Campbell Biology Chapter 35 Review: Plant Structure, Growth, and Development

35.1 Organs, tissue systems and cell types

pp. 738–745

On the AP exam? Background

Plant organs and cell types aren't tested in the current course, but they turn up as examples: cell walls and central vacuoles (Topics 2.1 and 2.4), root hairs and surface area (Topic 2.2), and leaves as the site of photosynthesis (Topic 3.4).

In the course: Topic 2.1 Cell Structure and Function, Topic 2.2 Cell Size, Topic 2.4 Membrane Permeability, Topic 2.7 Tonicity and Osmoregulation, Topic 3.4 Photosynthesis (notes, videos and more questions)

Key points

A flowering plant has two linked parts. The root system, below ground, collects water and minerals; the shoot system (stems, leaves and flowers) collects light and CO₂. Roots can't make their own food and shoots can't get their own water, so each depends on the other.
Roots hold the plant in place, absorb water and minerals, and in many plants act as food stores. Many eudicots grow one deep main root (a taproot) with branches, while grasses and most other monocots grow a shallow mat of similar-sized roots (a fibrous root system). Most uptake happens near root tips, where root hairs, each a long extension of one epidermal cell, add a huge amount of surface.
Stems hold leaves up to the light. Leaves attach at nodes, the stretches between nodes are internodes, and each leaf has an axillary bud in the angle above it that can grow into a branch. An active shoot tip tends to keep the buds near it dormant (apical dominance), so losing the tip lets side shoots grow. Many underground storage organs are really stems, which you can tell from their nodes and buds.
Leaves are usually the main site of photosynthesis. Most have a flat blade on a stalk called a petiole; monocot leaves usually have parallel veins and eudicot leaves a branching net. A compound leaf is split into leaflets, and you can tell a leaflet from a leaf because only a whole leaf has an axillary bud at its base. Some leaves are modified into tendrils, spines or storage organs.
Every organ contains three tissue systems that run continuously through the whole plant. The dermal system is the outer covering: an epidermis, usually coated with a waxy cuticle, replaced by periderm on older woody parts. The vascular system is xylem (water and minerals, moving up from the roots) and phloem (sugars, moving from sources such as leaves to sinks such as roots, fruits and growing tips). The ground system is everything else: pith inside the vascular tissue and cortex outside it.
Three basic cell types make up most tissues. Parenchyma cells are living, thin-walled, all-purpose cells that photosynthesize, store food and can divide to repair wounds. Collenchyma cells are living, with unevenly thickened walls that give bendable support to growing parts. Sclerenchyma cells (long fibers and stubby sclereids) have thick walls stiffened with lignin and are often dead at maturity, giving rigid support to parts that have stopped growing.
Xylem's water-carrying cells, tracheids and the wider vessel elements, are dead when working, leaving hollow tubes with lignified walls and thin spots called pits. Phloem's sieve-tube elements stay alive but lose their nucleus and ribosomes, so each one depends on a neighboring companion cell, linked to it by plasmodesmata.

Key terms (15)

taproot: A single dominant root that grows deep, with smaller side roots branching off it. Typical of many eudicots, like dandelions.
fibrous root system: A shallow, spreading tangle of many similar-sized roots with no main root, as in grasses and most other monocots.
root hair: A long, narrow projection from one epidermal cell near a root tip. Thousands of them multiply the surface that absorbs water and minerals.
node: A spot on a stem where one or more leaves grow out. The bare stretch of stem from one node to the next is an internode.
axillary bud: A bud tucked just above the point where a leaf joins the stem. If it becomes active, it grows into a side shoot.
apical dominance: The way an actively growing shoot tip keeps the axillary buds near it from growing. Remove the tip and side branches start growing.
petiole: The stalk that attaches a leaf blade to the stem.
dermal tissue system: The skin-like layer around the whole plant: epidermis on young parts, replaced by periderm on older woody parts.
cuticle: A film of wax and fatty polymers coating the epidermis of leaves and young stems. It cuts down evaporation.
vascular tissue system: The xylem and phloem that carry water, minerals and sugars between roots and shoots.
ground tissue system: Everything in a plant organ besides its skin and its plumbing, such as pith and cortex. Its jobs include making and storing food and holding the organ up.
parenchyma cell: A living, thin-walled, general-purpose plant cell with a big central vacuole. Parenchyma cells photosynthesize, store food and can divide to heal wounds.
collenchyma cell: A living support cell whose primary walls are thick in patches, so young stems and leaf stalks can bend and keep growing.
sclerenchyma cell: A support cell with a thick wall hardened by lignin, often dead at maturity. Fibers and sclereids are the two kinds.
sieve-tube element: A living phloem cell with no nucleus that carries sugary sap. It depends on a companion cell beside it.

