Organismal Biology · Unit 6: How plants work

Plant Body and Growth

How is a plant built, and how does it keep growing for as long as it lives? This lesson sorts the plant body into organs, tissue systems and cell types, follows growth in length from the apical meristems and growth in girth from the cambia, and reads a leaf as an organ that captures light and trades water for carbon dioxide. Afterwards you can name the tissue a cell belongs to, say which meristem made it, calculate how division and elongation each contribute to root growth, and explain why an oak gets wood and maize does not.

Watch first A Root Grows by Cells Times Length · 2:37

A 300-year-old oak still has shoot tips that formed this spring, and a ring of wood laid down this summer. No adult mammal grows a new limb each year, but a tree keeps adding leaves, branches, roots and wood for as long as it lives. To see how, you need two things: a map of what a plant is made of, and an account of where its new cells come from.

Organs, tissue systems and the cells that build them

A flowering plant has three kinds of organ, a structure made of several tissues that works as a unit. The root anchors the plant and absorbs water and minerals. The stem holds up leaves, buds and flowers and connects them to the roots. The leaf is the main site of photosynthesis in most plants.

Each organ is built from the same three tissue systems, sets of tissues that run continuously through the whole plant. The dermal system covers and protects it (the epidermis of a young plant). The vascular system carries water and dissolved material. The ground system is everything else: it stores, supports and, in green parts, photosynthesizes.

Below that sit the cells. Keep three levels apart: a cell type is one kind of cell, a tissue is a group of cells working together, and a tissue system is a plant-wide set of tissues. Think of bricks, walls and the house’s whole plumbing.

  • Parenchyma cells are living, thin-walled general workers. Some photosynthesize and some store starch.
  • Collenchyma cells are living, with unevenly thickened walls, and give flexible support to young stems and leaves (the strings in celery are mostly collenchyma).
  • Sclerenchyma cells have thick secondary walls hardened with lignin, a tough polymer. They come as long fibers and as shorter sclereids (the grit in a pear).

Xylem and phloem are not cell types. Each is a complex tissue made of several. In xylem, water moves through tracheids and vessel elements, which are dead at maturity: their walls form open pipes. In flowering plants, phloem carries sugars and amino acids through sieve-tube elements, which stay alive at maturity although they lose their nucleus, each supported by a neighbouring companion cell. How water and sugar actually move is the subject of plant transport.

Growing from the tips: apical meristems, division and elongation

New cells come from meristems, regions of cells that keep dividing. Apical meristems sit at the tip of every root and shoot and produce primary growth, growth in length. (For the cell division itself, see mitosis.)

Primary growth is not division alone. Behind a root tip, under a protective root cap, you can see three zones. In the zone of division, small cells divide. In the zone of elongation, they stop dividing and lengthen many times over as water fills their vacuoles. In the zone of maturation, they become their final types, and root hairs appear on the epidermis. A shoot tip works the same way, adding leaves at nodes, with stem internodes between them and a bud in the angle of each leaf.

That is the big contrast with animals. Most animals lay down their body plan early in development (see animal development), though many keep growing in size. A plant keeps its meristems for life and adds whole new organs indefinitely.

Do the cells in an apex mix as they divide? Sophia Satina, Albert Blakeslee and Amos Avery tested this in the Jimson weed, Datura stramonium, and published the answer in 1940. They put colchicine, a drug that blocks chromosome separation and so doubles the chromosome number, into Datura seeds by soaking them. Some shoots became periclinal chimeras: one whole cell layer of the apex was polyploid while the others kept the normal number. Polyploid cells are larger, so a layer’s descendants could be traced by cell size in sections. If apex cells mixed freely, polyploid cells should turn up scattered at random. Instead the team found that each layer responded to colchicine independently, which showed three stable layers. Later work assigns them their fates: L1 the epidermis, L2 most of the tissue under it, and L3 the inner core of the stem and its vascular tissue. That rules in clonally separate layers that persist through primary growth in Datura. It does not show three layers in every plant (many grasses have two), and neighbouring cells occasionally invade a layer.

