A mutation caused on purpose, for example by treating seeds with radiation or chemicals, to give breeders new variation to pick from.
- plant biotechnology
- Any way people put plants to work for human needs, from ancient breeding to its most common meaning today: changing crop DNA in the lab.
- genetically modified organism (GMO)
- An organism whose DNA has been changed by genetic engineering rather than only by breeding.
- transgene
- A gene from one species that has been inserted into the genome of another.
- Agrobacterium
- A soil bacterium that naturally slips part of its DNA into plant cells. Scientists use disarmed versions to deliver new genes into crops.
- gene editing
- Changing specific bases in an organism's own DNA, often with CRISPR, without having to add a gene from another species.
- biofuel
- Fuel such as ethanol made from recently grown plant material instead of from fossil fuels.
- transgene escape
- A transgene spreading from a crop into wild or weedy relatives, usually through cross-pollination.
- male sterility
- A trait that stops a plant from making working pollen. It's used to make hybrid seed and could keep transgenes from spreading by pollen.
- maternal inheritance
- Inheritance through the egg only. In many crops chloroplast DNA is passed on this way, so pollen doesn't carry it.
Check yourself: 38.3 Breeding and engineering crops
4 questions on 38.3 Breeding and engineering crops. Pick an answer to see if you got it, and why.
Wild grasses drop their seeds as soon as they ripen, which spreads them. Early farmers harvested wild grasses by cutting the stalks, carried them home, and planted some of the grain the next year. Over centuries, their crops came to hold on to ripe seeds. What best explains this change?
Breeders find a wild relative of lettuce that resists a leaf mildew. They cross it with a high-yielding lettuce variety, then cross the resistant hybrids back to the lettuce variety. They repeat this, crossing resistant offspring back to the lettuce variety three times in all. On average, what share of the final plants' genome comes from the lettuce variety?
A gene from a soil bacterium codes for an enzyme that breaks down a weedkiller. Researchers attach the gene's coding sequence to a plant promoter, insert it into soybean cells, and regrow whole plants. The plants make the bacterial enzyme. What makes this possible?
A crop is engineered with a gene for herbicide tolerance. Researchers cross it both ways with a wild relative and test the offspring (invented data): Cross (female × male) | Offspring tolerant to herbicide (%) Engineered crop × wild relative | 100 Wild relative × engineered crop | 0 Where was the gene most likely placed, and why does that matter?
0 of 4 answered