A Tree Grows… How?

Valorie’s currently attending a seminar about teaching science at the middle-school level.

Yesterday, her instructor asked the following question:

“I have in my hand a Douglass Fir tree seed that masses 1 gram [I’m making this number up, it’s not important].  I plant it in my yard, water it regularly, and wait for 20 years.

At the end of that time, I have a 50 foot tree that masses 1,000 kilograms [again, I’m making this exact number up, it’s not important].


My question is: Where did the 999,999 grams of mass come from?”


I’m going to put the answer out to the group.  Where DID the 999,999 grams of mass come from in the tree.

The answer surprises a lot of people.  And it brings to question how much we actually know about science.



I’m going to change my comment moderation policy for this one.  I’m NOT going to approve people whose comments have the right answer, until I post my follow-up post tomorrow, because once the right answer’s been given, it’s pretty clear.  But I’ll be interested in knowing the percentage of comments that have the right answer vs. the wrong answer.


Comments (100)

  1. Jimbo says:

    From the air.  The tree takes carbon from the air.

  2. Michael says:

    I would think it came from some combination of nutrients in the soil, carbon dioxide in the air, and water.

  3. isaac says:

    The air.

    More specifically, most of the tree is made from carbon fixed from the CO2 in the air.

  4. Chris Slatt says:

    If I remember my High School science correctly, the mass comes from the Carbon in the Carbon Dioxide it takes in from the air.

  5. Ryan Phelps says:

    Mostly water I bet.  Probably some carbon from the atmosphere to make it burn later.  There’s certainly some minerals and stuff from the ground, but if I remember 9th grade biology correctly, it’s mostly water.

  6. Chad says:

    Mostly mass displaced by the root system?

  7. Fun-damentalist says:

    God. It came from God. πŸ™‚

  8. dispensa says:

    We are all made of stars. πŸ™‚

  9. Edward says:

    Isn’t it mostly water, carbon and nitrogen from the soil?

    Basically rainwater and dead plant matter.

  10. Energy from the sun turned into mass via photosynthesis?

  11. Richard says:

    Carbon, from carbon dioxide in the air. It’s an important part of the carbon cycle, and is the exact opposite of the release of carbon when fossil fuels are burnt (answering the question "why are ashes lighter than the wood they came from?" which plagued the philosophers who came up with the four-elements model of matter).

  12. Justin Bowler says:

    The majority of the mass comes from the air and the water.

    Photosynthesis converts carbon-dioxide from the air into oxygen and carbon. The majority of the oxygen is released back to the atmosphere, and the carbon is then combined with water to form various fibrous tissues (essentially long chain carbohydrates) that make up the cell walls. Also, the inner liquid of cells are (like animals) mostly water.

    There are of course other elements (phosphates, nitrogens, traces of sulphates and various metals) that are used as well, but the majority of the mass of a tree is carbohydrates and water.

  13. Jonny D says:

    It gets it’s mass from:

    Water! (most of the mass is water, taken from the ground)

    Stuff from dirt! (disolved in the water, it takes in minerals & other nutrients from the soil)

    Air! (trees breath air, and the gases get used in the tree.)

    Of course, Water is far and away the largest contributor, behind that soil, and the rest is really far behind…

  14. Kumar says:

    photosynthesis ?

  15. Ave says:

    CO2 from atomsphere during the photosynthesis process?

  16. Donnie Russell says:

    The tree takes in carbon dioxide from the surrounding air, water, and through photosynthesis creates glucose. The carbon gets "fixed" in the plant over time, so I would say most of the plant’s mass comes from C building up in it over time. The reversal of this process, moving carbon from plants and their remains, back into the atmosphere is of course at the root of the problem of the greenhouse effect.

  17. Doug McClean says:

    Essentially, it came from "thin air." About 2/3 (60/96 may be a better estimate) of it came from atmospheric carbon dioxide and the other 1/3 came from water (I assume mostly groundwater). Some small and probably insignificant fraction came from various other things in the soil.