Worked problem. These numbers are illustrative, not a measured species. Assume steady state and that every cell file (a lengthwise row of cells) in the root behaves alike.

root growth rate = rate of new cells per file x final cell length

  1. One cortical cell file delivers 1 new cell per hour from the meristem. Cells leave the meristem 15 micrometers long and mature at 300 micrometers. Growth = 1 cell per hour x 300 micrometers = 300 micrometers per hour. Over 24 hours that is 7,200 micrometers, or 7.2 mm per day.
  2. Now block elongation but leave division alone. Cells stay 15 micrometers long: 1 x 15 = 15 micrometers per hour, or 0.36 mm per day. That is one twentieth of normal, so elongation supplies most of the length.
  3. Instead, halve division with normal elongation: 0.5 x 300 = 150 micrometers per hour, or 3.6 mm per day.
Root growth = cells made x final cell length⚙ original · interactive
Growth per hour
300 µm per h
Growth per day
7.2 mm per day

In this sketch: illustrative numbers, not a measured species; steady state; every cell file alike; zones not to scale. Primary growth is division plus elongation, and growth = rate of new cells per file x final cell length.

Growing wider: cambium, wood and bark

Many seed plants also grow in girth, which is secondary growth, from two lateral meristems, thin cylinders of dividing cells that run along the stem and root.

The vascular cambium lies between xylem and phloem. In plants with conventional secondary growth, which include most woody eudicots, it adds secondary xylem toward the inside and secondary phloem toward the outside. Secondary xylem is wood. In an oak (Quercus) or a poplar (Populus) in a temperate climate, the cambium works in spring and summer and rests in winter. Spring wood has wider cells than late-summer wood, so each year leaves one visible annual ring.

As the trunk widens, the epidermis splits. The cork cambium forms beneath it and makes cork (phellem), with waterproofed walls, toward the outside and living phelloderm toward the inside. Together these three layers are the periderm, which replaces the epidermis. Bark is everything outside the vascular cambium: the periderm and the secondary phloem. Bark is not only dead cork. Its inner part is living phloem.

AI-generated illustration: a flat radial strip of a woody trunk from centre to surface showing wood, a thin vascular cambium, secondary phloem, phelloderm, cork cambium and cork. A bracket marks bark as everything outside the vascular cambium, and an inset shows wide spring cells and narrower late-summer cells in one annual ring.
From the centre of an oak trunk outward: wood, the vascular cambium, secondary phloem, then the periderm of phelloderm, cork cambium and cork. Bark is everything outside the vascular cambium, so it includes living phloem. Each year leaves one annual ring.

Worked problem, part 4. Suppose a cambium adds a ring 2 mm wide in one year. On a trunk of radius 200 mm the new wood in cross-section is pi x (202 squared - 200 squared) = pi x 804, about 2,530 square millimeters. On a sapling of radius 20 mm it is pi x (22 squared - 20 squared) = pi x 84, about 264 square millimeters. The same ring width gives about 9.6 times more wood on the big trunk, because area added scales with radius.

A leaf as a gas-exchange and light-capture organ

A leaf is a thin, flat organ. Photosynthesis in it needs light, water and carbon dioxide, and the anatomy reflects all three. In a typical eudicot leaf:

  • A waxy cuticle over the epidermis reduces water loss.
  • Stomata (singular stoma) are pores, each between a pair of guard cells that open and close it. In many eudicots most are on the lower surface.
  • Under the upper epidermis, palisade mesophyll is a layer of tightly packed column-shaped cells full of chloroplasts.
  • Below it, spongy mesophyll has loosely arranged cells, also with chloroplasts, separated by air spaces that connect to the stomata.
  • Veins carry xylem and phloem through the blade.

An open stoma lets carbon dioxide in and water vapour out, so every gain of carbon costs water. That trade-off is in organism problems, and the large internal surface of the mesophyll in size and scaling. Oxygen leaves through the same pores as a by-product of splitting water in the light reactions (light reactions).

AI-generated illustration: schematic cross-section of a eudicot leaf with a cuticle over the upper epidermis, tightly packed palisade cells, loosely arranged spongy cells with air spaces, a vein with xylem and phloem, and stomata with guard cells in the lower epidermis. An inset shows a stoma from below.
A typical eudicot leaf: a waxy cuticle over the epidermis, palisade mesophyll under the upper surface, spongy mesophyll with air spaces connecting to the stomata, and veins carrying xylem and phloem. An open stoma lets carbon dioxide in and water vapour out.