Check yourself: 35.1 Organs, tissue systems and cell types

4 questions on 35.1 Organs, tissue systems and cell types. Pick an answer to see if you got it, and why.

Question 1 of 4

A student digs up a swollen underground storage organ and wants to know whether it is a modified stem or a modified root. Which observation would show most clearly that it is a stem?

Question 2 of 4

On a twig, a student finds a single stalk carrying seven flat green blades. There is a bud where the stalk joins the twig, but no bud at the base of any of the seven blades. How should this be described?

Question 3 of 4

Cells in the storage tissue of a potato are alive, have thin primary walls with no secondary wall, hold a large vacuole and are packed with starch-filled plastids. Which cell type are they?

Question 4 of 4

Which feature is shared by tracheids and vessel elements but NOT by sieve-tube elements?

0 of 4 answered

35.2 Meristems: where new cells come from

pp. 746–747

On the AP exam? Not tested

Meristems aren't part of the current course. What carries over is mitosis making new cells (Topic 4.5) and stem cells that stay unspecialized while their descendants specialize (Topic 6.6).

In the course: Topic 4.5 Cell Cycle, Topic 6.6 Gene Expression and Cell Specialization (notes, videos and more questions)

Key points

Most animals stop growing once they reach adult size, which is determinate growth. Plants keep growing for as long as they live, which is indeterminate growth. A few plant organs are determinate too, like leaves and flowers, which stop at a set size.
Plants can do this because they keep meristems: pockets of unspecialized, dividing cells that make new cells for as long as the plant lives.
Apical meristems sit at the tip of every root and shoot and inside axillary buds. Their new cells make the plant longer (primary growth), pushing roots into fresh soil and lifting shoots toward the light. Nonwoody (herbaceous) plants are made almost entirely this way.
Woody plants add two lateral meristems, thin sleeves of dividing cells that run the length of stems and roots. One, the vascular cambium, builds wood (secondary xylem) and secondary phloem; the other, the cork cambium, builds periderm, the corky layer that takes over from the epidermis. Together they make the plant thicker, which is secondary growth.
Each time an initial (stem cell) in a meristem divides, one daughter usually remains an initial, so the meristem never runs out. The other daughter becomes a derivative: it moves out of the meristem, may divide a few more times, and then specializes. Animal stem cells work in a similar way.
A plant always has organs of many ages at once: brand-new leaves at its tips, mature leaves farther back and, in woody plants, wood laid down years ago.
Plants are often grouped by life span. Annuals go from seed to seed and die within one year, biennials take two growing seasons and flower in the second, and perennials live for many years.

Key terms (15)

meristem: A region of unspecialized plant cells that keep dividing, making the new cells that let a plant grow.
indeterminate growth: Growth that continues throughout an organism's life, as in plant shoots and roots.
determinate growth: Growth that ends when an organ or organism reaches a fixed final size, as in most animals and in plant leaves and flowers.
apical meristem: A meristem at the tip of a root or shoot, or in a bud, that makes the plant longer.
primary growth: Growth in length, made by apical meristems. It produces all of a nonwoody plant's body.
lateral meristem: A thin sleeve of dividing cells along a woody stem or root that adds to its thickness. The vascular cambium and cork cambium are the two kinds.
secondary growth: Growth in thickness, made by lateral meristems in woody plants.
vascular cambium: A lateral meristem that adds secondary xylem (wood) to its inside and secondary phloem to its outside.
cork cambium: A lateral meristem close to the surface of woody stems and roots. It builds the protective periderm.
initial (stem cell): A meristem cell that stays behind each time it divides, keeping the meristem supplied with new cells for years.
derivative: A cell pushed out of a meristem that may divide a little more before specializing into part of a tissue.
herbaceous plant: A plant with soft, green stems that doesn't make wood, so its body comes almost entirely from primary growth.
annual: A plant that sprouts, flowers, makes seeds and dies within one year.
biennial: A plant that needs two growing seasons to finish its life cycle, usually flowering only in the second.
perennial: A plant that lives for many years, such as a tree, a shrub or many grasses.