  18. Ummm… the only unlimited physical inputs are air and water, so lets take a look.  I assume the composition of organic matter is probably about 50 % carbon and the rest oxygen and hydrogen.  There is obviously water in a tree as well.  So the inputs are the carbon from the carbon dioxide a tree respirates plus some hydrogen stolen from water and some oxygen stolen from water and/or carbon dioxide.  Obviously, most of the tree comes from carbon dioxide.

  19. JST says:

    Lots of water, mainly via roots, some via dew.

    Lots of carbon from atmospheric CO2.

    Lots of hydrogen from water via roots.

    Bits of nitrogen, potassium, etc. via roots.

  20. Tomas Särnhammar says:

    Mostly, from the air (carbon dioxide for example).

  21. Christian says:

    Tiny bit comes from the earh (minerals).

    The rest: from water and the CO2 in the air:

    6 H2O+6 CO2= C6H12 + 9 O2

    (if I remember correctly)

  22. Nar says:

    Carbon from the air, other stuff from the soil?

  23. Yuliy Pisetsky says:

    Most of that mass comes from water and carbon dioxide, as those are used to make sugars. Some nitrogen for proteins comes from the ground, but that isn’t too much overall.

  24. Naresh says:

    The tree grows by forming new branches and leaves.  The stems grows thicker and stonger.  All these are madeup of celss.  So the cells of the tree multiply.  The roots absorb nutrients from the soil and provide it to the tree.  This helps in the formation of the cells.  This is how a 1 gram seed becomes a 1000 kilogram tree.

  25. alanpa says:

    Are you looking for an answer as simple as Photosynthesis? If I were to be more specific, I’d say that the growth comes primarily from air (and even more specific – carbon dioxide). Light, water, and soil (or nitrogen and minerals from the soil) just enable them to process the carbon dioxide…I guess.

  26. Mike says:

    Simple: The nutrients from the organic matter in the ground. Where else?

  27. Diffuse says:

    I think most of it come from carbon dioxide in the air during the photosynthesis process… but I’m guessing here…

  28. Gwyn says:

    Mostly it comes from the air. Trees are carbon based lifeforms (like us), air contains carbon (in the form of carbon dioxide).

  29. Markus Gölzner says:

    Quite easy, it’s Photosynthesis: Water, carbon dioxide and sunlight are the main ingredients to let our plants grow.


  30. Michael Cook says:

    I think the answer is simple. We know that matter can neither be created nor destroyed, so it must come from somewhere.

    What do trees get? Water, dirt, air, and sun.

    Sunlight provides energy, but nothing else.

    Water provides a few things. Water provides raw building blocks (Hydrogen and Oxygen), as well as impurities (minerals and such) that the plant needs. The water also carries other chemicals (like from fertilizer and organic matter in dirt) into the tree.

    All that leaves us with is air, which I would suspect would make up the bulk of the added mass. Trees, like everything we know, are carbon based life forms. And while trees may be able to get some carbon from the soil (I don’t know) most of it comes from their changing of CO2 into O2. After all, trees don’t create Oxygen and blocks of Carbon, which means they must retain that Carbon to be used as building blocks.

    So I would say all that "new" mass comes from the air, the soil (this includes minerals transported by the water), and possibly the water, in that order.

    Interesting question though. I won’t what most people’s answers would be (like a random poll on the street).

  31. Nathan Lewis says:

    I’m pretty sure that (most of) the mass comes from the sugar produced during photosynthesis converting CO2 and H2O into C6H12O6.

    Every time this reaction occurs:

    6 CO2 + 12 H2O + light -> C6H12O6 + 6 O2 + 6 H2O

    6 Carbon Dioxide + 12 Water + Light -> 1 Glucose -> 6 Oxygen + 6 Water

    The oxygen is released, and some water is also released which may stay in the plant.  But the glucose certainly stays.

  32. J.Marsch says:

    Here’s my best shot (I’m definitely not a biologist, so I probably have this all wrong).

    The important thing to realize is that vast majority of the mass did NOT come from the the soil (ok, a few elements, like the nitrogen did).

    Some of it is water.

    The solids are organic molecules (largely protiens) made up of Carbon, Hydrogen, Sulphur, Potasium, Nitrogen, and some other stuff that I can’t remember what it is.