Maize (Zea mays) leaves look different. Each vein is wrapped in a ring of large bundle-sheath cells, with mesophyll cells around them, a wreath pattern called Kranz anatomy. In this C4 arrangement the mesophyll captures carbon first and the bundle-sheath cells, rich in the enzyme Rubisco, fix it into sugar (Calvin cycle has the chemistry). Kranz anatomy is not required for C4: Bienertia sinuspersici separates the two steps within single cells.

Four bodies, one toolkit: herb, tree, grass, cactus

The same organs, tissue systems and meristems build very different bodies.

Arabidopsis thaliana, a small herbaceous eudicot, grows mostly by primary growth, though its hypocotyl (the stem below the seed leaves) forms a cambium.

Oak or poplar adds wood every year and replaces its epidermis with periderm.

Maize is a monocot. Its vascular bundles are scattered through the stem and its leaf veins run parallel. It has no conventional vascular cambium, so it makes no wood: it gets taller from its shoot apex and from meristems at the bases of its leaves and internodes.

A prickly pear cactus (Opuntia) photosynthesizes in its flattened green stems. Its spines are modified leaves. It uses CAM, opening its stomata mainly at night (chemistry in Calvin cycle).

None of these is more advanced than the others; each lineage has had the same time to evolve. The life cycles behind them are in plant diversity and life cycles, and how they respond to their surroundings is in plant signals.

Design problem

Shi, Lebovka, Lopez-Salmeron, Sanchez and Greb (Development, 2019) switched on a permanent fluorescent mark in a few scattered cells of the Arabidopsis hypocotyl cambium and followed their descendants for 10 days. Of 99 marked clones, 35 spanned xylem, cambium and phloem. Does this show that one stem cell makes both xylem and phloem? What would you check before you believed it, and what can you not conclude?

One way to do it

Start with the method. A permanent lineage mark is inherited by every descendant of the marked cell, so a clone that reaches both xylem and phloem means one cell’s lineage made both. A pulse label such as EdU, which dilutes as cells divide, could not show that.

Next, the main alternative: two neighbouring cells marked at once, one feeding xylem and one phloem, would look like a single bifacial clone. Controls: keep induction sparse enough that marked cells are rarely adjacent, count how often two marks appear close together in short-term samples, and check that the reporter is off without the inducer.

Then the number: 35 of 99 is 35.35 percent, about 35 percent. That is the fraction of this sample of clones, not the fraction of cambium cells that are stem cells, because a clone’s type depends on which cell happened to be marked and on how long it was followed.

Finally, scope. This is the Arabidopsis hypocotyl over 10 days, not the cambium of an oak. The data support a bifacial stem cell, which is what the authors concluded, but they do not say whether each daughter’s fate is set by an asymmetry at division or by its position afterward.

How we measure it

Induced polyploid periclinal chimeras

Treat seeds or a growing shoot apex with colchicine so that cells in one layer double their chromosome number, then follow that layer's descendants by their larger cells, which can be told apart by size. It shows which apical layer gives rise to which tissues, in the species tested.

Inducible genetic lineage tracing

Use a drug-inducible switch, such as Cre recombinase acting on a reporter gene, to turn on a permanent fluorescent mark in a few scattered cells. Because every descendant inherits the mark, the clone that forms shows which tissues one cell's lineage produced.

EdU pulse labeling

Supply the thymidine analogue EdU for a short period so that cells copying their DNA take it up, then detect it in sections. It shows where cells were dividing during the pulse, but the label dilutes as cells keep dividing, so it is a snapshot rather than a lineage.

Sectioning and light or confocal microscopy

Cut thin cross-sections of root, stem or leaf, stain or image them, and identify cells by wall thickness, shape, contents and position. This is how tissue systems, cambia and annual rings are recognized.

Finish line

Test yourself

8 questions

Practice and MCAT-style, with explained answers.

Next · Lesson 23

Moving Water and Sugar in Plants

A coast redwood lifts water more than 100 m with no pump at all. Where does the force come from?

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Katherine Esau

Katherine Esau

Studied how phloem and other plant tissues develop from meristems, and wrote the textbooks Plant Anatomy (1953) and Anatomy of Seed Plants that taught plant structure as a developmental sequence from dividing cells to mature tissues.