Check yourself: 35.2 Meristems: where new cells come from

4 questions on 35.2 Meristems: where new cells come from. Pick an answer to see if you got it, and why.

Question 1 of 4

Which of these shows indeterminate growth?

Question 2 of 4

Which tissue is made by a lateral meristem?

Question 3 of 4

In a meristem, a cell divides by mitosis. One daughter cell stays in place and keeps dividing for years. The other is pushed aside, divides a few more times and its descendants become xylem cells. What are these two daughter cells called?

Question 4 of 4

In its first year, a plant grows a low cluster of leaves and a thick storage root. It survives the winter, then in its second summer it sends up a flowering stalk, makes seeds and dies. How is this plant classified?

0 of 4 answered

35.3 Primary growth: building roots, stems and leaves

pp. 747–751

On the AP exam? Background

Root and leaf anatomy isn't tested on its own, but it backs up tested ideas: root hairs and surface area (Topic 2.2), stomata and water movement (Topic 2.7), and leaf structure for capturing light and CO₂ (Topic 3.4).

In the course: Topic 2.2 Cell Size, Topic 2.7 Tonicity and Osmoregulation, Topic 3.4 Photosynthesis, Topic 6.6 Gene Expression and Cell Specialization (notes, videos and more questions)

Key points

A root tip is protected by a root cap, which shields the apical meristem and releases slippery slime that eases its way through soil. Behind it are three overlapping zones: cell division, where new cells are made; elongation, where the young cells lengthen many-fold and drive the tip deeper (most of the root's lengthening happens here); and differentiation (or maturation), where cells finish specializing and root hairs appear.
Inside a young root, water and minerals pass from the epidermis through the cortex, mostly storage parenchyma. The innermost cortex layer, the endodermis, is a one-cell-thick checkpoint for what enters the central vascular cylinder (the stele). A typical eudicot root has a star of xylem in its center, with phloem tucked between the star's arms; many monocot roots have a ring of xylem and phloem around a central pith.
New side roots start deep inside, in the pericycle (the ring of cells at the edge of the vascular cylinder), and have to break out through the tissues around it. That keeps their xylem and phloem joined to the parent root's from the start. Stems branch differently, from buds sitting on the surface.
The tip of every shoot holds a tiny, rounded shoot apical meristem. New leaves appear as bumps (leaf primordia) along its sides, and the stem gets longer mostly because the internodes beneath it stretch. Grasses also have intercalary meristems at the bases of their leaves and stems, so they can regrow after their tips are cut or grazed.
In a young stem, xylem and phloem run in vascular bundles. Eudicot stems usually have the bundles in a ring, with xylem facing the pith and phloem facing the cortex. Monocot stems have bundles scattered all through the ground tissue, so there's no separate pith and cortex.
A leaf is a thin sandwich. Its epidermis has stomata, pores opened and closed by pairs of guard cells, where CO₂ enters and water vapor escapes. Inside is the mesophyll: tightly packed, chloroplast-rich palisade cells near the top and loosely arranged spongy cells with air spaces below. Branching veins bring water in and carry sugars away, and a sleeve of bundle-sheath cells surrounds each vein (in C₄ plants these cells do special photosynthetic work).

