Campbell Biology · Chapter 29
Plant Diversity I: How Plants Colonized Land
pp. 600–617 · 3 sections
Before plants, the continents were mostly bare rock with thin crusts of microbes. This chapter follows how plants evolved from freshwater green algae, the traits that let them survive out of water, and the first two big groups to succeed there: the low-growing mosses and their kin, then the ferns and lycophytes that grew the first forests. Plant diversity isn't part of the current AP course, but the chapter is good practice for reading phylogenies, tracking haploid and diploid stages, and following carbon through ecosystems.
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29.1 From green algae to land plants
pp. 600–606
Plant groups and their traits aren't in the current course, but the reasoning is: building trees from shared derived traits and DNA (Topic 7.9), fossil and molecular evidence (Topic 7.6), and the haploid and diploid stages of a life cycle (Topic 5.1).
In the course: Topic 7.9 Phylogeny, Topic 7.6 Evidence of Evolution, Topic 5.1 Meiosis, Topic 7.2 Natural Selection, Topic 2.2 Cell Size, Topic 3.4 Photosynthesis (notes, videos and more questions)
Key points
- Land plants evolved from within the charophytes, a group of mostly freshwater green algae. Several cell features point there, such as ring-shaped clusters of the enzymes that build cellulose, the way a dividing cell lays down its new wall, similar swimming sperm and shared peroxisome enzymes. Nuclear and chloroplast DNA agree.
- Since this edition, genome studies have narrowed it further: plants' nearest living relatives seem to be the conjugating green algae (Zygnematophyceae), a charophyte group that includes Spirogyra and desmids. Either way, today's algae are cousins of plants, not their ancestors.
- Algae at the edges of ponds that sometimes dry out had a head start. A tough, waterproof polymer called sporopollenin coats charophyte zygotes and plant spores, so they survive drying.
- Life out of water had perks: sunlight not dimmed by water, CO₂ that diffuses thousands of times faster in air than in water, mineral-rich shoreline soils and, at first, almost no plant-eaters or plant pathogens. The trade-offs were constant risk of drying out and nothing to buoy a plant up.
- Four traits set land plants apart from charophytes: alternation of generations, with a multicellular embryo fed by its parent (so land plants are also called embryophytes); spores with sporopollenin walls, made in multicellular sporangia; eggs and sperm made in multicellular gametangia (archegonia and antheridia); and apical meristems that keep root and shoot tips growing.
- Many plants add a waxy cuticle that slows water loss, partnerships with mycorrhizal fungi that help gather soil minerals (fossils show these more than 400 million years ago), and secondary compounds that repel plant-eaters or screen out UV light.
- Fossil spores show plants on land by about 470 million years ago. Plants then split into bryophytes, which lack an extensive vascular system; seedless vascular plants, including lycophytes and ferns; and seed plants, the gymnosperms and the flowering plants (which are about 90% of plant species today).
Key terms (15)
- charophytes
- The group of mostly freshwater green algae that land plants evolved from within. Its living members are plants' closest relatives among the algae.
- sporopollenin
- A very tough, long-lasting polymer that coats plant spores, pollen and some algal zygotes, keeping them from drying out or breaking down.
- embryophyte
- Another name for a land plant, because every land plant keeps its young embryo inside the parent's tissue and feeds it there.
- alternation of generations
- A life cycle with two multicellular stages that take turns: a haploid stage that makes gametes and a diploid stage that makes spores.
- gametophyte
- The multicellular haploid stage of a plant life cycle. It grows from a spore and makes eggs, sperm or both by mitosis.
- sporophyte
- The multicellular diploid stage of a plant life cycle. It grows from a zygote and makes haploid spores by meiosis.
- spore
- A haploid cell, made by meiosis in plants, that divides by mitosis to build a gametophyte. Unlike a gamete, it needs no partner to start growing.
- sporangium
- A many-celled structure on the sporophyte where cells go through meiosis to make spores. Plural: sporangia.
- gametangium
- A many-celled organ on the gametophyte that makes gametes inside a protective jacket of cells. Plural: gametangia.
- archegonium
- The female gametangium, a flask-shaped organ holding one egg. Fertilization happens inside it, and the embryo starts growing there.