    The carbon that is used to make of the solids comes from Carbon Dioxide.  (incidentally, plants don’t really release oxygen by splitting carbon dioxide, they release oxygen by splitting water).

    Most of the mass comes from the manufacture of protiens and sugars from Carbon Dioxide from the air, hydrogen, extracted from water from the ground, and a few trace minerals and elements from the soil.  All powered by photosynthesis.

  33. J.Marsch says:

    Here’s my best shot (I’m definitely not a biologist, so I probably have this all wrong).

    The important thing to realize is that vast majority of the mass did NOT come from the the soil (ok, a few elements, like the nitrogen did).

    Some of it is water.

    The solids are organic molecules (largely protiens) made up of Carbon, Hydrogen, Sulphur, Potasium, Nitrogen, and some other stuff that I can’t remember what it is.

    The carbon that is used to make of the solids comes from Carbon Dioxide.  (incidentally, plants don’t really release oxygen by splitting carbon dioxide, they release oxygen by splitting water).

    Most of the mass comes from the manufacture of protiens and sugars from Carbon Dioxide from the air, hydrogen, extracted from water from the ground, and a few trace minerals and elements from the soil.  All powered by photosynthesis.

  34. Euro says:

    Oh, the immense majority of the tree mass has to come from gaseous Nitrogen, Carbon, Oxygen and Hydrogen, absorbed from the atmosphere, in the form of N2, O2, CO2, as well as water from the soil and the air.

    A small amount of other "stuff" must be absorbed from the soil, which is why you need a "fertile" soil or to add fertilizers. "Stuff", is mainly Phosphorus and other trace components such as iron and sulphur. The natural source of those components are minerals in the soil’s rocky components and decomposed organic material from prior life forms, such as other trees.

    I believe that trees can absorb Nitrogen from the soil too, but I don’t think there is enough there — that’s why it must get the rest from the air.

    (FYI: No biology classes since high school 20 years ago, from a foreign country)

  35. Adam says:

    Mostly from the air and water in the ground. From photosynthesis:

    16 x CO2 (carbon dioxide) + 6 x H2O (water) -> C16H12O6 (glucose) + 16 x O2 (oxygen)

    Or couse, there are a lot of things you can make other than glucose, but that’s a start. Most of the tree that’s not liquid water is going to be some carbohydrate or other built up in a similar fashion. Either directly from carbon dioxide and water, or possibly from smaller carbohydrates that started out that way. And the sun provides all the power.

    There’s a reasonable amount of other things. Nitrogen is probably next – which IIRC doesn’t come from the air but is absorbed through the roots after it’s been fixed into some other compound or other from the soil by bacteria. Then stuff like Iron and Sodium and tiny amounts of a whole bunch of other things which is probably absobed in solution from the water sucked up by the roots. I’m less sure about that though – biology wasn’t my strong point. More of a Maths/Physics/Chem/Comp Sci type.

    I’m *fairly* sure that’s mostly right … your phrasing leads me to believe that it may be different. I’m looking forward to your next post though.

  36. Rich says:

    The tree is approx 98.6% comes from water… and the rest is carbon and trace elements… (just a guess)

  37. Jason says:

    From the nutrients in the soil and the carbon dioxide used in photosynthesis?

  38. PaulV says:

    Water and CO2 …

    6 CO2 + 12 H2O + light β†’ C6H12O6 + 6 O2 + 6 H2O

  39. Andreas Johansson says:

    It grows because of photosynthesis.

    It takes carbon dioxide from the air and gives out oxygen, what is left from the process stays inside and makes out the actual tree.

    It needs sunshine to work.

  40. Mat Noguchi says:

    The sun and oxygen? There has to be a chemical reaction somewhere, and the only chemical reaction that new when you add the seed is photosynthesis.


  41. BrianK says:

    The new mass comes from the cells of the tree. As they grow and produce more cells, their bodies are the wood. Also they contain water which adds to the weight.

  42. Matt Lee says:

    CO2 from the air as well as the various kites, frisbees, and balls that get stuck in them when you play under them.

