Roots, xylem and phloem
All three working zones of a growing root tip fit into roughly its first centimetre.
What roots do
Roots anchor the plant, take up water and minerals, and store food made by photosynthesis. Most are underground, but some plants also grow roots from the shoot above the soil. There are two main kinds of root system. Dicots usually have a tap root: one main root grows straight down and smaller side roots branch off it. That is why a dandelion snaps when you pull it, and why it can grow back from the piece left behind. Monocots such as lawn grasses, wheat, rice and corn have a fibrous root system, a dense mat of thin roots near the surface that also holds soil in place.
The tip of a growing root is protected by a root cap, which is constantly worn away and replaced as the root pushes through soil. Behind it come three zones: a zone of cell division, a zone of elongation where the new cells lengthen and push the root forward, and a zone of maturation where cells take on their final jobs. OpenStax notes that all three fit in about the first centimetre. In the maturation zone the epidermal cells grow root hairs, thin outgrowths that enormously increase the surface area for absorbing water and minerals.
Water that enters a root has to cross a checkpoint before it reaches the xylem. The endodermis is a ring of cells around the central vascular tissue, and its walls carry a waxy band called the Casparian strip. Water and dissolved minerals cannot slip between these cells; they must pass through the cell membranes, which lets the root control what enters and keep out many toxic substances.
Two pipelines
A plant has two transport systems running side by side. Xylem carries water and minerals in one direction only: from soil to root to leaf and out into the air, pulled by transpiration as the last unit showed. Phloem carries sugar, and it can flow either way. Parts of the plant that make or release sugar are called sources, usually mature leaves. Parts that use or store it are sinks: roots, young shoots, developing seeds and fruits, tubers. The top leaves send sugar up to the growing shoot tip, the lower leaves send it down to the roots, and the middle leaves send it both ways.
Leaves load sucrose into the phloem’s sieve-tube elements using energy. Phloem sap can be up to 30 percent sugar. All that sugar draws water in from the nearby xylem by osmosis, raising the pressure, and the pressure pushes the sap towards the sinks, where the sugar is used or stored and the water leaves again. Living companion cells next to each sieve tube supply the energy it needs.
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Partners in the soil
Roots rarely work alone. Most land plants form mycorrhizae, partnerships with fungi whose thin threads reach far beyond the roots and into tiny soil pores. The plant gains mainly phosphate and other minerals; the fungus can take up to 20 percent of the carbon the plant fixes. OpenStax says endomycorrhizae are found in the roots of more than 80 percent of land plants. Legumes such as peas, beans, peanuts and soybeans have a second partner: soil bacteria called rhizobia that live in root nodules and turn nitrogen gas from the air into ammonia the plant can use. Biological processes supply 65 percent of the nitrogen used in agriculture.
| Root hair | Outgrowth of a root epidermal cell; adds surface for absorption. |
| Casparian strip | Waxy band in the endodermis that forces water through membranes. |
| Source | A part that makes or releases sugar, such as a mature leaf. |
| Sink | A part that uses or stores sugar: roots, fruits, seeds. |
| Mycorrhiza | Partnership between a fungus and plant roots. |
| Rhizobia | Bacteria in legume root nodules that fix nitrogen. |
- Tap roots (dicots) and fibrous roots (monocots).
- Root cap, then division, elongation and maturation zones; root hairs absorb.
- Xylem: water one way, soil to air. Phloem: sugar from sources to sinks.
- Mycorrhizal fungi and nitrogen-fixing rhizobia help roots.
- Which plant has a fibrous root system?
- Wheat
- Dandelion
- Carrot
- Beet
- What do root hairs do?
- Increase the surface area for absorbing water and minerals
- Carry sugar
- Protect the root tip
- Make chlorophyll
- Order the parts of a root tip from the very end backwards.
- What is the job of the Casparian strip?
- It forces water and minerals to pass through cell membranes
- It carries sugar to the leaves
- It makes root hairs
- It stores starch
- Which tissue can carry its contents both up and down?
- Phloem
- Xylem
- Epidermis
- Cuticle
- Which of these is a sink?
- A developing fruit
- A mature sunlit leaf
- The cuticle
- A stoma
- Phloem sap can be up to what percentage sugar?