Key terms (15)

root cap: A cap of loosely attached cells covering the root tip. Its cells wear away and are replaced as the root pushes on, shielding the meristem and releasing slippery slime.
zone of cell division: The region at a root tip, just behind the root cap, where the apical meristem makes new cells.
zone of elongation: The part of a growing root just behind the dividing cells, where cells lengthen dramatically. It supplies most of the root's increase in length.
zone of differentiation: The region behind the elongation zone where root cells finish specializing and root hairs form. Also called the zone of maturation.
cortex: Ground tissue lying outside the vascular tissue and just inside the epidermis of roots and stems. It often stores food.
endodermis: A single ring of cells wrapped around a root's vascular cylinder, at the inner edge of the cortex. It screens which dissolved substances get into the xylem.
pericycle: A thin sheath of cells forming the outside edge of a root's vascular cylinder, right against the endodermis. Side roots are born from it.
stele: The name for all the vascular tissue in a root or stem taken together. In a root it forms one central core, called the vascular cylinder.
leaf primordium: A small bump on the side of a shoot apical meristem that develops into a leaf. Plural: primordia.
intercalary meristem: A meristem at the base of a grass leaf or stem segment. It lets grasses regrow after their tips are cut or eaten.
vascular bundle: A strand of xylem and phloem running through a stem or leaf. Bundles form a ring in eudicot stems and are scattered in monocot stems.
pith: Ground tissue in the center of a stem, inside the ring of vascular bundles.
stomata: Pores in the leaf epidermis that let CO₂ in and O₂ and water vapor out. Singular: stoma.
guard cells: The pair of cells around each stoma that change shape to open or close it.
mesophyll: A leaf's inner photosynthetic tissue: packed palisade cells near the top and loose, airy spongy cells below.

Check yourself: 35.3 Primary growth: building roots, stems and leaves

4 questions on 35.3 Primary growth: building roots, stems and leaves. Pick an answer to see if you got it, and why.

Question 1 of 4Calculator allowed

A student marks a young bean root with ink lines 1.0 mm apart, starting at the root tip, and measures the spaces again 24 hours later (invented data). Space between marks | Length after 24 h (mm) 0–1 mm from tip | 1.2 1–2 mm | 4.5 2–3 mm | 3.1 3–4 mm | 1.1 4–5 mm | 1.0 Which conclusion is best supported?

Question 2 of 4

Along a growing root, root hairs first appear in which region, and why there?

Question 3 of 4

In a cross section of an older root, a student spots a young lateral root that hasn't reached the soil yet. Its xylem and phloem are joined to the parent root's vascular cylinder, and crushed cortex cells surround its tip. Which description fits?

Question 4 of 4

A cross section shows a solid central core of xylem shaped like a four-armed star, with phloem filling the gaps between the arms. Around this core is a one-cell-thick ring of cells, then a wide zone of starch-filled parenchyma, then a single outer layer of cells, some with long thin outgrowths. What is the section most likely from?

0 of 4 answered

35.4 Secondary growth: wood and bark

pp. 751–755

On the AP exam? Not tested

Wood and bark aren't in the current course. Useful links: ring widths that change with rainfall or temperature are an environmental effect on phenotype (Topic 5.5), and wood is a long-term store of carbon taken from the air (Topic 8.2).

In the course: Topic 5.5 Environmental Effects on Phenotype, Topic 8.2 Energy Flow Through Ecosystems, Topic 4.5 Cell Cycle (notes, videos and more questions)

Key points

Secondary growth makes woody stems and roots thicker. It's found in conifers and many eudicots but rarely in monocots. It happens in older parts while primary growth goes on at the tips.
The vascular cambium is a sleeve of dividing cells, frequently a single cell layer, sitting between the wood and the bark. It adds secondary xylem to its inside and secondary phloem to its outside, much more xylem than phloem. Some of its cells divide to widen the cambium itself as the trunk grows.
Other cambium cells build vascular rays: ribbons of living parenchyma running from the center outward like wheel spokes. Rays are the trunk's sideways highways between wood and inner bark, and they also hold starch reserves and help seal off injuries.
Wood is secondary xylem. In places with seasons, the cambium rests in winter. Early wood made in spring has wide, thin-walled cells that move lots of water; late wood made later is denser and stronger. The contrast makes one visible ring per year, and wide rings usually mean good growing years. Reading rings to date wood or study past climates is called dendrochronology.
Only the newest outer wood, the sapwood, still carries water. The older center, the heartwood, no longer conducts and is often darkened by compounds that resist rot and insects. The inner bark works the same way: just the most recent layer of secondary phloem moves sugar, and older layers get crushed and eventually flake away.
As the stem widens, the epidermis splits and is replaced by periderm, made by the cork cambium. Cork cells are pushed outward, lay down waxy suberin and die, making a waterproof, germ-resistant layer; a thin layer of phelloderm forms inward. New cork cambiums keep forming deeper inside. Lenticels, spots of loosely packed cork, let the living tissues inside exchange gases.
Bark is everything outside the vascular cambium: secondary phloem plus all the layers of periderm. Because periderm blocks water, most uptake by roots happens at their young tips.

