Campbell Biology · Chapter 30

Plant Diversity II: The Evolution of Seed Plants

pp. 618–635 · 4 sections

Seeds and pollen let plants reproduce without water and spread into dry places, and seed plants now cover most of the land. This chapter compares the two groups of seed plants, the cone-bearing gymnosperms and the flowering plants, follows their life cycles, and ends with how much people depend on them. Plant groups aren't in the current AP course, but the chapter gives practice with haploid and diploid stages, reading trees, artificial selection and human impacts on biodiversity.

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Chapter quiz (42 questions)Flashcards (56 terms)

30.1 What seeds and pollen added

pp. 618–621

On the AP exam? Not tested

Ovules, pollen and seed structure aren't in the current course. What carries over is tracking haploid and diploid stages (Topic 5.1) and explaining a trait as an adaptation (Topic 7.2).

In the course: Topic 5.1 Meiosis, Topic 7.2 Natural Selection, Topic 7.6 Evidence of Evolution (notes, videos and more questions)

Key points

Seed plants, meaning the gymnosperms and the flowering plants, added five features to what earlier land plants had: tiny gametophytes, two kinds of spores, ovules, pollen and seeds. Together they let these plants reproduce without liquid water and shield the next generation from drought and strong sunlight.
Across plant groups the gametophyte keeps shrinking. In mosses it's the main plant; in ferns it's small but lives on its own; in seed plants it's microscopic and develops inside the parent sporophyte, which keeps it moist, screens out UV and feeds it.
All seed plants are heterosporous. Microsporangia make huge numbers of microspores, which become male gametophytes, while each megasporangium keeps a single working megaspore, which becomes a female gametophyte. Their closest seedless relatives make one kind of spore, so heterospory probably evolved along the way.
An ovule is a megasporangium and its megaspore wrapped in protective layers called integuments: one layer in gymnosperms and usually two in flowering plants. The female gametophyte grows inside it and makes one or more eggs.
A pollen grain is a male gametophyte inside a wall toughened with sporopollenin. Wind or animals carry it to the ovule (pollination), and a pollen tube grows to bring the sperm right to the egg. Cycads and Ginkgo still have swimming sperm, but in most seed plants the sperm have no flagella.
After fertilization the ovule turns into a seed: an embryo, stored food and a seed coat made from the integuments. Compared with a one-celled spore, a seed is many-celled, stocked with food and well protected, can stay dormant for a long time, and can be spread by wind, water or animals.

Key terms (15)

seed plant: Any plant that makes seeds. The two living groups are the gymnosperms and the flowering plants (angiosperms).
heterosporous: Making two kinds of spores: microspores that become male gametophytes and megaspores that become female gametophytes. Every seed plant works this way.
microspore: The smaller kind of spore, made by meiosis. In seed plants it grows into a pollen grain.
megaspore: The larger kind of spore, made by meiosis. In seed plants it stays inside the ovule and grows into a female gametophyte.
microsporangium: A spore case that makes microspores. In seed plants these are the pollen sacs.
megasporangium: A spore case that makes megaspores. In seed plants it never lets its spore go; it stays inside the ovule.
integument: A layer of the parent plant's tissue wrapped around the megasporangium in an ovule. After fertilization it becomes the seed coat.
ovule: In seed plants, a megasporangium and its megaspore, together with the integuments that wrap them. Once fertilized, it develops into a seed.
pollen grain: A male gametophyte of just a few cells, packed in a tough wall so it can travel by wind or on an animal.
sporopollenin: An extremely tough, decay-resistant polymer in the walls of spores and pollen. It protects them from drying out and damage.
pollination: Moving pollen onto or close to a seed plant's ovules, carried by wind, water or animals.
pollen tube: A tube that grows out of a pollen grain and carries the sperm to the egg, so no outside water is needed.
seed: A plant embryo packed with a food supply inside a protective coat. It forms from a fertilized ovule.
seed coat: The protective outer layer of a seed, which develops from the ovule's integuments.
dormancy: A resting state in which a seed's growth and metabolism nearly stop until conditions are right for it to sprout.