- antheridium
- The male gametangium, which makes lots of sperm and releases them.
- apical meristem
- A small zone of constantly dividing cells at each root tip and shoot tip. The new cells it adds let roots and shoots keep getting longer year after year.
- cuticle
- A waxy layer over a plant's leaves and stems that keeps water in and makes it harder for fungi and bacteria to get in.
- mycorrhizae
- Partnerships between plants and soil fungi. The fungus's fine threads gather minerals for the plant and get sugars in return.
- secondary compound
- A molecule a plant makes through a side pathway that isn't needed for basic growth but helps it, for example by poisoning plant-eaters or absorbing UV light.
Check yourself: 29.1 From green algae to land plants
4 questions on 29.1 From green algae to land plants. Pick an answer to see if you got it, and why.
In a plant whose life cycle alternates between two generations, the gametophyte makes the eggs and sperm. Which process produces these gametes, and why?
A student scores four traits in land plants and three algal lineages, X, Y and Z (invented data). Trait 1 is found in land plants, X, Y and Z. Trait 2 is found in land plants and Z only. Trait 3 is found in Y only. Trait 4 is found in land plants only. Which trait is evidence about which alga is the closest relative of land plants?
Researchers compare one chloroplast gene from a moss with the same gene from four green algae. The share of positions that differ from the moss (invented data): alga P, 21%; alga Q, 9%; alga R, 17%; alga S, 24%. Only algae Q and R build the new wall of a dividing cell using a phragmoplast, as plants do. Which conclusion is best supported?
Researchers compare spores from normal moss plants with spores from a mutant that can't add sporopollenin to the spore wall. Half of each batch is kept moist, and half is held in dry air for 48 hours; then all are placed on wet growth medium. Germination (%, invented data): Spores | Kept moist | Dried 48 h Normal | 86 | 81 Mutant | 84 | 7 Which conclusion do these data best support?
0 of 4 answered
29.2 Bryophytes: life led by the gametophyte
pp. 606–610
Bryophyte groups and life cycles aren't in the current course. Only the haploid and diploid logic (Topic 5.1) and mosses' part in the carbon cycle (Topic 8.2) connect to what's tested.
In the course: Topic 5.1 Meiosis, Topic 8.2 Energy Flow Through Ecosystems, Topic 7.9 Phylogeny, Topic 3.4 Photosynthesis, Topic 2.2 Cell Size, Topic 7.2 Natural Selection (notes, videos and more questions)
Key points
- Bryophytes are the liverworts, mosses and hornworts: small plants with no well-developed network of lignified vascular tissue. 'Bryophyte' is an informal name for all three, while the phylum Bryophyta means mosses alone.
- In bryophytes the haploid gametophyte is the big, green, long-lived plant you notice. The diploid sporophyte is small, lasts a shorter time and stays attached, drawing water, minerals and sugars from the gametophyte through its foot.
- A moss spore grows into a branching green thread, the protonema, which buds into upright leafy gametophores. Rhizoids hold the plant in place, but unlike roots they do little of the absorbing.
- Bryophytes stay low because their parts are thin and they lack lignin-stiffened plumbing. Water and nutrients move only short distances by diffusion and along surfaces, and a few mosses have basic water-conducting cells.
- Sperm have flagella and need a thin layer of water to swim to the egg inside an archegonium. So bryophytes thrive in damp places, and where the male and female gametophytes are separate plants, they must grow close together.
- The sporophyte is a foot, a stalk (seta) and a capsule (sporangium) where meiosis makes spores. In most mosses, a ring of teeth around the capsule's mouth releases spores bit by bit in dry weather. Moss and hornwort sporophytes have stomata; liverworts don't.
- Update: this edition says bryophytes aren't a clade, but most recent genome studies find that they do form one, the sister group of vascular plants. Mosses also matter ecologically: they colonize bare ground and survive drying out, and peatlands, many built largely from Sphagnum, store roughly a quarter to a third of all carbon held in soils while covering only about 3% of the land. Draining or digging peat releases that carbon as CO₂.
Key terms (14)
- bryophyte
- An informal name for the liverworts, mosses and hornworts, the land plants with no well-developed network of vascular tissue.