Katherine Esau was born in 1898 in the Russian Empire, studied agriculture in Germany, and settled in California in the 1920s. Her early research was on the sugar beet and a viral disease of it called curly top. Her work turned to how the virus is transmitted and how it alters the phloem, so understanding the disease meant understanding the phloem, and that question held her for decades. Working at the University of California, Davis, she followed sieve-tube elements from their origin in the developing tissue to maturity, when they lose their nucleus but stay alive, fed by neighbouring companion cells. She also described what viruses do to those cells. In 1953 she published Plant Anatomy, a book that explained each tissue by tracing how it develops, and later a shorter Anatomy of Seed Plants for students. She was elected to the National Academy of Sciences in 1957. Her books gave plant biologists a shared way to read a cross-section: find the meristem, then follow its products outward and inward, which is the approach this lesson uses.

✓ verbatim Esau, Plant Anatomy (John Wiley and Sons, New York, 1953); Ray F. Evert, Katherine Esau, Biographical Memoirs of the National Academy of Sciences.
Sophia Satina

Sophia Satina

With Albert Blakeslee and Amos Avery, made periclinal chimeras in Datura shoot apices with colchicine and used them to show, in 1940, that the Datura shoot apex is built of three independently behaving cell layers; this and later papers traced the tissues each gives rise to.

Sophia Satina trained as a biologist in Moscow, left Russia in the early 1920s, and joined Albert Blakeslee's group studying the Jimson weed, Datura stramonium. By the late 1930s the group could double chromosome numbers in Datura with colchicine. Satina saw that this tool could answer a question about the shoot tip. Do the cells of the apex mix as they divide, or do they stay in separate layers? When colchicine reached only part of the shoot apex, some shoots became periclinal chimeras: one layer of cells polyploid, the others with the normal chromosome number. Polyploid cells are larger, so a layer's descendants could be followed by cell size in sections. Satina and her co-authors found, in this and later papers, three stable layers, each with its own fate. Layer-tracing with chimeras became a standard way to ask which cells of a meristem build which tissues.

✓ verbatim Satina, Blakeslee and Avery, Demonstration of the three germ layers in the shoot apex of Datura by means of induced polyploidy in periclinal chimeras, American Journal of Botany 27:895-905 (1940).
Albert F. Blakeslee

Albert F. Blakeslee

Built Datura stramonium into an experimental plant for chromosome studies and, with Amos Avery, reported in 1937 that colchicine doubles chromosome number in plants, the tool behind the 1940 chimera experiment.

Albert Blakeslee began as a mycologist. Early in his career he showed that some bread moulds of the genus Mucor come in two mating types, so that a culture forms sexual spores only when the two types meet. He later moved into plant genetics with the Carnegie Institution's laboratory at Cold Spring Harbor, New York, and chose the Jimson weed, Datura stramonium, as his plant. Datura grows quickly and its seed capsules change shape in visible ways when a plant carries an extra chromosome, so his group could link whole chromosomes to traits in the field. In 1937 he and Amos Avery reported that colchicine, an alkaloid from the autumn crocus, could double the chromosome number of plant cells. Breeders took up the method for making polyploid crops. Within three years the same drug, applied to Datura, gave Satina, Blakeslee and Avery the marked cell layers that showed how the Datura shoot apex is organized.

✓ verbatim Blakeslee and Avery, Methods of inducing doubling of chromosomes in plants by treatment with colchicine, Journal of Heredity 28:393-411 (1937); Satina, Blakeslee and Avery, American Journal of Botany 27:895-905 (1940).
Thomas Greb

Thomas Greb

His laboratory, with Dongbo Shi as first author, combined pulse labeling and inducible lineage tracing in the Arabidopsis hypocotyl and found evidence for a single bifacial cambium stem cell that produces both xylem and phloem.

The vascular cambium sits between wood and bark, adding xylem on one side and phloem on the other. A long-standing question is whether one kind of cell does both jobs or whether separate cells feed each side. The laboratory of Thomas Greb addressed it in the hypocotyl, the short stem below the seed leaves, of Arabidopsis thaliana, which makes a cambium even though the plant is a small herb. Dongbo Shi and colleagues used two methods. A pulse of the DNA label EdU marked cells that copied their DNA during the labeling period. An inducible genetic switch turned on a fluorescent reporter in a few scattered cells and in all of their descendants. After 10 days, 35 of 99 marked clones spanned xylem, cambium and phloem. The authors concluded that their data support a bifacial stem cell, and that central stem cells divided relatively rapidly compared with tissue-specific progenitors. Whether each division is intrinsically asymmetric they left open. The work gives a cellular picture of how wood and inner bark arise from one thin layer.