Check yourself: 30.1 What seeds and pollen added

4 questions on 30.1 What seeds and pollen added. Pick an answer to see if you got it, and why.

Question 1 of 4

Before fertilization, which set of parts makes up an ovule in a seed plant?

Question 2 of 4

A paleobotanist finds a fossil plant with two kinds of sporangia on the same branch. One kind held thousands of tiny spores; the other held just a few large spores. No seeds or seed coats are preserved anywhere on the plant. Which conclusion is best supported?

Question 3 of 4

In one living gymnosperm, pollen lands at the opening of an ovule and grows a short tube into the ovule's tissue over several months. The tube then bursts and releases two large sperm, each covered in thousands of flagella, which swim less than a millimeter through fluid made by the ovule to reach the egg. What does this best show?

Question 4 of 4

Seeds of one pine species are sown at three depths in soil. Some seeds are left whole; in others, about two-thirds of the food tissue around the embryo is cut away first (invented data): Sowing depth | Whole seeds emerging (%) | Seeds with food cut away emerging (%) 1 cm | 90 | 85 3 cm | 84 | 40 6 cm | 61 | 5 Which conclusion do the data best support?

0 of 4 answered

30.2 Gymnosperms and the pine life cycle

pp. 621–625

On the AP exam? Not tested

Gymnosperm groups and the pine life cycle aren't in the current course. The useful links are meiosis making haploid cells (Topic 5.1), reading trees (Topic 7.9) and using fossils as evidence (Topic 7.6).

In the course: Topic 5.1 Meiosis, Topic 7.9 Phylogeny, Topic 7.6 Evidence of Evolution, Topic 7.2 Natural Selection (notes, videos and more questions)

Key points

The seed plants alive today split into two sister clades, the gymnosperms and the angiosperms. A gymnosperm's seeds develop out in the open on modified leaves, usually the scales of a cone, instead of inside an ovary.
The first seed plants show up in fossils about 360 million years ago and gymnosperms a little over 300 million years ago. As the climate dried in the Permian, gymnosperms spread while the swamp-loving lycophytes and ferns declined.
Gymnosperms were the main land plants for much of the Mesozoic era (about 252 to 66 million years ago), the age of dinosaurs. Flowering plants overtook them in many places late in that era, but conifers still cover huge northern forests.
About 1,000 gymnosperm species survive in four groups: cycads (palmlike leaves and big cones), Ginkgo (one living species, with fan-shaped leaves), gnetophytes (three very different genera), and conifers (about 600 species, such as pines, spruces, firs and redwoods). DNA studies suggest gnetophytes belong close to, or even inside, the conifers.
Most conifers are evergreen. Narrow needles with a thick cuticle lose little water, and keeping leaves all year lets them photosynthesize as soon as spring weather allows. A few, like larches, drop their needles every fall.
A pine tree is the diploid sporophyte. Its small pollen cones make microspores by meiosis, which become winged pollen spread by wind. Its larger ovulate cones carry two ovules per scale; in each ovule, meiosis makes four cells, only one survives as the megaspore, and it grows into a female gametophyte with a few archegonia that hold eggs.
Pine reproduction is slow: fertilization usually comes about a year after pollination, and the seeds ripen months after that. A pine seed holds a diploid embryo, food made of haploid female gametophyte tissue, and a seed coat from the parent tree's integument.

Key terms (14)

