Kurzgesagt – In a Nutshell 

Sources – Trees II

We thank the following experts for their comments: 

• Prof. dr. ir. Kathy Steppe
  Faculty of Bioscience Engineering, Ghent University

• Dr. Sofia van Moorsel
  Department of Geography, University of Zurich
  
  

• Prof. em. Roger R. Lew
  York University
  
  

• Dr. Abu Imran Baba
  Faculty of Forest Sciences, Swedish University of Agricultural Sciences 

—Trees are the heaviest and largest living things on Earth, with the most massive tree weighing almost 2,000 tons – as much as ten blue whales. 

For this statement we are excluding clonal species, though there are also very large clonal 

trees

#Fishlake National Forest: “Pando” (retrieved 2025)

https://www.fs.usda.gov/r04/fishlake/recreation/explore-forest/pando 

Quote: “Pando is believed to be the largest, most dense organism ever found at nearly 13 million pounds. The clone spreads over 106 acres, consisting of over 40,000 individual trees. The exact age of the clone and its root system is difficult to calculate, but it is estimated to have started at the end of the last ice age.”

These clonal trees have been argued to be the largest even among other clonal organisms, like the honey fungus Armillaria ostoyae

#Walking Mountains Science Center (2021): “Curious Nature Largest Organisms: Aspen vs. Fungi?”

https://blog.walkingmountains.org/curious-nature/aspen-vs-fungi-colorado 

However, here we are talking about the largest single-stem tree, called General Sherman. 

#Guiness World Records (2020): “Largest living hardwood tree (by weight)”

https://www.guinnessworldrecords.com/world-records/634276-largest-living-hardwood-tree-mass 

Quote:“The world 's largest individual tree (by both volume and mass) is a giant sequoia (Sequoiadendron giganteum) named General Sherman, located in Sequoia National Park in the Sierra Nevada Mountains of California, USA.”

Estimates for its weight vary significantly, some reach 2,000 tons. 

#Zinke, Paul J.; Stagenberger, Alan G. (1992): “Soil and Nutrient Element Aspects of

Sequoiadendron Giganteum”, Proceedings of the Symposium on Giant Sequoias: Their Place in the Ecosystem and Society

https://www.fs.usda.gov/psw/publications/documents/psw_gtr151/psw_gtr151.pdf 

Quote:“Stagner (1952) reports trunk weights of 625 short tons for the General Sherman Tree, and 565 tons for the General Grant tree assuming a specific gravity of 0.4. Fry and White (1938) reported the wet field weight of the General Sherman tree at 2105 tons. By using the density of 0.3 (Cockrell and Stangenberger 1971), their volume of 42,650 cubic feet (1208 m3) yields an estimated dry weight of 623 tons (trunk, 362; bark, 54; limbs, 37; roots, 142; and foliage, 28 metric tons).”

while some of them consider it to be even larger

#Encyclopedia Britannica: “Giant sequoia” (retrieved 2025)

https://www.britannica.com/plant/giant-sequoia#ref225641 

Taking the estimate of 2,000 and a maximum weight for a blue whale of 200 tonnes

#Natural History Museum: “The blue whale in The Ocean” (retrieved 2025)

https://www.nhm.ac.uk/bluewhale/ocean/record-breaker/ 

Quote:“Blue whales can weigh up to 180 tonnes, although most adults weigh between 72 and 135 tonnes.”

We conclude that General Sherman weighs approximately the same as ten blue whales. 

—But instead of flowing weightlessly in the ocean it reaches 25 stories into the sky, held in place by surprisingly shallow roots.

#Sequoia & Kings Canyon National Parks California: “The General Sherman Tree” (retrieved 2025)

https://www.nps.gov/seki/learn/nature/sherman.htm 

#California State Parks: “Big Tree Life Cycle: Growth & Development” (retrieved 2025)

https://www.parks.ca.gov/?page_id=1151 

Quote: “The entire root system is likely to be within four or five feet of the soil surface. This is an astonishingly delicate foundation for an above the ground structure that may tower upward two hundred fifty to three hundred feet (twenty to twenty-five stories) and weigh twelve million pounds (as much as a small ocean going freighter).”

—What makes plants so incredibly successful is that over a billion years ago their ancestors became better than any other living thing at harvesting carbon.  

#Bowles, Alexander M.C. et al. (2023): “The origin and early evolution of plants”, Trends in Plant Science, vol. 28, 3

https://www.cell.com/action/showPdf?pii=S1360-1385%2822%2900271-0

 Quote:“Plants (Archaeplastida; see Glossary) have transformed our planet, increasing energy input to the biosphere, altering the atmosphere, and forever changed global biogeochemical cycles [1–4].[...] All living plants belong to Archaeplastida [...], named for the primary endosymbiotic union between a eukaryote and a cyanobacterium, from which the major group of photosynthetic eukaryotes arose (Box 2) [7].

(D) Simplified phylogeny and timescale of archaeplastid evolution (based on Figure 4).” 

—Today plants make up 80% of the biomass on earth —and are the basis for all complex life – all animals eat either plants or animals that eat plants, to get the carbon they need. 

#Our World in Data (2019): “Humans make up just 0.01% of Earth's life — what's the rest?”

https://ourworldindata.org/life-on-earth 

#National Geographic Education: “Food Web”

https://education.nationalgeographic.org/resource/food-web/

#Kimball, John W.: “Food Chains and Food Webs” (retrieved 2025)

https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Biology_(Kimball)/17%3A_Ecology/17.01%3A_Energy_Flow_through_the_Biosphere/17.1B%3A_Food_Chains_and_Food_Webs 

—But the atmosphere is only about 0.04% CO2. 425 CO2 molecules per million molecules in the air. 

Atmospheric levels of CO2 have been rising due to human emissions since the onset of the industrial era. At the time of writing they are at 425 ppm. 

#NASA: “Carbon Dioxide” (retrieved 2025)

https://climate.nasa.gov/vital-signs/carbon-dioxide/?intent=121 

—To get a SINGLE tonne of carbon a tree has to process 6000 tons, or 5 million cubic meters, of air! 

Trees fixate carbon from CO2 through photosynthesis. The basic equation of photosynthesis is:

6CO2 + 6H2O + Light Energy → C6H12O6 + 6O2

So it uses one molecule of CO2 for every carbon atom it fixates. Taking into consideration the molar masses,

#Royal Society of Chemistry: Periodic Table (retrieved 2025)

https://periodic-table.rsc.org/

we conclude that 44g of CO2 are exchanged for 12g of fixated carbon.

Since the mass air concentration (not to be confused with the concentration in ppm)
of CO2 in air is 0.063% 

#The Engineering Toolbox (2003): “Air - Composition and Molecular Weight” (retrieved 2025)

https://www.engineeringtoolbox.com/air-composition-d_212.html

Then, per every ton of fixated carbon a tree must process: 

1 ton C × (44g CO2/12g C) × (100 mass of air/0.063 mass of CO2)= 5.8 × 103 ton of air.

At 20 °C and one atmosphere of pressure, this mass of air would occupy a volume of: 

#The Engineering ToolBox (2003): “Air Density, Specific Weight, and Thermal Expansion Coefficients at Varying Temperatures and Pressures.” (retrieved 2025)

https://www.engineeringtoolbox.com/air-density-specific-weight-d_600.html

5.8 × 103 ton × (103 kg/1 ton) × (1 m3/1.2 kg) = 4. 8 × 106  m3

—A huge industrial park network, made from dozens of branches, subbranches and hundreds of thousands of twigs that can sense the sun and shape the tree to grow towards it in slow motion. 

#Liscum, Emmanuel et al. (2014): “Phototropism: Growing towards an Understanding of Plant Movement”, The Plant Cell, vol. 26, 1, 38-55.

https://pmc.ncbi.nlm.nih.gov/articles/PMC3963583/ 

—They are carrying up to a million leaves.

The number of leaves on a tree depends heavily on the species and size of the tree. The tree shown here is a European beech (Fagus sylvatica). It is a relatively small specimen with around 10 meters of crown diameter

#USDA Forest Service (1993):  “Fact Sheet ST-244: Fagus sylvatica. European Beech” https://hort.ifas.ufl.edu/database/documents/pdf/tree_fact_sheets/fagsyla.pdf 

Given the leaf area index for this species

#Leuschner, Cristoph; et al. (2006): ”Variation in leaf area index and stand leaf mass of European beech across gradients of soil acidity and precipitation”, Plant Ecology, vol.186, 247–258
https://link.springer.com/article/10.1007/s11258-006-9127-2

and an estimated area of the leaf of 16 cm2

#Thomas, Frank M. et al. (2024): “Leaf traits of Central-European beech (Fagus sylvatica) and oaks (Quercus petraea/robur): Effects of severe drought and long-term dynamics”, Forest Ecology and Management, vol. 559, 121823 https://www.sciencedirect.com/science/article/pii/S037811272400135X 

It has around a million leaves 

#Czernia, Dominik (2024): “Tree Leaves Calculator”

https://www.omnicalculator.com/biology/tree-leaves 

—It is made from hundreds of millions of factory cells and optimized to have as much surface area and be as thin as possible to harvest sunlight. 