Key terms (15)

secondary xylem: Xylem made by the vascular cambium, building up year after year as wood.
secondary phloem: Phloem made by the vascular cambium on its outer side. Only the youngest layer carries sugar.
vascular ray: A spoke-like band of living parenchyma crossing the wood and inner bark. It ferries water and nutrients sideways, stores starch and helps wall off damage.
early wood: Wood laid down in spring, built of wide cells with thin walls that pass water easily. Also called spring wood.
late wood: Wood made later in the growing season, with narrow, thick-walled cells that add strength. Also called summer wood.
growth ring: One year's layer of wood, seen as a ring in a cross section. Its width reflects that year's growing conditions.
dendrochronology: The study of tree-ring patterns to date wood and to learn about past climates.
heartwood: The older, central wood of a trunk that no longer carries water and is often darker.
sapwood: The younger, outer wood of a trunk that still carries water and minerals.
cork cells: Cells made on the outer side of the cork cambium. They fill their walls with suberin, then die, leaving a protective layer.
suberin: A fatty, wax-like polymer in the walls of cork cells that keeps water from passing through.
phelloderm: Living parenchyma cells that the cork cambium adds on its inner side, usually only a few cell layers thick.
periderm: The protective covering that replaces the epidermis on woody stems and roots: cork cambium plus the cork and phelloderm it makes.
lenticel: A pore-like patch in bark where cork cells are loosely arranged, so the living tissue inside can still exchange gases with the air.
bark: Every tissue outside the vascular cambium: secondary phloem plus all the layers of periderm.

Check yourself: 35.4 Secondary growth: wood and bark

4 questions on 35.4 Secondary growth: wood and bark. Pick an answer to see if you got it, and why.

Question 1 of 4

In the trunk of a 50-year-old tree, where is the youngest secondary xylem?

Question 2 of 4

A researcher measures the width of growth rings in a core from a tree in a dry valley and compares them with rainfall records for each growing season (invented data). Year | Ring width (mm) | Growing-season rainfall (mm) 2016 | 2.6 | 410 2017 | 3.2 | 450 2018 | 1.3 | 190 2019 | 2.4 | 390 2020 | 3.0 | 430 2021 | 1.0 | 160 2022 | 2.9 | 400 Which conclusion do these data best support?

Question 3 of 4

Why can you usually see a sharp line between one growth ring and the next in a temperate tree?

Question 4 of 4

A section of a maple trunk is stood upright in water containing a red dye. After an hour, the dye has moved up only through the outer 6 rings of wood. The inner rings are darker in color and stay unstained. What best explains this?

0 of 4 answered

35.5 How genes and position shape a plant

pp. 755–761

On the AP exam? Yes

Topic 6.6 tests the main idea (cells with the same genes specialize by expressing different ones), and Topic 5.5 tests how environment shapes phenotype. The ABC model, microfibrils and Arabidopsis are background you won't be asked to name.

In the course: Topic 6.6 Gene Expression and Cell Specialization, Topic 6.5 Regulation of Gene Expression, Topic 5.5 Environmental Effects on Phenotype, Topic 4.1 Cell Communication, Topic 2.7 Tonicity and Osmoregulation (notes, videos and more questions)