gymnosperm: A seed plant whose seeds aren't enclosed in an ovary; they sit exposed, usually on cone scales. Cycads, Ginkgo, gnetophytes and conifers are gymnosperms.
progymnosperm: An extinct, seedless plant, mostly from the Devonian, that already had wood and, in some, two kinds of spores. These plants show seed-plant traits arriving before seeds did.
cone (strobilus): A cluster of spore-bearing scales on a short stem. Gymnosperm cones make either pollen or ovules.
pollen cone: A small, short-lived cone whose scales carry microsporangia that release pollen.
ovulate cone: A larger, longer-lasting cone whose scales carry ovules, which develop into seeds.
microsporocyte: A diploid cell in a microsporangium that goes through meiosis to make four microspores. Also called a microspore mother cell.
megasporocyte: A diploid cell in an ovule that goes through meiosis. In most seed plants only one of its four products survives as the megaspore.
archegonium: A small, flask-shaped structure on a female gametophyte that holds an egg. Pines and other gymnosperms still have them; flowering plants don't.
conifer: The largest group of gymnosperms, about 600 species of mostly cone-bearing trees with needle- or scale-like leaves, such as pines, firs and cedars.
cycad: A gymnosperm with stiff, palmlike leaves and large cones. Cycads still have swimming sperm and were common in the Mesozoic.
Ginkgo biloba: The only living species of its gymnosperm group, a tree with fan-shaped leaves and separate male and female trees.
gnetophyte: A small, varied group of gymnosperms in three genera, from desert shrubs to tropical vines. DNA places them near the conifers.
evergreen: Keeping leaves all year instead of dropping them in one season. Most conifers are evergreen.
Mesozoic era: The span of Earth's history from about 252 to 66 million years ago, when gymnosperms and dinosaurs dominated the land.

Check yourself: 30.2 Gymnosperms and the pine life cycle

4 questions on 30.2 Gymnosperms and the pine life cycle. Pick an answer to see if you got it, and why.

Question 1 of 4

The name gymnosperm means "naked seed." What does "naked" refer to?

Question 2 of 4

Inside a pine's cones, 25 megasporocytes and 25 microsporocytes each finish meiosis normally. How many functional megaspores and how many microspores result?

Question 3 of 4

A student opens young pine ovulate cones in October, five months after pollen landed on them in spring. Pollen tubes have begun growing into the ovules, but no egg has been fertilized yet. What is the best explanation?

Question 4 of 4

0 of 4 answered

30.3 Flowers, fruits and flowering plants

pp. 625–632

On the AP exam? Not tested

Flower parts, double fertilization and monocot versus eudicot traits aren't on the exam. Pollinators keeping species apart is a good example of reproductive isolation (Topic 7.10), and the plant-pollinator partnership is mutualism (Topic 8.5).

In the course: Topic 5.1 Meiosis, Topic 7.10 Speciation, Topic 7.9 Phylogeny, Topic 8.5 Community Ecology (notes, videos and more questions)

Key points

Angiosperms, the flowering plants, are seed plants whose ovules are sealed inside an ovary that later becomes a fruit. With roughly 300,000 known species, they make up about 90% of living land plant species. (Older books put them in a phylum called Anthophyta; today they're simply treated as one clade.)
A flower is a short shoot whose leaves have been reshaped into as many as four whorls. Sepals protect the bud, petals attract pollinators, stamens make pollen in their anthers, and carpels hold the ovules: pollen lands on the stigma, a sticky surface on top, and the style leads down to the ovary.
A fruit is a ripened ovary, sometimes with other flower parts attached. It protects the seeds and helps spread them: animals eat fleshy fruits and drop the seeds elsewhere, while dry fruits may have wings, plumes or hooks, or may float. Cereal grains like rice and wheat are actually tiny dry fruits.
Meiosis in the anther makes microspores that become pollen grains, each holding a tube cell plus a generative cell, which later splits into two sperm. In each ovule, meiosis yields four megaspores, but only one lasts, growing into the embryo sac (the female gametophyte), usually just seven cells, including the egg and a large central cell with two nuclei.
Double fertilization: the pollen tube grows down the style and releases two sperm into the embryo sac. One fertilizes the egg, making a diploid zygote; the other fuses with the central cell, which in most species becomes triploid endosperm, the seed's food. A likely payoff: because endosperm forms only after fertilization, the plant doesn't stock food in ovules that won't become seeds.
Flowering-plant fossils go back about 130–140 million years, and by about 100 million years ago these plants were taking over many landscapes. The oldest surviving lineages are Amborella, the water lilies and the star anise group. Most species are magnoliids, monocots (about a quarter) or eudicots (roughly 70%). The old "dicot" group isn't a clade, because it leaves out the monocots that branched from within it.
Monocots usually have one cotyledon, parallel leaf veins, scattered vascular bundles, fibrous roots, flower parts in threes and pollen with one opening. Eudicots usually have two cotyledons, netlike veins, bundles in a ring, a taproot, parts in fours or fives and pollen with three openings. Animals shaped this group's evolution: pollinators and seed dispersers are partners, plant-eaters drive new defenses, and specialized pollinators can keep populations apart and speed up speciation.