We are simplifying here. The main function of most leaves is photosynthesis, so most of them are flat and present a wide area towards the sky to catch light, but this is not always the case. Other leaves can have different functions and not even be involved in photosynthesis:

#Virginia Tech. & Virginia State University (2018): “Virginia Cooperative Extension Gardener Handbook”, Chapter 1: Botany, Anatomy: Plant Parts and Functions

https://pressbooks.lib.vt.edu/emgtraining/chapter/1/#chapter-24-section-2
Quote: “Scale leaves, or cataphylls, are found on rhizomes and are the small, leathery, protective leaves that enclose and protect buds. Seed leaves, or cotyledons, are modified leaves that are found on the embryonic plant and commonly serve as storage organs. Spines as found on barberry and cactus, are specialized modified leaves that protect the plant. Storage leaves, as found on bulbous plants and succulents, serve as food storage organs. Other specialized leaves include bracts, which are often brightly colored. The showy structures on dogwoods and poinsettias are bracts, not petals.”

The number of cells in a leaf depends on the species of the plant and the size of the leaf. On screen we show a leaf from an European beech (Fagus sylvatica) with a estimated area of 16 cm2

#Thomas, Frank M. et al. (2024): “Leaf traits of Central-European beech (Fagus sylvatica) and oaks (Quercus petraea/robur): Effects of severe drought and long-term dynamics”, Forest Ecology and Management, vol. 559, 121823
https://www.sciencedirect.com/science/article/pii/S037811272400135X 

Taking into account  the typical leaf cell size 

#BioNumbers: “Typical cell diameter (Plant)” (retrieved 2025)

https://bionumbers.hms.harvard.edu/bionumber.aspx?s=n&v=2&id=108685 

16 cm2/ (10µm)2 ~ 107

and if we consider the leaf to have a thickness of around 10 cells, that results in a total of around hundreds of millions of cells per leaf. 

—While your skin is hundreds of cells thick, a leaf can be just ten cells top to bottom. 

The thickness of skin varies widely in the human body.

#Faculty of Biological Sciences, University of Leeds (2003): “Skin functions and Layers”

https://www.histology.leeds.ac.uk/skin/skin_layers.php

Quote:“The thickness of skin varies from 0.5mm thick on the eyelids to 4.0mm thick on the heels of your feet.”

Taking the thicker parts of skin and the typical size of a human skin cell: 

#Ismail, Mohd M. et al. (2011) : “An investigation of electromagnetic field effect on a human skin cell using numerical method approaches”, RF and Microwave Conference (RFM), 2011 IEEE International 

https://www.researchgate.net/figure/Human-skin-cell-size_tbl1_261384224

Then human skin is around:

4mm/40µm=100

layers of cells thick.

 

The number of cells in a leaf depends on many things including the species of the plant, its adaptations, and the environment conditions, but many leaves have only a few layers of cells.  

#Virginia Tech. & Virginia State University (2018): “Virginia Cooperative Extension Gardener Handbook”, Chapter 1: Botany, Anatomy: Plant Parts and Functions

https://pressbooks.lib.vt.edu/emgtraining/chapter/1/ 

Quote:“The leaf blade is composed of several layers. On both the top and bottom is a layer of thickened, tough cells called the epidermis. The primary function of the epidermis is protection of leaf tissue. [...]

Part of the epidermis is the cuticle, which is composed of a waxy substance called cutin that protects the leaf from dehydration and prevents penetration of some diseases. [...]

Stomata (singular: stoma) are openings in leaves that allow passage of water and gasses into and out of the leaf. Guard cells are epidermal cells located around a stoma. They help regulate gas exchange by opening and closing in response to weather conditions. [...]

The middle layer of the leaf is the mesophyll and is located between the upper and lower epidermis. This is the layer where photosynthesis occurs. The mesophyll is divided into a dense upper layer called the palisade, and a spongy lower layer that contains a great deal of air space, called the parenchyma layer. The cells in these two layers contain chloroplasts which are the actual site of the photosynthetic process.

Figure 1-16: Cross section of a leaf showing the cuticle as the outer surface of top and bottom of the leaf (protecting it from dehydration), the epidermis, palisade and spongy mesophyll, and the vascular bundle containing the xylem and phloem. The lower epidermis contains stomata (plural of stoma) surrounded by guard cells.”

#Crèvecoeur, Michèle (2022): “Plant Microscopy: Anatomy of leaf lamina” 

https://michelecrevecoeur.ch/?page_id=1704 

Quote:“The outermost layer of the leaf is epidermis present on both faces of the lamina. The epidermis is usually a monolayer of cells without intercellular spaces.

Some cells of the epidermis are specialized (stomata, trichomes) and the cell wall in contact with the environment is characterized by a cuticle whose thickness varies according to the environment in which the plant lives.

Between the two epidermis there is the mesophyll term used to designate the anatomical region filled with parenchyma of variable organization (homogeneous or heterogeneous). It contains vascular bundles with superposition of xylem (towards abaxial face) and phloem (towards adaxial face) as in the stem. Some leaves contain secretory tissues and supporting tissues (collenchyma, sclerenchyma) are frequently observed.

In a bifacial leaf the mesophyll is heterogeneous with two different parenchyma: on the adaxial face there is palisade parenchyma that is close to the upper epidermis of the leaves, and consists of a few rows of narrow cells, perpendicular to the leaf surface exposed to light. They are tightly packaged without intercellular spaces and contain many chloroplasts arranged around the central vacuole. In case of intensive light, chloroplasts move on the side of the cell opposite to the light exposition.

The spongy parenchyma is located on the abaxial face less exposed to light. It also contains chloroplasts but less numerous and is characterized by either small or large intercellular spaces, assuring communication with outside (O2, CO2, …).”

—On their top leaves have only a single, ultra thin layer of protective transparent “skin” cells that let light through and keep water in. 

#Crèvecoeur, Michèle (2022): “Plant Microscopy: Anatomy of leaf lamina” 

https://michelecrevecoeur.ch/?page_id=1704 

Quote:“The outermost layer of the leaf is epidermis present on both faces of the lamina. The epidermis is usually a monolayer of cells without intercellular spaces.”

#Virginia Tech. & Virginia State University (2018): “Virginia Cooperative Extension Gardener Handbook”, Chapter 1: Botany, Anatomy: Plant Parts and Functions

https://pressbooks.lib.vt.edu/emgtraining/chapter/1/ 

Quote:“Part of the epidermis is the cuticle, which is composed of a waxy substance called cutin that protects the leaf from dehydration and prevents penetration of some diseases.”

—Below them are layers of factory cells, filled to the brink with chloroplasts that do the actual work. Beneath them a spongy layer of loose cells enables gases to travel around. 

#Virginia Tech. & Virginia State University (2018): “Virginia Cooperative Extension Gardener Handbook”, Chapter 1: Botany, Anatomy: Plant Parts and Functions

https://pressbooks.lib.vt.edu/emgtraining/chapter/1/ 

Quote:“The middle layer of the leaf is the mesophyll and is located between the upper and lower epidermis. This is the layer where photosynthesis occurs. The mesophyll is divided into a dense upper layer called the palisade, and a spongy lower layer that contains a great deal of air space, called the parenchyma layer. The cells in these two layers contain chloroplasts which are the actual site of the photosynthetic process.”

#Crèvecoeur, Michèle (2022): “Plant Microscopy: Anatomy of leaf lamina” 

https://michelecrevecoeur.ch/?page_id=1704 

Quote:“In a bifacial leaf the mesophyll is heterogeneous with two different parenchyma: on the adaxial face there is palisade parenchyma that is close to the upper epidermis of the leaves, and consists of a few rows of narrow cells, perpendicular to the leaf surface exposed to light. They are tightly packaged without intercellular spaces and contain many chloroplasts arranged around the central vacuole. In case of intensive light, chloroplasts move on the side of the cell opposite to the light exposition.

The spongy parenchyma is located on the abaxial face less exposed to light. It also contains chloroplasts but less numerous and is characterized by either small or large intercellular spaces, assuring communication with outside (O2, CO2, …).”