✓ verbatim Shi, Lebovka, Lopez-Salmeron, Sanchez and Greb, Bifacial cambium stem cells generate xylem and phloem during radial plant growth, Development 146(1):dev171355 (2019), open access under CC BY 4.0.
Test yourself
Question 1 of 8 Score 0 / 0

Which of these is a single cell type rather than a tissue or a tissue system?
Sources & Integrity

Every structure, slide, simulation, and clip on this page is listed below with the source it came from. None of the 3D structures is drawn by hand or by AI: each one loads by accession ID from RCSB PDB or PubChem. This panel is generated from the lesson's own data file, so what it lists is what the page loads.

Simulations

  • How a root tip grows · Original interactive — Biochemistrypedia (Deppmann Lab).

Scientist profiles & quotes

  • Katherine Esau — Esau, Plant Anatomy (John Wiley and Sons, New York, 1953); Ray F. Evert, Katherine Esau, Biographical Memoirs of the National Academy of Sciences.
  • Sophia Satina — Satina, Blakeslee and Avery, Demonstration of the three germ layers in the shoot apex of Datura by means of induced polyploidy in periclinal chimeras, American Journal of Botany 27:895-905 (1940).
  • Albert F. Blakeslee — Blakeslee and Avery, Methods of inducing doubling of chromosomes in plants by treatment with colchicine, Journal of Heredity 28:393-411 (1937); Satina, Blakeslee and Avery, American Journal of Botany 27:895-905 (1940).
  • Thomas Greb — Shi, Lebovka, Lopez-Salmeron, Sanchez and Greb, Bifacial cambium stem cells generate xylem and phloem during radial plant growth, Development 146(1):dev171355 (2019), open access under CC BY 4.0.

Quotes are verbatim from the cited source. Profile text is original to this site and was checked against the sources cited.

Animated explainer

A Root Grows by Cells Times Length · narration: an AI narrator voice (Gemini, Charon) reading narration written from this lesson's text · animation: AI-generated in code (Manim) by Claude, a schematic with no drawn molecular structures, plots computed from their equations · captions from the narration script.

Images & figures

2 illustrations in this lesson are AI-generated: drawn by an image model from a written spec, then checked by a separate AI judge against the spec and the lesson text. None shows a chemical structure or data.

  • a flat radial strip of a woody trunk from centre to surface showing wood, a thin vascular cambium, secondary phloem, phelloderm, cork cambium and cork. A bracket marks bark as everything outside the vascular cambium, and an inset shows wide spring cells and narrower late-summer cells in one annual ring.
  • schematic cross-section of a eudicot leaf with a cuticle over the upper epidermis, tightly packed palisade cells, loosely arranged spongy cells with air spaces, a vein with xylem and phloem, and stomata with guard cells in the lower epidermis. An inset shows a stoma from below.

Every 3D structure is rendered live from an accession ID, and every plot is drawn from its equation in the browser.

integrity Machines check the mechanics. The build confirms that every local file exists, that every question's answer key points at a real choice, and that any AI-generated scientific figure names the person who checked it; a weekly job re-resolves every accession ID against its database. Questions are original; MCAT-style items are written for this site and are not real AAMC questions. Banned sources (the Tymoczko textbook, publisher decks, copyrighted figures) are not used.

Like I'm five

A plant has little growing spots at the tips of its roots and shoots. Cells there split into more cells, then stretch long, and that makes the plant taller and the roots deeper. Trees also have a thin growing sheet under their bark that makes a new layer of wood every year, which is why a cut log shows rings. A plant keeps its growing spots all its life, so it can keep adding new leaves and branches.
High school