Key terms (15)

angiosperm: A flowering plant: a seed plant whose ovules develop inside an ovary, which ripens into a fruit.
flower: A short shoot built for sexual reproduction. Its leaves are reshaped into as many as four whorls: the sepals, petals, stamens and carpels.
sepal: One of the outermost flower parts, usually green, that cover and protect the bud before it opens.
petal: A flower part, often brightly colored, that helps attract pollinators. Wind-pollinated flowers often have small petals or none.
stamen: The pollen-making organ of a flower: a thin stalk (the filament) topped by an anther, where pollen forms.
carpel: The ovule-holding organ of a flower, made of a stigma, a style and an ovary. Some flowers have one; others have several, separate or fused.
stigma: The sticky tip of a carpel that catches pollen.
ovary: The swollen base of a carpel that holds one or more ovules. After fertilization it ripens into a fruit.
fruit: A ripened ovary, sometimes with other flower parts, that protects seeds and helps spread them.
embryo sac: The female gametophyte of a flowering plant: usually seven cells inside the ovule, including the egg.
double fertilization: The flowering-plant process in which one sperm fertilizes the egg and a second sperm fuses with the central cell to start the endosperm.
endosperm: Food-storing tissue in a flowering plant's seed, usually triploid, that forms from the central cell after double fertilization.
cotyledon: A seed leaf of a plant embryo. It may store food or absorb it from the endosperm.
monocots: A clade of flowering plants, including grasses, palms, lilies and orchids, whose embryos usually have one cotyledon.
eudicots: The largest clade of flowering plants, including roses, beans, oaks and sunflowers. Most have two cotyledons and pollen with three openings.

Check yourself: 30.3 Flowers, fruits and flowering plants

4 questions on 30.3 Flowers, fruits and flowering plants. Pick an answer to see if you got it, and why.

Question 1 of 4

A mutant plant makes flowers with normal sepals, petals and stamens, but no carpels ever form. Which statement about this plant is correct?

Question 2 of 4

In maize, allele Y gives yellow endosperm and allele y gives white. A yy plant is used as the female parent and is pollinated by a YY plant. What are the genotypes of the embryo and the endosperm in the kernels that form?

Question 3 of 4

One hypothesis says double fertilization keeps flowering plants from spending food on ovules that will never become seeds. A researcher blocks fertilization in some ovules of a gymnosperm and of a flowering plant, then measures the food-storing tissue in each ovule a month later (invented data, mg per ovule): Plant | Fertilized ovules | Unfertilized ovules Gymnosperm | 9.0 | 7.8 Flowering plant | 8.6 | 0.3 Which conclusion is best supported?

Question 4 of 4

Seeds of a shrub with red berries are tested for germination (invented data): Treatment | Seeds germinating (%) Whole berry planted | 6 Pulp removed by hand | 38 Seeds collected from bird droppings | 45 Which conclusion is best supported?

0 of 4 answered

30.4 Why seed plants matter to people

pp. 632–634

On the AP exam? Background

The products we get from plants aren't tested, but the ideas behind them are: crops as artificial selection (Topic 7.3), genetic diversity (Topic 7.11), energy loss between trophic levels (Topic 8.2), and biodiversity loss from human disruption (Topics 8.6–8.7).