#Ha, Melissa; Morrow, Maria; Algiers, Kammy (2025): “Internal Leaf Structure”, ASCCC Open Educational Resources Initiative

https://bio.libretexts.org/Bookshelves/Botany/Botany_(Ha_Morrow_and_Algiers)/03%3A_Plant_Structure/3.04%3A_Leaves/3.4.02%3A_Internal_Leaf_Structure 

Quote:“Below the palisade parenchyma are seemingly loosely arranged cells of an irregular shape. These are the cells of the spongy parenchyma (or spongy mesophyll). The intercellular air spaces found between mesophyll cells facilitate gaseous exchange.”

—The whole leaf is traversed by a network of vein-like superhighways, that bring water and minerals up from the roots and carry sugars back down. 

#Ha, Melissa; Morrow, Maria; Algiers, Kammy (2025): “Internal Leaf Structure”, ASCCC Open Educational Resources Initiative

https://bio.libretexts.org/Bookshelves/Botany/Botany_(Ha_Morrow_and_Algiers)/03%3A_Plant_Structure/3.04%3A_Leaves/3.4.02%3A_Internal_Leaf_Structure 

Quote: “Like the stem, the leaf contains vascular bundles composed of xylem and phloem (Figure  3.4.2.6−7 ). When a typical stem vascular bundle (which has xylem internal to the phloem) enters the leaf, xylem usually faces upwards, whereas phloem faces downwards. The conducting cells of the xylem (tracheids and vessel elements) transport water and minerals to the leaves. The sieve-tube elements of the phloem transports the photosynthetic products from the leaf to the other parts of the plant. The phloem is typically supported by a cluster of fibers (sclerenchyma) that increase structural support for the veins. A single vascular bundle, no matter how large or small, always contains both xylem and phloem tissues.

Figure 3.4.2.6 This scanning electron micrograph shows xylem (larger cells on top) and phloem (smaller cells on bottom) in the leaf vascular bundle from the lyre-leaved sand cress (Arabidopsis lyrata). (credit: modification of work by Robert R. Wise; scale-bar data from Matt Russell)”

#Nagwa: “Lesson Explainer: Specialized Plant Structures” (retrieved 2025) https://www.nagwa.com/en/explainers/393196591893/ 

Quote:

“Figure 5: A diagram of a cross section of a leaf. The key structures of the waxy cuticle, epidermis, palisade mesophyll layer, spongy mesophyll layer, xylem, phloem, stomata, and guard cells are highlighted.” 

—At the bottom is another protective layer of cells interrupted by hundreds of thousands of stomata – tiny mouths opened and closed by two guard cells that look a bit like lips. 

#Ha, Melissa; Morrow, Maria; Algiers, Kammy (2025): “Internal Leaf Structure”, ASCCC Open Educational Resources Initiative

https://bio.libretexts.org/Bookshelves/Botany/Botany_(Ha_Morrow_and_Algiers)/03%3A_Plant_Structure/3.04%3A_Leaves/3.4.02%3A_Internal_Leaf_Structure
Quote: “The epidermis helps in the regulation of gas exchange. It contains stomata (singular = stoma; Figure  3.4.2.2), openings through which the exchange of gases takes place. Two guard cells surround each stoma, regulating its opening and closing, and the guard cells are sometimes flanked by subsidiary cells. Guard cells are the only epidermal cells to contain chloroplasts. In most cases, the lower epidermis contains more stomata than the upper epidermis because the bottom of the leaf is cooler and less prone to water loss.”

#Virginia Tech. & Virginia State University (2018): “Virginia Cooperative Extension Gardener Handbook”, Chapter 1: Botany, Anatomy: Plant Parts and Functions

https://pressbooks.lib.vt.edu/emgtraining/chapter/1/ 

Quote: “Stomata (singular: stoma) are openings in leaves that allow passage of water and gasses into and out of the leaf. Guard cells are epidermal cells located around a stoma. They help regulate gas exchange by opening and closing in response to weather conditions.

Figure 1-17: Images of stoma: (a) electron micrograph shows closed stoma on a dicot; (b) shows stoma opening and closing; (c) diagram of stomal pore with guar cells on either side, entirely surrounded by epidermal cells that make up the leaf surface.”

Stomatal density also depends on other factors like the species: 

#Roberts, Bruce R. (1990): “What are Stomates and how do They Work?”, Arboriculture & Urban Forestry (AUF), vol. 16, 12

https://auf.isa-arbor.com/content/16/12/331 

Quote:“Although individual stomatal pores are very small (for most trees, about 400 could fit on the head of a pin), the total number of stomates on each leaf can be quite large (as many as 16,000 per square inch in some oaks). The combined area occupied by these pores when fully open is somewhere between 1 % to 3% of the total surface area in most leaves.”

On screen we show an European beech (Fagus sylvatica), whose approximate stomatal density is of around 200 stomata per square millimeter. 

#Kučerová, Jana et al. (2018): “Adaptive variation in physiological traits of beech provenances in Central Europe”, iForest - Biogeosciences and Forestry, vol. 11, 1, 24-31

https://iforest.sisef.org/contents/?id=ifor2291-010# 

If we consider a typical leaf area of around 16 cm2

#Thomas, Frank M. et al. (2024): “Leaf traits of Central-European beech (Fagus sylvatica) and oaks (Quercus petraea/robur): Effects of severe drought and long-term dynamics”, Forest Ecology and Management, vol. 559, 121823
https://www.sciencedirect.com/science/article/pii/S037811272400135X 

(200 stoma / 1 mm2) × (1 mm2/10-2 cm2) × (16 cm2/leaf) = 320,000

So we estimate that the order of magnitude of the number of stomata is in the hundreds of thousands.

—Each day an adult tree pulls up dozens of liters of water all the way from its roots in the ground to these veins, where about 95% of it is sweated out through hundreds of billions of these tiny mouths. 

How much water a tree absorbs daily depends on many factors, like the species of the tree, the soil, and the availability of water.

#Wullschleger, Stan D.; Meinzer, Frederick C.; Vertessy, Robert A. (1998): “A review of whole-plant water use studies in tree”, Tree Physiology, vol.18, 8-9, 499–512 https://academic.oup.com/treephys/article-abstract/18/8-9/499/1632547 

Quote: “A survey of 52 studies conducted since 1970 indicated that rates of water use ranged from 10 kg day−1 for trees in a 32-year-old plantation of Quercus petraea L. ex Liebl. in eastern France to 1,180 kg day−1 for an overstory Euperua purpurea Bth. tree growing in the Amazonian rainforest. The studies included in this survey reported whole-tree estimates of water use for 67 species in over 35 genera. Almost 90% of the observations indicated maximum rates of daily water use between 10 and 200 kg day−1 for trees that averaged 21 m in height.”

Most of the water is “sweated out” by the tree through transpiration, but a small percentage is stored: 

#Liu, Ziqiang et al. (2021): “Partitioning tree water usage into storage and transpiration in a mixed forest”, Forest Ecosystems, vol. 8, 72

https://forestecosyst.springeropen.com/counter/pdf/10.1186/s40663-021-00353-5.pdf

Quote:“Water migration and use are important processes in trees. However, it is possible to overestimate transpiration by equating the water absorbed through the plant roots to that diffused back to the atmosphere through stomatal transpiration. Therefore, it is necessary to quantify the water transpired and stored in plants. [...] The water stored in both species comprised 6%–7% of the total water fluxes and, therefore, should be considered in water balance models.”

Some trees can also exude water through guttation.

—The vapor from a forest of billions of trees can seed clouds and create rain.

#Wright, Jonathon S. et al. (2017): “Rainforest-initiated wet season onset over the southern Amazon”, Proceedings of the U.S.A. National Academy of Sciences, vol. 114, 32, 8481-8486

https://www.pnas.org/doi/10.1073/pnas.1621516114

—in a nutshell, with the energy from the sun, water molecules are split into hydrogen and oxygen. The oxygen is ejected, while the leftover hydrogen and CO2 are forged and reduced into glucose – a simple sugar that’s both battery and building block.  And the source of most carbon in the world for most animals.

#Encyclopedia Britannica: “Photosynthesis” (retrieved 2025)

https://www.britannica.com/science/photosynthesis 

#Nature Education (2014): “Photosynthetic Cells”

https://www.nature.com/scitable/topicpage/photosynthetic-cells-14025371/ 

Quote:“Photosynthesis consists of both light-dependent reactions and light-independent reactions. In plants, the so-called "light" reactions occur within the chloroplast thylakoids, where the aforementioned chlorophyll pigments reside. When light energy reaches the pigment molecules, it energizes the electrons within them, and these electrons are shunted to an electron transport chain in the thylakoid membrane. Every step in the electron transport chain then brings each electron to a lower energy state and harnesses its energy by producing ATP and NADPH. Meanwhile, each chlorophyll molecule replaces its lost electron with an electron from water; this process essentially splits water molecules to produce oxygen (Figure 5).