- nonvascular plant
- A plant that lacks an extensive network of lignified tubes for moving water and food, so it stays small and close to the ground.
- liverwort
- A bryophyte whose gametophyte is either a flat, lobed sheet or a creeping stem with small leaflike flaps. Its sporophytes have no stomata.
- hornwort
- A bryophyte whose sporophyte is a long, slender horn. It splits lengthwise, top first, to let its spores out.
- moss
- A bryophyte with small leafy shoots, often growing in soft mats; its sporophyte is usually a capsule on a long stalk.
- protonema
- The branching mat of green threads that first grows from a moss spore, before the leafy shoots appear. Plural: protonemata.
- gametophore
- An upright, leafy shoot of a moss gametophyte that carries the archegonia or antheridia.
- rhizoid
- A thin, rootlike strand, either one long cell or a chain of cells, that anchors a bryophyte or fern gametophyte but isn't a true root.
- foot
- The base of a bryophyte sporophyte, sunk into the gametophyte, which absorbs food and water from it.
- seta
- The stalk of a moss or liverwort sporophyte, which carries food up to the capsule and lifts it into the air. Plural: setae.
- capsule
- The sporangium at the top of a bryophyte sporophyte, where meiosis makes the spores.
- peristome
- The ring of toothlike flaps around a moss capsule's opening. The teeth open when dry and close when damp, letting spores out a few at a time.
- stomata
- Small pores in the outer layer of a plant that open for CO₂ and O₂ exchange and close to save water. Singular: stoma.
- peat
- Thick layers of partly decayed plant matter, often mostly Sphagnum moss, that build up in waterlogged, low-oxygen wetlands called peatlands, which are often cold and acidic.
Check yourself: 29.2 Bryophytes: life led by the gametophyte
4 questions on 29.2 Bryophytes: life led by the gametophyte. Pick an answer to see if you got it, and why.
A student uses a DNA-binding stain to measure the DNA in nuclei from two parts of a moss: the green, leafy carpet and a brown stalk topped by a capsule that rises out of it. All cells are measured before they copy their DNA. In this invented data set, the leafy cells average 1.0 unit of DNA, and the stalk cells average 2.0 units. Which interpretation is correct?
Researchers expose only the green, leafy part of a moss to air containing ¹⁴CO₂, while the attached stalk and capsule are shielded from it. A few days later, much of the radioactive carbon turns up in sugars inside the capsule. What do these results best show?
In a forest, an ecologist maps a moss species whose male and female gametophytes grow as separate plants. She counts sporophytes on female patches at different distances from the nearest male patch (invented data): Distance to nearest male patch | Sporophytes per female patch (average) 0–5 cm | 38 10 cm | 11 20 cm | 2 50 cm or more | 0 Which explanation fits these data best?
A biologist dries clumps of moss until they're brittle. She rewets one set by misting only the leafy shoots, and another by standing only the rhizoid-covered bases in water while keeping the shoots dry. After 15 minutes, the misted shoots are green and soft, while the shoots standing in water are still mostly dry. What do the results suggest?
0 of 4 answered
29.3 Ferns, lycophytes and the first tall plants
pp. 610–615
Plant tissues and the seedless plant groups aren't in the current course. What carries over is reading trees (Topic 7.9), haploid and diploid stages (Topic 5.1) and how ancient forests pulled CO₂ from the air and became coal (Topic 8.2).
In the course: Topic 7.9 Phylogeny, Topic 8.2 Energy Flow Through Ecosystems, Topic 5.1 Meiosis, Topic 3.4 Photosynthesis, Topic 7.2 Natural Selection (notes, videos and more questions)
Key points
- Vascular plants show up in fossils about 425 million years ago. The early ones were small, but their sporophytes branched, fed themselves and carried several sporangia.
- In vascular plants the life cycle flips: the diploid sporophyte is the big, long-lived plant. In ferns and their seedless relatives the gametophyte is tiny but still lives on its own, and the sperm still swim, so these plants do best in damp places.