In the course: Topic 7.3 Artificial Selection, Topic 7.11 Variations in Populations, Topic 8.2 Energy Flow Through Ecosystems, Topic 8.6 Biodiversity, Topic 8.7 Disruptions in Ecosystems (notes, videos and more questions)

Key points

Nearly all our food comes from flowering plants, and three grasses (rice, wheat and maize) supply roughly half the calories people eat worldwide. Grain also feeds livestock, and because most energy is lost at each step up a food chain, eating grain directly feeds far more people than turning it into meat.
Crops are the product of artificial selection that began about 10,000 to 12,000 years ago in several parts of the world. Early farmers kept seed from plants with bigger grains, seed heads that didn't shatter and quick, even sprouting. Comparing crop and wild-relative DNA shows that a few genes can explain big changes.
Seed plants also give us wood (thick-walled xylem) for fuel, building and paper, along with fibers, drinks and spices. Many medicines began as plant chemicals: morphine comes from the opium poppy, and the malaria drug artemisinin from sweet wormwood.
Plant diversity is under threat, mostly from habitat loss as forests are cleared for farming. About 10 million hectares of forest were cleared each year from 2015 to 2020, much of it in the tropics, where most species live. When plants disappear, so do many animals that rely on them.
Plants regrow, but species don't: an extinct species is gone for good. Only a small share of plant species have been checked for medicines or food, and the wild relatives of crops carry alleles for traits like drought or disease resistance that breeders may need later.
Clearing forests disrupts more than biodiversity. Burning and decay release stored carbon as CO₂, and with fewer trees, less water returns to the air through transpiration, so local climates can become hotter and drier.
Ways to protect plant diversity include saving habitat, harvesting at rates that let plants recover, and seed banks that keep seeds cold and dry so they stay alive for decades. The Svalbard Global Seed Vault in Norway stores backup copies of crop seeds from around the world.

Key terms (12)

domestication: Changing a wild species into one adapted to life with people, through many generations of human choice about which individuals breed.
artificial selection: Selection in which people, not nature, decide which individuals reproduce, so traits people want become more common.
crop wild relative: A wild species closely related to a crop. Its alleles, like ones for pest or drought resistance, can be bred into the crop.
monoculture: A large area planted with a single crop variety. Low genetic variation makes it easy for one pest or disease to wipe it out.
wood: Tough tissue made of layers of thick-walled xylem cells. Among living plants, only seed plants make it.
secondary compound: A chemical a plant makes that isn't needed for basic growth, often for defense. Many medicines, spices and drugs come from these.
deforestation: Clearing forest and turning the land over to other uses, such as farms, pasture or cities.
habitat loss: The destruction or breaking up of the places where species live. It's the leading cause of plant extinctions today.
extinction: The permanent loss of a species, when its last individual dies.
biodiversity: The variety of life in a place, from genes to species to whole ecosystems.
transpiration: Water evaporating from a plant's leaves, which pulls more water up from the roots and adds moisture to the air.
seed bank: A facility that stores seeds, usually cold and dry, to preserve plant species and crop varieties for the future.

Check yourself: 30.4 Why seed plants matter to people

4 questions on 30.4 Why seed plants matter to people. Pick an answer to see if you got it, and why.

Question 1 of 4

Wild ancestors of many grain crops make seeds that stay dormant for different lengths of time, so a batch sprouts over months or years. Seeds of the domesticated crops mostly sprout within days of planting. Which explanation best fits this difference?

Question 2 of 4

Most bananas sold in many countries come from one variety grown as genetically identical clones from cuttings. A soil fungus strain that kills this variety has been spreading across banana-growing regions. Why is this crop especially at risk?

Question 3 of 4Calculator allowed

A hectare of farmland grows grain holding 2.4 × 10⁷ kcal of food energy. A person needs about 2,000 kcal per day. If the grain is fed to cattle and about 10% of the feed's energy ends up in the beef people eat, how many person-days of food does the hectare provide as beef, compared with eating the grain directly?

Question 4 of 4

A wild relative of a crop, found only along one stretch of salty coastline, carries an allele that lets it grow in salty soil. Breeders cross it with the crop and pick offspring that keep the allele and still yield well. What does this example best show about conserving wild plants?

0 of 4 answered