Once the light reactions have occurred, the light-independent or "dark" reactions take place in the chloroplast stroma. During this process, also known as carbon fixation, energy from the ATP and NADPH molecules generated by the light reactions drives a chemical pathway that uses the carbon in carbon dioxide (from the atmosphere) to build a three-carbon sugar called glyceraldehyde-3-phosphate (G3P). Cells then use G3P to build a wide variety of other sugars (such as glucose) and organic molecules.

Figure 5: The light and dark reactions in the chloroplast

The chloroplast is involved in both stages of photosynthesis. The light reactions take place in the thylakoid. There, water (H2O) is oxidized, and oxygen (O2) is released. The electrons that freed from the water are transferred to ATP and NADPH. The dark reactions then occur outside the thylakoid. In these reactions, the energy from ATP and NADPH is used to fix carbon dioxide (CO2). The products of this reaction are sugar molecules and various other organic molecules necessary for cell function and metabolism. Note that the dark reaction takes place in the stroma (the aqueous fluid surrounding the stacks of thylakoids) and in the cytoplasm.”

Most of photosynthesis takes place in the chloroplast 

#Nature Education (2014): “Photosynthetic Cells”

https://www.nature.com/scitable/topicpage/photosynthetic-cells-14025371/ 

“Figure 3: Structure of a chloroplast”

#Virginia Tech. & Virginia State University (2018): “Virginia Cooperative Extension Gardener Handbook”, Chapter 1: Botany, Physiology: Plant Growth and Development

https://pressbooks.lib.vt.edu/emgtraining/chapter/1/#chapter-24-section-3 

Quote:“Plants first store the energy from light in simple sugars, such as glucose (C6H12O6). Some of these sugars are converted back to water and carbon dioxide, releasing the stored energy through the process called respiration. This energy released from respiration is required for all living processes and growth. Simple sugars are also converted to other sugars and starches (carbohydrates) which may be transported to the stems and roots for use or storage, or may be used as building blocks for more complex structures (e.g., oils, pigments, proteins, cell walls).”

#Jeckelmann, Jean-Marc; Erni, Bernhard (2020): “Transporters of glucose and other carbohydrates in bacteria”, Pflugers Archiv : European journal of physiology, vol.472, 9, 1129–1153

https://pubmed.ncbi.nlm.nih.gov/32372286/ 

Quote:“Glucose arguably is the most important energy carrier, carbon source for metabolites and building block for biopolymers in all kingdoms of life. The proper function of animal organs and tissues depends on the continuous supply of glucose from the bloodstream. Most animals can resorb only a small number of monosaccharides, mostly glucose, galactose and fructose, while all other sugars oligosaccharides and dietary fibers are degraded and metabolized by the microbiota of the lower intestine”

—Oxygen is not just garbage to the tree though. To actually use the energy stored in the glucose, the tree has to burn the sugar, just like we humans do, with cellular respiration. So all living cells in the tree suck in oxygen – through the tiny leaf mouths, cracks in the bark, and even root tips tapping in tiny air pockets hidden in the soil.

#Virginia Tech. & Virginia State University (2018): “Virginia Cooperative Extension Gardener Handbook”, Chapter 1: Botany, Physiology: Plant Growth and Development 

https://pressbooks.lib.vt.edu/emgtraining/chapter/1/#chapter-24-section-3 

Quote:“Carbohydrates made during photosynthesis are of value to the plant when they are converted into energy. This energy is used in the process of building new tissues (plant growth). The chemical process by which sugars and starches produced by photosynthesis are converted into energy is called respiration. It is similar to the burning of wood or coal to produce heat (energy). This process in cells is shown most simply as:

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + Energy released

This equation is precisely the opposite of that used to illustrate photosynthesis, although more is involved than just reversing the reaction. However, it is appropriate to relate photosynthesis to a building process, while respiration is a breaking-down process. [...]

The release of accumulated carbon dioxide and the uptake of oxygen occurs at the cell level. In animals, blood carries both carbon dioxide and oxygen to and from the atmosphere by means of the lungs or gills. In plants, there is simple diffusion into the open spaces around the cells, and exchange occurs through the stomata on leaves and stems, or through root hairs.”

—This respiration runs nonstop, and especially at night when the leaf factories stop production. 

#Virginia Tech. & Virginia State University (2018): “Virginia Cooperative Extension Gardener Handbook”, Chapter 1: Botany, Physiology: Plant Growth and Development

https://pressbooks.lib.vt.edu/emgtraining/chapter/1/#chapter-24-section-3 

Quote: “Unlike photosynthesis, respiration occurs at night as well as during the day. Respiration occurs in all life forms and in all cells.”

—Trees actually reabsorb a lot of the oxygen they produce and almost all of the rest gets used up by microbes and everything else breathing nearby.

#Malhi, Yadvinder (2019): “Does the Amazon provide 20% of our oxygen?”

http://www.yadvindermalhi.org/blog/does-the-amazon-provide-20-of-our-oxygen 

—Most of the world's free oxygen doesn't come from trees but from algae and cyanobacteria in the oceans

#Woods Hole Oceanographic Institution: ”Does the ocean produce oxygen?” (retrieved 2025)

https://www.whoi.edu/ocean-learning-hub/ocean-facts/does-the-ocean-produce-oxygen/ 

Quote:“Plants, algae, and cyanobacteria all create oxygen. They do this through photosynthesis. Using energy from sunlight, they turn carbon dioxide and water into sugar and oxygen. They use the sugars for food. Some oxygen is released into the atmosphere.

But oxygen is also used up. Most living cells use it to make energy in a process called cellular respiration. When organisms die, they decompose. Decomposition also uses oxygen. Most oxygen that's produced gets used up by these two processes. (Used up means the atoms become part of a different molecule. The oxygen atoms are still around, just in a different form.)

Over millions of years, tiny single-celled algae and cyanobacteria pumped out oxygen. Much of it got used up in respiration or decomposition. But some of those dead organisms didn't decompose. They sank deep into the ocean and settled on the bottom. That left a tiny bit of oxygen behind. Instead of being used up, it stayed in the air.

In this way, the oceans slowly built up the oxygen in our atmosphere. At the same time, they decreased the amount of carbon dioxide. (Remember that photosynthesis uses carbon dioxide.)

Today, the process continues. We now know that more than half the planet's oxygen comes from the ocean. Not the entire ocean-just the top 200 meters (656 feet) or so. That's about as far as sunlight can travel through water to power photosynthesis. In this photic zone we find all kinds of photosynthetic organisms.”

—Most of the water a tree needs comes from rainfall, which soaks mainly into the upper layers of soil. 

#Critical Zone Observatories U.S. (2016): “Where do trees get their water?” https://czo-archive.criticalzone.org/national/blogs/post/where-do-trees-get-their-water/
Quote:“The reason behind this phenomenon is simple: unless there is a nearby stream or groundwater is close enough to the surface to be accessed by the roots, precipitation from above is the easier to access. Thus, rain and melting snow is the major source of water used by trees. We know this is true beyond just reasoning, it is actually possible to determine where trees are getting their water by the molecular nature of the water in their leaves and vascular tissues. [...]

Dr. Katie Gaines, a recent PhD recipient from the Pennsylvania State University, studied the source of waters in northern hardwood tree species at the Shale Hills Critical Zone Observatory using stable isotope ratios of water. In her study, Dr. Gaines observed oak (Quercus spp.), hickory (Carya spp), and maple (Acer spp.) were primarily utilizing water held in the top 12 inches (30 cm) of the soil (Gaines et al., 2015). In addition, the isotopic signature of the water within the trees closely resembled that of summer precipitation. However, Dr. Gaines also found that oak and hickory were using a much deeper soil water and water ‘tightly-bound’ to surfaces that require more effort to access.”

—About 50% of their roots are packed into the top 25 centimeters of soil. They are not a mirror image of the crown, but a dense, tangled mat, deeply intermingled with their neighbours’. 

#Jackson, Robert B. et al. (1996): “A global analysis of root distributions for terrestrial biomes”, Oecologia, vol.108, 389–411

https://link.springer.com/article/10.1007/BF00333714

Quote:“Overall, the globally averaged root distribution for all ecosystems was β=0.966 (r 2=0.89) with approximately 30%, 50%, and 75% of roots in the top 10 cm, 20 cm, and 40 cm, respectively.” 