- Xylem moves water and minerals upward, mostly through long tracheid cells with lignin-hardened walls. Phloem carries sugars and other organic molecules. Stiff, lignified plumbing let plants grow tall, shade out shorter rivals and release spores higher in the wind, and the first forests appeared about 385 million years ago.
- Roots took over from rhizoids, both absorbing water and minerals and anchoring the plant. Fossils suggest roots evolved separately in lycophytes and in the line leading to ferns and seed plants.
- Lycophytes have microphylls, small leaves with one unbranched vein. Ferns and seed plants instead grow megaphylls, whose branching veins can supply a bigger photosynthetic surface. Sporophylls are leaves that bear sporangia, in clusters called sori on ferns and in cones called strobili on many lycophytes. Most seedless plants make one kind of spore (homosporous); a few, like all seed plants, make two (heterosporous).
- Living seedless vascular plants form two clades. Lycophytes include quillworts, club mosses and spike mosses. Ferns in the broad sense (also called monilophytes) include horsetails and whisk ferns too. Ferns share a more recent ancestor with seed plants than with lycophytes, so 'seedless vascular plants' is a grade: a group defined by a trait its members lack (seeds), not by shared ancestry.
- In Carboniferous swamp forests of giant lycophyte trees, horsetails and ferns, faster photosynthesis drew CO₂ out of the air and helped cool the climate. Plant matter that didn't fully decay in the waterlogged swamps became peat and then coal. Burning that coal now returns the carbon to the air.
Key terms (15)
- vascular tissue
- A plant's plumbing: long conducting cells lined up end to end that move water, minerals and sugars between roots and shoots.
- xylem
- Vascular tissue that carries water and dissolved minerals up from the roots. Its cells have walls stiffened with lignin.
- phloem
- Vascular tissue that moves sugars and other organic molecules, such as amino acids, from where they're made to where they're needed.
- tracheid
- A long, thin, lignified water-conducting cell in xylem, found in nearly all vascular plants.
- lignin
- A hard polymer that stiffens the walls of water-conducting cells, letting stems stand up against gravity and pipes resist collapse.
- root
- An underground organ of a vascular plant that anchors it and absorbs water and minerals from the soil.
- microphyll
- A small, often needle-shaped leaf with a single unbranched vein. Among living plants, only lycophytes have them.
- megaphyll
- A leaf with a branching network of veins, found in ferns and seed plants. Its veins can supply a large surface for photosynthesis.
- sporophyll
- A leaf specialized to carry sporangia.
- sorus
- A cluster of sporangia, usually on the underside of a fern leaf. Plural: sori.
- strobilus
- A cone-shaped cluster of sporophylls, found in most gymnosperms and many lycophytes. Plural: strobili.
- homosporous
- Making just one kind of spore, which usually grows into a gametophyte that makes both eggs and sperm.
- heterosporous
- Making two kinds of spores: small microspores that become male gametophytes and large megaspores that become female gametophytes.
- lycophytes
- The oldest living lineage of vascular plants, made up of quillworts, spike mosses and club mosses. All have microphylls, and despite the names, none are true mosses.
- ferns (monilophytes)
- The clade of seedless vascular plants that includes ferns, horsetails and whisk ferns. They share a more recent ancestor with seed plants than with lycophytes.
Check yourself: 29.3 Ferns, lycophytes and the first tall plants
4 questions on 29.3 Ferns, lycophytes and the first tall plants. Pick an answer to see if you got it, and why.
A lab spreads spores from the underside of a fern frond on moist agar. Weeks later the plate is covered with small, flat green plants a few millimeters wide. After the plate is flooded with a thin film of water, some of these small plants grow a tiny frond and root. Which statement about these structures is correct?
A fern's roots are placed in water containing a red dye, and one of its fronds is given ¹⁴CO₂ for an hour. Later, the dye shows up in the veins of the fronds, and sugar carrying the ¹⁴C shows up in the underground stem and roots. Which tissues most likely carried each substance?
In a fern species, a mutation sharply reduces the lignin in the walls of the water-conducting cells. Compared with normal plants grown in the same conditions, which outcome is most likely?
A paleobotanist finds a fossil plant with tracheids and no seeds. Which additional observation would most strongly suggest it belongs with the lycophytes rather than the ferns?
0 of 4 answered