Key points

Development has three overlapping parts. Growth means getting permanently bigger; morphogenesis is the shaping of tissues and organs and the placing of cell types within them; and cell differentiation is cells with identical genomes becoming different kinds of cells. Plants are also unusually flexible: one genotype can build differently shaped organs in different surroundings (developmental plasticity), for example thick sun leaves and thin shade leaves on one tree.
Arabidopsis thaliana, a small, fast-growing relative of mustard and cabbage, is plant biology's main model organism. It goes from seed to seed in about six weeks, makes thousands of seeds, has a small genome (about 27,000 protein-coding genes on 5 chromosome pairs) that was the first plant genome sequenced, and is easy to modify. Inserting DNA at random with Agrobacterium can break genes, and studying the resulting knockout mutants shows what each gene does.
Cell division adds cells, but a plant grows mainly as cells expand. A ring of microtubules, the preprophase band, marks where a cell will divide. Unequal divisions, in which one daughter gets more cytoplasm, often give daughter cells different fates and help set up polarity, the difference between a plant's root end and its shoot end.
Plant cells expand mostly by taking in water, which fills the central vacuole; that costs far less than building new protein-rich cytoplasm, so plants can grow fast. The direction of growth is set by the wall: cellulose microfibrils barely stretch, so a cell expands at right angles to them, and microtubules just inside the membrane guide where new microfibrils are laid.
In plants, a cell's fate depends mainly on its position and on signals from its neighbors, rather than on its ancestry. Every cell keeps the full genome, so differentiation comes from switching different genes on and off. Animals use position cues too, but cell ancestry and master genes such as Hox genes play a bigger part in laying out their bodies; plants place their floral organs with a different family of transcription factors, the MADS-box genes.
A shoot meristem goes through phase changes, from juvenile to adult vegetative to reproductive, often marked by a change in leaf shape. Each node keeps the phase it formed in. Floral meristem identity genes switch a shoot tip from indeterminate growth to a determinate flower.
Organ identity genes, which encode MADS-box transcription factors, decide what each of a flower's four rings (whorls) becomes. In the ABC model, A alone makes sepals, A + B petals, B + C stamens and C alone carpels, and A and C repress each other. Later work added E genes, which the others need in order to work; without them, the flower's organs develop as sepal-like or leafy structures.

Key terms (15)

growth: A permanent gain in size. In plants it comes mostly from cells taking in water and expanding.
morphogenesis: The shaping of a developing tissue, organ or body, including where each cell type ends up.
cell differentiation: The process that turns genetically identical cells into specialists with different structures and jobs.
developmental plasticity: The ability to change form in response to the environment, like a tree growing thick sun leaves and thin shade leaves.
model organism: A species that is easy to grow and study, used to learn principles that apply widely. Arabidopsis is plant biology's main one.
preprophase band: A ring of microtubules that forms just before mitosis and marks where a plant cell's new wall will form.
asymmetrical cell division: A cell division that splits the cytoplasm unevenly, so the two daughters start out different and often follow different paths.
polarity: A built-in difference between the two ends of a cell or body, such as a plant's shoot end versus its root end.
cellulose microfibril: A bundle of cellulose chains in the cell wall. A growing cell expands at right angles to its microfibrils.
pattern formation: The process that puts each kind of tissue or organ in its proper place, for example vascular tissue inside a leaf and sepals on the outside of a flower.
positional information: Signals that tell a cell where it is in a developing organ, which help decide what it becomes.
phase change: A shift in what a shoot meristem makes, such as from juvenile leaves to adult leaves or from leaves to flowers.
floral meristem identity gene: A gene whose product switches a shoot tip from making leaves and stem to making a flower.
organ identity gene: A gene, usually from the MADS-box family, that decides which kind of floral organ (sepal, petal, stamen or carpel) a region of a flower becomes.
ABC model: A model in which three classes of genes (A, B and C), switched on in overlapping rings, decide which floral organ forms where.

Check yourself: 35.5 How genes and position shape a plant

4 questions on 35.5 How genes and position shape a plant. Pick an answer to see if you got it, and why.

Question 1 of 4

Question 2 of 4

Question 3 of 4

Segments cut from the elongation zone of young roots were floated for 6 hours in solutions of mannitol, a sugar alcohol that root cells barely take up or use (invented data). Mannitol (M) | Length increase (%) 0 | 18 0.1 | 11 0.2 | 4 0.3 | 0 When segments from the 0.3 M solution were moved into plain water, they started lengthening again. Which explanation fits these results best?

Question 4 of 4

In a shoot tip, cells of the outermost layer normally become epidermis, and cells of the layer beneath become mesophyll. Now and then, an outer-layer cell divides in an unusual plane so that one daughter ends up in the layer below. Its descendants develop into mesophyll like their new neighbors. What does this suggest?

0 of 4 answered