#New York State Urban Forestry Council (2022): ”Urban Forestry Fundamentals, Part 3: Roots Grow Like Plates, Not Mirrors. How to Protect Them.”
https://nysufc.org/urban-forestry-fundamentals-part-3-roots-grow-like-plates-not-mirrors-how-to-protect-them/2022/09/26/
Quote:“Most species grow as depicted [below]: large, shallow plates of mostly fine roots extending well beyond the canopy[...]

Most of us grew up with—and still often see—illustrations of a tree’s root system depicted as a mirror image of the tree’s canopy. However tall and wide the canopy, that’s how deep and wide the roots grow, right? Turns out, that’s not the case. Though they were not the first to examine the issue, more than 25 years ago, researchers at Cornell’s Urban Horticulture Institute (UHI) definitely debunked the “mirror image” folklore around tree roots.

The UHI researchers observed root growth with the help of a rhizotron, a simple belowground viewing chamber that gives a literal window into a tree’s roots as they grow. (More recently, scientists are using ground-penetrating radar to find out where tree roots are.) The UHI researchers found that the “mirror of the canopy” portrayal of how tree roots grow is incorrect.”

—Only if it’s very dry do roots grow straight down to tap hidden water reserves, in extreme cases more than 20 stories deep. But this is a rare exception. Most roots reach down to 7 m deep. 

#Canadell, Josep et al. (1996): “Maximum rooting depth of vegetation types at the global scale”, Oecologia, vol.108, 583–595

https://link.springer.com/article/10.1007/BF00329030
Quote: “The depth at which plants are able to grow roots has important implications for the whole ecosystem's hydrological balance, as well as for carbon and nutrient cycling. Here we summarize what we know about the maximum rooting depth of species belonging to the major terrestrial biomes. We found 290 observations of maximum rooting depth in the literature which covered 253 woody and herbaceous species. Maximum rooting depth ranged from 0.3 m for some tundra species to 68 m for Boscia albitrunca in the central Kalahari; 194 species had roots at least 2 m deep, 50 species had roots at a depth of 5 m or more, and 22 species had roots as deep as 10 m or more. [...] Grouping all the species across biomes (except croplands) by three basic functional groups: trees, shrubs, and herbaceous plants, the maximum rooting depth was 7.0±1.2 m for trees[.]”

As mentioned in the source above, the maximum measured rooting depth is 68 m. If we consider that a storey is 3.3 meters:

 68 m/3.3 = 20 storeys

https://www.guinnessworldrecords.com/world-records/634276-largest-living-hardwood-tree-mass 

Quote:“The world 's largest individual tree (by both volume and mass) is a giant sequoia (Sequoiadendron giganteum) named General Sherman, located in Sequoia National Park in the Sierra Nevada Mountains of California, USA.”

https://www.parks.ca.gov/?page_id=1151 

Quote:“The entire root system is likely to be within four or five feet of the soil surface. This is an astonishingly delicate foundation for an above the ground structure that may tower upward two hundred fifty to three hundred feet (twenty to twenty-five stories) and weigh twelve million pounds (as much as a small ocean going freighter).”

—Phosphorus to build DNA, nitrogen for proteins and many more. And all of these are stealthily buried underground.

#Zhao, Boyu et al. (2024): “Microbe-dependent and independent nitrogen and phosphate acquisition and regulation in plants”, New Phytologist, vol. 242, 4, 1507-1522

https://nph.onlinelibrary.wiley.com/doi/10.1111/nph.19263

Quote:“Direct and indirect uptake pathways for plants to obtain nitrogen and phosphorus. The plants directly absorb N and P from the soil through root epidermal cells, called the direct uptake pathway. There are many ways for plants to obtain nutrients indirectly, called the micobe‐dependent pathway. For example, most plants can also form a symbiotic relationship with arbuscular mycorrhizal (AM) fungi to obtain nutrients from farther afield, called the mycorrhizal/symbiotic pathway. Part of the CO2 fixed by plant photosynthesis is released into the rhizosphere environment in the form of root exudates, recruiting specific rhizosphere microbiomes and obtaining N and P in synergy with plants. In addition, legumes can form a nodule symbiosis with rhizobia, which directly utilizes N2 in the atmosphere. C, carbon; LMWOA, low‐molecular‐weight organic acid; Ni, inorganic nitrogen; No, organic nitrogen; Pi, inorganic phosphorus; Po, organic phosphorus.”

—Each cap is filled with gravity-sensing cells, in which, tiny dense particles sink like pebbles settling in a jar of water. So the root always knows which way is down.

#Ha, Melissa; Morrow, Maria; Algiers, Kammy (2025): “Internal Root Structure”, ASCCC Open Educational Resources Initiative
https://bio.libretexts.org/Bookshelves/Botany/Botany_(Ha_Morrow_and_Algiers)/03%3A_Plant_Structure/3.02%3A_Roots/3.2.03%3A_Internal_Root_Structure
Quote:“If you were to cut a root down longitudinally, you would see the various layers inside. The tip of the root is protected by the root cap, a structure exclusive to roots and unlike any other plant structure. The root cap is continuously replaced because it gets damaged easily as the root pushes through soil. The root tip can be divided into three zones: a zone of cell division, a zone of elongation, and a zone of maturation and differentiation (Figure  3.2.3.1). The zone of cell division is a continuation of the root cap; it is made up of the actively dividing cells of the root meristem. The zone of elongation is where the newly formed cells begin to increase in length, thereby lengthening the root.

Figure  3.2.3.1 : A longitudinal view of the root reveals the zones of cell division, elongation, and maturation. Cell division occurs in the apical meristem.”

#Su, Shih-Heng; Keith, Marie A., Masson, Patrick H. (2020): “Gravity Signaling in Flowering Plant Roots”, Plants (Basel, Switzerland), vol. 9, 10, 1290
https://pmc.ncbi.nlm.nih.gov/articles/PMC7601833/
Quote:“In roots, gravity sensing occurs primarily in the cap, an organ that covers the root apical meristem. The root cap serves both as a sensor for several environmental cues and as a protective structure preventing meristem damage as the root grows in soil. Several experiments have documented a role for the root cap in gravity sensing. For instance, deleting the root cap [2], altering groups of cap cells with heavy-ion microbeam irradiation [3], or killing the cap by genetic ablation [4], all lead to defects in root gravitropism. In fact, ablating distinct layers of central columella cells in the cap of Arabidopsis thaliana roots demonstrated a predominant role for the top two layers in gravity sensing [5]. So, what makes those cells capable of gravity sensing?

A possible answer to this question can be found in the unique organization of these cells. Indeed, root-cap columella cells are rather large and characterized by a central cytoplasm mostly devoid of visible organelles or bundled cytoskeleton elements. The endoplasmic reticulum (ER) and most other organelles line their periphery, with the exception of dense starch-filled plastids (amyloplasts), which are free to sediment to the bottom under the influence of gravity [6,7,8]. When a plant is reoriented within the gravity field, columella amyloplasts settle to the new bottom-side of the cells, triggering a transduction pathway that leads to altered cellular polarization with subsequent lateral auxin transport, as described below [9].

Several lines of evidence support a role for amyloplast sedimentation and/or repositioning within these cells in gravity sensing. First, studies with starchless and starch-deficient mutants have revealed an important role for amyloplast sedimentation in gravitropism. Starch is denser than the surrounding cytoplasm, providing enough weight for amyloplasts to settle to the lower side of the columella cells. Upon plant reorientation, amyloplasts settle to the new bottom side in wild type plants. On the other hand, in starchless mutants, columella amyloplasts cannot sediment upon gravistimulation, being much lighter. Interestingly, these starchless mutants display substantially altered gravitropic responses, supporting a role for amyloplast sedimentation in gravity sensing [10,11,12,13].”

#Baba, Abu I. et al. (2022): “Plants in Microgravity: Molecular and Technological Perspectives”, International Journal of Molecular Sciences, vol.23, 18, 10548
researchgate.net/publication/363506988_Plants_in_Microgravity_Molecular_and_Technological_Perspectives?_tp=eyJjb250ZXh0Ijp7ImZpcnN0UGFnZSI6Il9kaXJlY3QiLCJwYWdlIjoiX2RpcmVjdCJ9fQ 

Quote: “Representation of gravity-sensing according to the statolith hypothesis. (A) Root tip (primary site of gravity-sensing) representing the horizontal orientation of root and auxin flow. Yellow arrows show the auxin flow after stimulation by gravity. (B) Gravity-sensing in the columella cell (primary site of gravity-sensing or perception in the root) along the gravity vector, indicated by the blue arrow pointing downwards. On reorientation of the seedling, amyloplasts (black circles) sediment in the cell along the new gravity vector, causing the root bending. The green region represents the nucleus of the columella cell and the orange region the vacuoles. (Some segments of the figure were created with BioRender.com) (accessed on 2 June 2022).”