- What do rhizobia give a bean plant?
- Ammonia made from nitrogen in the air
- Sugar
- Water
- Carbon dioxide
- According to OpenStax, endomycorrhizae occur in the roots of more than what percentage of land plants?
- Put the path of water in order.
Sources for this course (16)
- -fix5: gravity calculation corrected using rho*g*h = 1000*9.8*h Pa
- the cited textbook has a tenfold coefficient error. Sources (checked 2026-10-17): OpenStax Biology 2e 8.1 Overview of Photosynthesis (photoautotrophs, mesophyll, stomata on the underside, guard cells, chloroplast, thylakoid, granum, stroma, two stages; openstax.org/books/biology-2e/pages/8-1-overview-of-photosynthesis)
- 8.2 The Light-Dependent Reactions (400 to 700 nm, chlorophyll a absorbs blue and red, carotenoids, 300 to 400 chlorophylls per antenna, water split at PSII, about 10 percent of the oxygen used by the leaf's mitochondria; .../8-2-the-light-dependent-reactions-of-photosynthesis)
- 8.3 Using Light Energy to Make Organic Molecules (RuBisCO, 3-PGA, G3P, three turns per G3P and six per glucose, "dark reaction" outdated, cacti fix CO2 at night; .../8-3-using-light-energy-to-make-organic-molecules)
- 30.3 Roots (tap and fibrous roots, root cap, three zones in the first centimetre, root hairs, endodermis and Casparian strip; .../30-3-roots)
- 30.4 Leaves (lamina, petiole, midrib, parallel and reticulate venation, Ginkgo, cuticle, trichomes, palisade in one to three layers, spongy mesophyll, guard cells the only epidermal cells with chloroplasts, xylem and phloem, needles and spines; .../30-4-leaves)
- 30.5 Transport of Water and Solutes (redwoods near 116 m, turgor 0.6 to 0.8 MPa and up to 1.5 MPa = 210 psi vs tyres 30 to 34 psi, gravity -0.1 MPa per metre, -2 MPa at the leaf surface, cohesion-tension, up to 90 percent of the water lost, sources and sinks, phloem sap up to 30 percent sugar; .../30-5-transport-of-water-and-solutes-in-plants)
- 31.3 Nutritional Adaptations (rhizobia, nodules, nitrogenase, biological fixation 65 percent of nitrogen used in agriculture, mycorrhizae in more than 80 percent of land plants, up to 20 percent of the carbon; .../31-3-nutritional-adaptations-of-plants)
- 32.2 Pollination and Fertilization (self and cross pollination, bees cannot see red, nectar guides, bats, birds, wind, water, at least 25,000 orchid species, double fertilization 2n zygote and 3n endosperm, seed parts, scutellum, fruit types, dispersal, dormancy; .../32-2-pollination-and-fertilization)
- Britannica, Photosynthesis: history (Priestley 1771, Ingenhousz 1779, 1782: oxygen formed at the expense of fixed air, identified the year before as carbon dioxide) (britannica.com/science/photosynthesis/History-of-the-study-of-photosynthesis)
- Smithsonian Libraries, Ingenhousz, Experiments upon Vegetables, 1779 (library.si.edu/digital-library/book/experimentsuponv00inge)
- van Helmont's account (200 pounds of earth, 5 pound willow, 169 pounds and about 3 ounces after five years, soil short by about 2 ounces)
- quoted from Howe 1965 by S. G. Saupe, College of St. Benedict and St. John's University (employees.csbsju.edu/ssaupe/biol115/van_helmont.htm)
- F. F. Blackman, Optima and Limiting Factors, Annals of Botany 19:281-296, April 1905 (doi.org/10.1093/oxfordjournals.aob.a089000)
- NOAA Global Monitoring Laboratory, Trends in CO2 (gml.noaa.gov/ccgg/trends/: Mauna Loa September 2026 monthly mean 426.07 ppm)
- NOAA Climate.gov, Climate change: atmospheric carbon dioxide (280 ppm or less before the Industrial Revolution of the mid-1700s, from ice cores; climate.gov/news-features/understanding-climate/climate-change-atmospheric-carbon-dioxide).
Checked: 2026-10-17