A flowering plant has three organs (root, stem and leaf) built from three tissue systems: dermal tissue covers it, vascular tissue (xylem and phloem) carries water and food, and ground tissue fills the rest. New cells come from meristems, patches of cells that keep dividing. Apical meristems at the root and shoot tips make the plant longer: cells divide, then elongate, then mature into their final type. In woody plants, lateral meristems make the stem thicker. The vascular cambium adds wood (secondary xylem) on the inside and secondary phloem on the outside, and the cork cambium makes cork for the bark. Leaves are thin, flat organs with a waxy cuticle, pores called stomata, and chloroplast-rich mesophyll cells around air spaces.
College

Plant cell types (parenchyma, collenchyma, sclerenchyma, tracheids and vessel elements, sieve-tube elements and companion cells) assemble into simple and complex tissues, which make up the dermal, ground and vascular tissue systems of each organ. Primary growth at apical meristems produces the primary body; at steady state, root elongation rate equals the rate of cell production per file times final cell length, so elongation supplies most of the length. In Datura, induced polyploid periclinal chimeras showed that the shoot apex consists of three clonally distinct layers (L1, L2, L3) with separate fates. Secondary growth comes from lateral meristems: the vascular cambium makes secondary xylem inward and secondary phloem outward, and the cork cambium makes phellem outward and phelloderm inward; bark is everything outside the vascular cambium. Monocots such as maize lack a conventional vascular cambium. In a typical eudicot leaf, palisade mesophyll lies under the upper epidermis, spongy mesophyll with large air spaces lies above the lower epidermis, and most stomata are on the lower surface; C4 grasses such as maize add Kranz bundle sheaths around their veins.
Graduate

Growth in a root at steady state follows a kinematic relation: elongation rate = F x L, where F is the flux of cells leaving the meristem per file (cells per hour) and L the final cell length. A change in growth rate can therefore come from F, from L, or from both, and measuring both is the only way to tell which. Radial growth adds area as the annulus pi(r2 squared - r1 squared), so a fixed ring width adds area in proportion to radius, and wood production per ring rises as a trunk grows even at constant cambial activity. Datura chimeras established clonal apical layers; later clonal analyses in other species found layers occasionally invaded by neighbouring cells, and many grasses have two tunica-like layers rather than three. In the Arabidopsis hypocotyl, Shi and colleagues (2019) used EdU pulse labeling and inducible Cre-lox lineage tracing; 35 of 99 clones spanned xylem, cambium and phloem, supporting a bifacial stem cell. An open question is whether the fate of each daughter of a cambial stem cell is set by an intrinsic asymmetry at division or by its position after division, and whether the Arabidopsis picture holds in the cambium of a long-lived tree.
Acid at dawn

CAM plants such as cacti take up carbon dioxide at night and store it as malic acid, so their tissue should be more acid at dawn than at dusk. Klaus Winter and Andrew Smith tested this as a diagnostic across 70 vascular plant species sampled in Panama, titrating extracts to pH 6.5. All 30 species classed as CAM acidified overnight; one, Clusia uvitana, gained 452 millimoles of hydrogen ion per kilogram of fresh mass. None of the 40 C3 species tested showed a significant dawn-dusk difference. One acid reading, at one time of day, would not have told them apart.

— Winter and Smith, New Phytologist 233:599-609 (2022; online 2021).
A herb that switches

Talinum triangulare is a leafy herb, not a cactus, yet under drought it can switch from ordinary C3 photosynthesis to a weak form of CAM, and back again when water returns. Dominik Brilhaus and colleagues followed the switch with RNA sequencing and metabolite measurements during water deprivation. Transcripts for CAM enzymes rose sharply in the leaves, and the authors listed candidate transcription factors that might control the change. Candidates are not proof: the study shows which genes changed together, and leaves it to later experiments to show which ones cause the switch.

— Brilhaus, Brautigam, Mettler-Altmann, Winter and Weber, Plant Physiology 170:102-122 (2016; online 2015).
Faster pores, bigger plants

Stomata open and close more slowly than light changes under a passing cloud, so a leaf can lose water while its pores lag behind. Maria Papanatsiou and colleagues expressed BLINK1, a light-gated potassium channel, in the guard cells of Arabidopsis. Stomata in these plants opened faster in light and closed faster afterwards. Under the study's fluctuating-light conditions the plants accumulated 2.2 times as much biomass as controls. That is one laboratory system, and it does not predict a yield gain in a field crop.

— Papanatsiou, Petersen, Henderson, Wang, Christie and Blatt, Science 363:1456-1459 (2019).

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