—As it pushes forward, specialized cells detect moisture, temperature, chemical gradients and the smallest vibrations from water.

#Giehl, Ricardo F. H.; Wirén, Nicolaus von (2018): “Hydropatterning—how roots test the water”, vol. 362, 6421, 1358-1359
https://www.science.org/doi/10.1126/science.aav9375
Quote:“Where there are small-scale differences in water availability around soil particles, water potential gradients are sensed in roots (red cells). Hydrotropism guides roots towards water, whereas hydropatterning alters the distribution of root hairs and lateral roots along the root circumference (not to scale). ARF7-dependent asymmetric LBD16 expression triggers lateral root initiation on the side in contact with water.” 

#Ai, Haiyue et al. (2023): “Auxin‐dependent regulation of cell division rates governs root thermomorphogenesis”, The EMBO Journal, vol. 42, 11
https://www.embopress.org/doi/full/10.15252/embj.2022111926
Quote:“We here show that roots are able to sense and respond to elevated temperature independently of shoot‐derived signals. This response is mediated by a yet unknown root thermosensor that employs auxin as a messenger to relay temperature signals to the cell cycle.”

#Canarini, Alberto (2019): “Root Exudation of Primary Metabolites: Mechanisms and Their Roles in Plant Responses to Environmental Stimuli”, Frontiers in Plant Science, vol. 10
https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2019.00157/full Quote:“[W]e propose a novel conceptual framework for root exudates. This framework is built upon two main concepts: (1) root exudation of primary metabolites is driven by diffusion, with plants and microbes both modulating concentration gradients and therefore diffusion rates to soil depending on their nutritional status; (2) exuded metabolite concentrations can be sensed at the root tip and signals are translated to modify root architecture. The flux of primary metabolites through root exudation is mostly located at the root tip, where the lack of cell differentiation favors diffusion of metabolites to the soil. We show examples of how the root tip senses concentration changes of exuded metabolites and translates that into signals to modify root growth. Plants can modify the concentration of metabolites either by controlling source/sink processes or by expressing and regulating efflux carriers, therefore challenging the idea of root exudation as a purely unregulated passive process. Through root exudate flux, plants can locally enhance concentrations of many common metabolites, which can serve as sensors and integrators of the plant nutritional status and of the nutrient availability in the surrounding environment.”

#Gagliano, Monica et al. (2017): “Tuned in: plant roots use sound to locate water”, Oecologia, vol.184, 1, 151-160
https://pubmed.ncbi.nlm.nih.gov/28382479/
Quote:“Plants use moisture gradients to direct their roots through the soil once a water source is detected, but how they first detect the source is unknown. We used the model plant Pisum sativum to investigate the mechanism by which roots sense and locate water. We found that roots were able to locate a water source by sensing the vibrations generated by water moving inside pipes, even in the absence of substrate moisture. When both moisture and acoustic cues were available, roots preferentially used moisture in the soil over acoustic vibrations, suggesting that acoustic gradients enable roots to broadly detect a water source at a distance, while moisture gradients help them to reach their target more accurately.”

—This raw data flows into the root’s command center just behind the tip, where cells produce electrical pulses and move transmitter chemicals around. Signals from the soil are processed, interpreted and turned into decisions about where to grow.

#Kong, Xiangpei et al. (2018): “The Root Transition Zone: A Hot Spot for Signal Crosstalk”, Trends in Plant Science, vol. 23, 5, 403-409 https://www.cell.com/trends/plant-science/abstract/S1360-1385(18)30023-2
Quote: “The root transition zone (TZ), located between the apical meristem and basal elongation region, has a unique role in root growth and development. The root TZ is not only the active site for hormone crosstalk, but also the perception site for various environmental cues, such as aluminum (Al) stress and low phosphate (Pi) stress. We propose that the root TZ is a hot spot for the integration of diverse inputs from endogenous (hormonal) and exogenous (sensorial) stimuli to control root growth.”

#Baluška, František; Mancuso, Stefano (2013): “Root Apex Transition Zone As Oscillatory Zone”, Frontiers in Plant Science, vol. 4 https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2013.00354/full
Quote: “Root apex of higher plants shows very high sensitivity to environmental stimuli. The root cap acts as the most prominent plant sensory organ; sensing diverse physical parameters such as gravity, light, humidity, oxygen, and critical inorganic nutrients. However, the motoric responses to these stimuli are accomplished in the elongation region. This spatial discrepancy was solved when we have discovered and characterized the transition zone which is interpolated between the apical meristem and the subapical elongation zone. Cells of this zone are very active in the cytoskeletal rearrangements, endocytosis and endocytic vesicle recycling, as well as in electric activities. Here we discuss the oscillatory nature of the transition zone which, together with several other features of this zone, suggest that it acts as some kind of command center. In accordance with the early proposal of Charles and Francis Darwin, cells of this root zone receive sensory information from the root cap and instruct the motoric responses of cells in the elongation zone.[...]

Our recent discoveries revealed that the transition zone, although negligible with respect of cell growth, is the most active zone in the whole root apex with respect of oscillating electric spike activities (Masi et al., 2009), endocytosis-driven vesicle recycling (Mancuso et al., 2005, 2007; Schlicht et al., 2006), and oxygen demands (Mancuso et al., 2000; Mugnai et al., 2012). Electric activity peaks at the transition zone (Collings et al., 1992; Masi et al., 2009; Baluška and Mancuso, 2013a; Figure 4), and perhaps this makes this root apex zone also for an attractive target of pathogenic and symbiotic organisms (Miller et al., 1986). Until now, these synaptic and electric activities represent a mystery, but these would be rather expected on the basis of the Darwin “root-brain” hypothesis (Baluška et al., 2009b; Kutschera and Niklas, 2009; Sahi et al., 2012) first postulated by Charles and Francis Darwin more than 150 year ago (Darwin, 1880; Barlow, 2006).”

#Baluška, František et al. (2010): “Root apex transition zone: a signalling-response nexus in the root”, Trends in plant science, vol. 15, 7, 402–408.
https://pubmed.ncbi.nlm.nih.gov/20621671/
Quote: “Longitudinal zonation, as well as a simple and regular anatomy, are hallmarks of the root apex. Here we focus on one particular root-apex zone, the transition zone, which is located between the apical meristem and basal elongation region. This zone has a unique role as the determiner of cell fate and root growth; this is accomplished by means of the complex system of a polar auxin transport circuit. The transition zone also integrates diverse inputs from endogenous (hormonal) and exogenous (sensorial) stimuli and translates them into signalling and motoric outputs as adaptive differential growth responses. These underlie the root-apex tropisms and other aspects of adaptive root behaviour.”

—A single tree has hundreds of thousands of these command centers and they seem to share information with each other. 

#Perry, Thomas O. (1989): “Tree Roots: Facts and Fallacies”
https://www.arborcaresolutions.com.au/treerootfacts.pdf
Quote:“A tree possesses thousands of leaves and hundreds of kilometers of roots with hundreds of thousands of root tips.” 

#Baluška, František et al. (2004): “Root apices as plant command centres: The unique 'brain-like' status of the root apex transition zone”; Biologia, 59 https://www.researchgate.net/publication/291968255_Root_apices_as_plant_command_centres_The_unique_'brain-like'_status_of_the_root_apex_transition_zone 

—Once a root has chosen a path, fuzzy little drinking straws called root hairs, loaded with enzymes and transport proteins, begin soaking up water and dissolved minerals. 

#Ha, Melissa; Morrow, Maria; Algiers, Kammy (2025): “Internal Root Structure”, ASCCC Open Educational Resources Initiative
https://bio.libretexts.org/Bookshelves/Botany/Botany_(Ha_Morrow_and_Algiers)/03%3A_Plant_Structure/3.02%3A_Roots/3.2.03%3A_Internal_Root_Structure
Quote: “Root hairs, which are extensions of root epidermal cells, increase the surface area of the root, greatly contributing to the absorption of water and minerals.”

#Lew, Roger R. (2000): “Electrobiology of Root Hairs”  In: Ridge, R.W., Emons, A.M.C. (eds) Root Hairs. Springer, Tokyo.
https://link.springer.com/chapter/10.1007/978-4-431-68370-4_8
Quote: “Root hairs are not an obligatory cell type on the root surface. However, when present, they have highly active respiration (Connoly and Berlyn 1996), and are known to preferentially express a number of transport proteins, such as the plasma membrane H+ ATPase (Samuels et al. 1992), ammonium and nitrate transporters (Lauter et al. 1996), and a phosphate transporter (Daram et al. 1998). In fact, root hair length increases when phosphorus levels are low (Bates and Lynch 1996). The preferential expression of ion transporters and morphological responses to nutrient conditions both suggest that root hairs have special and unique functions in ion transport.”

#Nature Education (2013): “Plant-Soil Interactions: Nutrient Uptake” https://www.nature.com/scitable/knowledge/library/plant-soil-interactions-nutrient-uptake-105289112/

Quote: “Under conditions of potassium limitation, in contrast, plants usually induce high affinity K+ transport systems. There are likely many proteins involved in high affinity potassium transport, but in Arabidopsis, two proteins have been identified as the most important transporters in this process. Interestingly, one of these transporters, AtHAK5, is a carrier protein and thought to mediate active transport of potassium into plant roots, whereas the other protein, AKT1, is a channel protein and likely mediates a passive transport mechanism with an increased affinity for K+ under conditions of potassium limitation (Pyo et al., 2010). More recent work shows that plants contain a number of different transport systems to acquire potassium from the soil and distribute it within the plants. Although much remains to be learned about potassium uptake and translocation in plants, it is clear that the mechanisms involved are complex and tightly controlled to allow the plant to acquire sufficient amounts of potassium from the soil under varying conditions.” 

—But many essential nutrients are locked away in solid rock. So roots evolved to move into the finest cracks. Once in, they fill with water and swell like tiny hydraulic jacks, reaching enough pressure to break even the hardest rock. 

#Panchuk, Karla (2019): “Mechanical Weathering”, Physical Geology, First University of Saskatchewan Edition https://www.saskoer.ca/physicalgeology/chapter/8-1-mechanical-weathering-2/
Quote: “The effects of plants are significant in mechanical weathering. Roots can force their way into even the tiniest cracks. They exert tremendous pressure on the rocks as they grow, widening the cracks and breaking the rock. This is called root wedging (Figure 8.7).

Figure 8.7 Root wedging along a quarry wall. Left: Rocks beneath the thick red beds have been split into sheets by tree roots. Right: A closer examination reveals that tree roots are working into vertical cracks as well. Source: Karla Panchuk (2018) CC BY 4.0”

#Pawlik, Łukasz; Phillips, Jonathan D.; Šamonil, Pavel (2016): “Roots, rock, and regolith: Biomechanical and biochemical weathering by trees and its impact on hillslopes—A critical literature review”, Earth-Science Reviews, vol. 159, 142–159
https://geography.as.uky.edu/sites/default/files/RootsRockReg.pdf
Quote: “Biomechanical interactions between tree roots and bedrock are more obvious when growing roots encounter gaps in or weaker portion of the rock: e.g., elongated fissures, chemically altered joints, etc. When weathered, the bedrock zone can be entered through fractures as quickly as 2–3 years; the time documented for P. ponderosa on b1 m deep soil (Witty et al., 2003; p. 398). However, does the story continue? Are growing and expanding tree roots opening joints and cracks in fresh or weakly weathered bedrock? If yes, in what conditions; under the soil cover, within joints in rock outcrops? Here we refer specifically to the widening of joints or rock partings by the radial pressure of root growth, resulting in the splitting of rock masses. This mechanism is far from obvious, mainly due to the monitoring constraints, and sometimes doubted (Yatsu, 1988). Even so, several observations have been already made which pointed to such possibility (Little and Field, 2003), and tree roots can frequently be observed growing in bedrock joints that appear to have been widened. For instance, Little and Field (2003) speculated on bedrock breaking along small cracks caused by growing plant roots. Root and trunk growth is clearly capable of displacing even large rock fragments where the roots can grow under or along the rock (e.g., Phillips and Marion, 2006), but the tensile strength of most unweathered rock greatly exceeds the radial pressure than growing roots can achieve. [...] Once roots have entered rock via a joint or crack, biochemical weathering is enhanced by moisture fluxes along the root, root respiration, LMW acid production, and rhizosphere processes. Although root growth pressure cannot break up intact, unweathered rock, via radial growth roots tend to fill partings widened by biochemical weathering, thus keeping moisture and rhizosphere processes in contact with the rock.”

#Anderson, Suzanne P. (2019) : “Breaking it Down: Mechanical Processes in the Weathering Engine Available to Purchase”, Elements, vol.15, 4, 247-252 https://pubs.geoscienceworld.org/msa/elements/article/15/4/247/572796/Breaking-it-Down-Mechanical-Processes-in-the
Quote: “From a mechanical perspective, it seems unlikely that roots can break rock. Roots generate radial pressures from 0.51–0.9 MPa, while the tensile strength of rock ranges from 1–25 MPa (Pawlik et al. 2016). However, as emphasized herein, subcritical cracking occurs at stresses as low as one-tenth of the tensile strength of rock (Eppes and Keanini 2017). Over the decades-to-centuries of the growth of individual trees, subcritical cracking coupled with the undisputed geochemical interactions of the root rhizo-sphere may, indeed, promote rock fracture.”

—Next they release a mix of acids that seep into the fractures and dissolve the bonds that hold nutrients in place.

#Oburger, Eva et al. (2009): “Interactive effects of organic acids in the rhizosphere”, Soil Biology and Biochemistry, vol. 41, 3, 449-457
https://www.sciencedirect.com/science/article/abs/pii/S0038071708003714
Quote:“Organic acids released into the rhizosphere may perform many beneficial functions to the plant including metal detoxification and enhancement of nutrient acquisition. [...]Plants actively modify conditions in the rhizosphere by releasing substances from their roots. Among known root exudates, low molecular weight organic acids have drawn considerable interest due to their potential to stimulate microbial growth, detoxify potentially toxic metals (e.g., Al3+), mobilize poorly soluble nutrients (e.g., P, Fe, Zn) and accelerate mineral weathering (Jones, 1998, Dakora and Phillips, 2002, Neumann and Römheld, 1999, Ryan et al., 2001).”

—Claw-like molecules grab them and pull them in before they can slip away. 

#Parker, David; Reichman, Suzie M.; Crowley, David E. (2005): “Metal Chelation in the Rhizosphere”. In: Root and Soil Management: Interactions Between Roots and the Soil Interactions Between Roots and the Soil American Society of Agronomy Monograph No. 48 (pp.57-93) (eds) R. W. Zobel,  S.F. Wright. American Society of Agronomy, Crop Science Society of America, Soil Science Society of America
https://www.researchgate.net/publication/236160509_Metal_Chelation_in_the_Rhizosphere
Quote: “Siderophores are iron-chelating ligands that are secreted by  microorganisms and the roots of graminaceous monocots in response to Fe deficiency. Given the almost universal essentiality of Fe for life, these compounds play a key role in Fe nutrition, and are believed to be central to the overall ecology of the rhizosphere, and thus may be important in areas such as plant-disease suppression (Crowley,2001).”

—The underground networks of fungi can stretch for kilometers. 

The hyphae of even a single clonal individual can extend for an area of square kilometers.

#Guinness World Records: “Heaviest organism” (retrieved 2025)
https://www.guinnessworldrecords.com/world-records/106978-heaviest-organism
Quote: “The heaviest living "being" on Earth is a single gigantic specimen of Armillaria ostoyae honey mushroom, discovered in the Malheur National Forest of Oregon, USA, which is estimated to weigh somewhere between 7,500 and 35,000 US tons (6,800–31,750 tonnes). Known colloquially as the "Humongous Fungus", it occupies a total area of 965 hectares (2,385 acres, [10 square kilometers]), equivalent to 1,350 soccer fields, also making it the largest fungus by area.”

—They are so small that they can go where roots can’t, slipping between grains of soil to reach distant pockets of nutrients. 

#Allen, Michael F. (2007): “Mycorrhizal Fungi: Highways for Water and Nutrients in Arid Soils”, Vadose Zone Journal, vol. 6,  291-297.
https://acsess.onlinelibrary.wiley.com/doi/10.2136/vzj2006.0068
Quote: “Macropores [in soil] are larger than 80 μm, and mesopores go down to 30 μm. Many fine roots can penetrate macropores, and most root hairs penetrate mesopores. When macropores and mesopores are water filled, water transport along mycorrhizal hyphae is likely to be negligible. Fine root hairs, such as grasses, even penetrate the larger micropores. But mycorrhizal fungal hyphal tips are as small as 2 μm, capable of growing into the largest of the ultramicropores.”

—So hundreds of millions of years ago roots and fungi formed a trade alliance. The trees provide a cut from the sugars they produce far up in the sky and fungi collect and give them nutrients and water back in return.

#Cairney, John W. G. (2000): “Evolution of mycorrhiza systems”, Naturwissenschaften, vol. 87, 467–475
https://link.springer.com/article/10.1007/s001140050762
Quote: “Most terrestrial plants live in mutualistic symbiosis with root-infecting mycorrhizal fungi. Fossil records and molecular clock dating suggest that all extant land plants have arisen from an ancestral arbuscular mycorrhizal condition. Arbuscular mycorrhizas evolved concurrently with the first colonisation of land by plants some 450–500 million years ago and persist in most extant plant taxa.”

#Strullu-Derrien, Christine et al.  (2018): “The origin and evolution of mycorrhizal symbioses: from paleomycology to phylogenomics”, New Phytologist, vol. 220, 4 https://nph.onlinelibrary.wiley.com/doi/10.1111/nph.15076
Quote: “Usually the fungus receives carbon from the photosynthetic host, which in return is supplied with essential soil elements (e.g. nitrogen (N), phosphorus (P)).

Figure 1: Geological timescale with oldest known fossils (right: Feist et al., 2005; Friis et al., 2011; Honegger et al., 2013; Krings et al., 2011a,c; LePage et al., 1997; Redecker et al., 2000; Remy et al., 1994; Rothwell et al., 2012; Stein et al., 2012; Strother et al., 1996; Strullu-Derrien et al., 2009, 2014; Taylor et al., 1999) and antiquity of genomic traits based on molecular clock estimates (left: Floudas et al., 2012; Kohler et al., 2015; Martin et al., 2016). The asterisk represents the Rhynie chert. AM, arbuscular mycorrhizas; CAZymes, Carbohydrate-Active enZYmes; CMm, coil-forming mycorrhizas in Mucoromycotina; MiSSPs, mycorrhiza-induced small secreted proteins; PCWDEs, plant cell wall-degrading enzymes.”

—Some fungi grow directly into the root’s cells, building tiny trade posts, where sugars and minerals change hands.

#Strullu-Derrien, Christine et al.  (2018): “The origin and evolution of mycorrhizal symbioses: from paleomycology to phylogenomics”, New Phytologist, vol. 220, 4
https://nph.onlinelibrary.wiley.com/doi/10.1111/nph.15076
Quote:“Usually the fungus receives carbon from the photosynthetic host, which in return is supplied with essential soil elements (e.g. nitrogen (N), phosphorus (P)).[...]
Mycorrhizal associations in living species take several forms that involve different plant and fungal clades (van der Heijden et al., 2015). Endomycorrhizas, the most widespread associations, include the arbuscular mycorrhizas (AM) formed by the Glomeromycotina. These fungi commonly colonize roots and sometimes also the rhizomes (Wang & Qiu, 2006; Smith & Read, 2008; Pressel et al., 2016) of vascular plants and the thalli of early-diverging land plants, namely liverworts (Marchantiophyta) and hornworts (Anthocerotophyta) (e.g. Strullu, 1985; Read et al., 2000; Selosse, 2005; Duckett et al., 2006; Pressel et al., 2010; Desirò et al., 2013). The AM symbiosis is thus phylogenetically widespread in plants. Hyphae grow in the apoplastic space between plant cells and they penetrate cells where they form arbuscules (i.e. branched structures involved in nutrient exchange between the plant and the fungus; Bonfante & Genre, 2010).” 

#Sessoms, Florence (2020) : “Arbuscular mycorrhizal fungi: tiny friends with big impact”, University of Minnesota Turfgrass Science News https://turf.umn.edu/news/arbuscular-mycorrhizal-fungi-tiny-friends-big-impact Quote:“Arbuscular mycorrhiza fungi (AMF) are soil microorganisms able to form mutualistic symbiosis with most terrestrial plants. Spores that are present in soil germinate, infect the root system, and form arbuscule structures inside the cells (Figure 1). Arbuscules are the site of nutrients exchange between the plant and the fungi. Another characteristic of this symbiosis is the presence of a large mycorrhizal network around the root system. 

Figure 1. Schematic representation of AMF establishment inside a host and the known exchange between the two partners. Figure credit: Florence Sessoms.”

—Others wrap themselves around root tips, weaving between its outer layers, insulating delicate tissues and protecting them against microorganisms.

#Strullu-Derrien, Christine et al.  (2018): “The origin and evolution of mycorrhizal symbioses: from paleomycology to phylogenomics”, New Phytologist, vol. 220, 4
https://nph.onlinelibrary.wiley.com/doi/10.1111/nph.15076
Quote:“Another type, the ectomycorrhiza (ECM), is distinguished on the basis of a sheath of fungal hyphae enveloping the root and an intercellular penetration pattern where the hyphae form a network between cortical cells called a Hartig net (Strullu, 1985; Smith & Read, 2008). Many species of Ascomycota, Basidiomycota and a few members of the genus Endogone (Mucoromycotina) form ECMs (Yamamoto et al., 2017). The associated plants are mostly shrubs and trees from temperate, boreal and Mediterranean regions, but there are also some ecologically important tropical families, including Dipterocarpaceae, Myrtaceae, Caesalpinioideae and Fagaceae (e.g. Alexander, 2006; Smith & Read, 2008; Tedersoo & Brundrett, 2017).”

#Gonzier, Paolo et al. (2019): “An ectomycorrhizal symbiosis differently affects host susceptibility to two congeneric fungal pathogens”, Fungal Ecology, vol. 39,  250-256 https://www.sciencedirect.com/science/article/abs/pii/S175450481830103X
Quote:“Ectomycorrhizal fungi can protect plants against root pathogens via mechanisms such as forming a physical barrier around (hyphal mantle) and within (Hartig net) the roots that restricts the pathogens access to infection sites and photosynthates, or by synthesis of antibiotics, volatile and non-volatile compounds, and cell wall degrading enzymes (Reviewed by Ghorbanpour et al., 2018).”

#Nature Education (2013): “Plant-Soil Interactions: Nutrient Uptake”

https://www.nature.com/scitable/knowledge/library/plant-soil-interactions-nutrient-uptake-105289112/ 

—Today there are thousands of fungal tree ally species, each with its own specialties. 

#Heijden, Marcel G. A. van der et al. (2015): “Mycorrhizal ecology and evolution: the past, the present, and the future”, New Phytologist, vol. 205, 1406-1423. https://nph.onlinelibrary.wiley.com/doi/10.1111/nph.13288
Quote:“Estimates suggest that there are c. 50 000 fungal species that form mycorrhizal associations with c. 250 000 plant species.”

—Some only partner with specific tree species, while others are happy to work with almost anyone. 

#Swiss Federal Institute for Forest, Snow and Landscape Research (2011): “Mycorrhiza – a fascinating symbiosis in the forest”
https://www.waldwissen.net/en/forest-ecology/forest-plants/plant-ecology/fascinating-mycorrhiza
Quote:“Many mycorrhizal fungi are host specific, which means that they only grow with specific tree species (i.e. larch bolete (Suillus grevillei) or oak milkcap (Lactarius quietus)). Others grow exclusively in deciduous forests, or coniferous forests.”

—These connections often knit the roots of many trees together  into vast underground networks. 

#SPUN: “Mycorrhizal fungi” (retrieved 2025)
https://www.spun.earth/networks/mycorrhizal-fungi 

—Under just one cubic meter of healthy forest floor, fine tree roots can stretch for several kilometers and for every kilometer of root, there can be hundreds of kilometers of fungal networks. 

As discussed above, most roots concentrate in the upper layers of the soil, so we expect the total number of kilometers of root and hyphae under one square meter to be of the same order of magnitude as the number of kilometers in the upper cubic meter.

#Germon, Amandine et al. (2018): “Consequences of mixing Acacia mangium and Eucalyptus grandis trees on soil exploration by fine-roots down to a depth of 17 m”, Plant and Soil, 424, 1-18
https://www.researchgate.net/figure/Root-length-index-km-m-a-and-root-area-index-mm-b-in-the-0-1m-1-2m-2-4m_fig5_320300192 

Quote: “Root length index (km m⁻²) a) and root area index (m² m⁻²) b) in the 0–1 m, 1–2 m, 2–4 m, 4–6 m, 6–9 m and 9–17 m soil layers for Acacia mangium monospecific stands (100A), Eucalyptus grandis monospecific stands (100E) and the mixed stands (50A50E). Different upper-case letters indicate significant differences between treatments for the cumulative indices and different lower-case letters indicate significant differences between treatments within each individual soil layer (p < 0.05)”

#Bioneers (2023): “The Wood Wide Web: The Intelligent Underground Mycelial Network”
https://bioneers.org/the-wood-wide-web-the-intelligent-underground-mycelium-network/
Quote: “There are thousands of kilometers, even under a square meter of soil, of fungi linking all these plants together.”