  43. Rex says:

    Cellular Division (aka growth)

    Though I bet the real answer surprises me πŸ˜›

  44. Joseph says:

    Much of the mass is water, a small portion is mineral content from the dirt, and the rest is carbon dioxide converted to sugars and other good stuff by way of photosynthesis.

    My guess, based on recent tree felling experience: 2/3 water, 1/100 minerals from the ground, and the rest is sequestered carbon dioxide.

  45. Alex says:

    From CO2 in the air, light and water?

  46. shivram says:

    Primarily from Carbon Dioxide in air and water which gets converted by photosynthesis using Sun’s energy into the carbohydrates that plants are composed of (mostly complex carbohydrates like cellulose).

    Looked another way all the mass in a tree comes from other parts of the earth.

  47. Gareth says:

    I’d guess that the mass contains some of the nutrients from the soil, but consists mostly of the sugars produced as the result of photosynthesis – so it consists mostly of stored up solar energy.

  48. BIM says:

    Most of it came from photosynthesis – carbon dixide from teh air plus water via the roots.

  49. Cory C says:

    The sun via photosynthesis.

  50. Sean says:

    produced by photosynthesis

  51. Barry Kelly says:

    Working backwards from burning, where most of the material turns into smoke / water vapor / carbon dioxide & monoxide, I would think that most of the mass comes from thin air – namely, carbon dioxide from the air to create carbon-based compounds, the building blocks of life etc etc. Probably a substantial amount of the mass comes from water in cells too, just like animals, but that ultimately comes from the air too, falling to ground as rain.

  52. Steve Downey says:

    C02 + H20 + light => carbohydrates + O2

    That’s most of it. Of course there’s also nutrient uptake for nitrogen and phosphorus, which are necessary for the plant to break down the carbohydrates to do work. But they don’t amount to that much.

  53. john k says:

    My guess is most of the mass (carbon) comes from CO2 in the air. The rest (hydrogen, oxygen) from water and other trace minerals from the soil.

  54. Dean Harding says:

    A tree uses photosynthesis to grow. Basically it uses enegry from the sun to make sugars from the carbon dioxide in the air. The actual grow only happens at the very tips of the branches – the trunk and branches themselves only grow in diameter (which is why you can hammer a nail into a tree and the nail will never move).

  55. Anony Moose says:

    I’m guessing that there’s some specific answer, rather than a generic "nutrients that provide material for cell growth" answer. (Do plants extract chemicals from the air, as well as the soil? I wouldn’t put it past the sneaky buggers.)

    Oh, and I would suspect that carbon-based nutrients are the big contributors – probably CO2 from the air split up and recombined with other nutrients to form the organic molecules we all know and love. When they convert CO2 to O2, the C has to go somewhere, right?

    Well, it’s either that or the whole theory of thermodynamics is flawed and plants actually create matter out of absolutely nothing, and I’m not betting on that being the correct answer. But it would be useful if they did.  πŸ˜‰

  56. It comes from photosynthesis removing the C from the CO2 in the atmosphere.  There are certainly minerals removed from the soil as part of the growing process but they are trace amounts.  Note that when a tree or a garden grows, the dirt level doesn’t sink, does it?

    This is fundamentally why plants are valued as carbon sinks – they hold C that was previously in the air.

  57. ty says:

    If the answer being looked for is "the sun," it’s highly incomplete.  The correct answer would be all the things mentioned by Jonny D, PLUS "the sun".

    The simple answer of "the sun" implies mass-energy equivalence a la Einstein.  This is, of course, incorrect.  The mass already existed.  No mass was added by the sun.  The energy captured by photosynthesis only supplies enough energy for *chemical* reactions, transforming CO2, N2, and other products found in the air and the ground into cell matter.  

    Given the phrasing of the question: "Where did the 999,999 grams of MASS come from," the answer is not the sun.  The sun added ZERO mass; all the mass was already present in the air and in the ground.  All it did was increase the energy present in chemical bonds.

    Chemical reactions, as we all know from high school chemistry (ah, that’s the problem, this is middle school), obey conservation of matter.  Only nuclear reactions can add or subtract mass, by converting from and to energy.

  58. Norman Diamond says:

    OK I’m amazed.  Since Mr. Osterman said he will pend correct answers but he completed the posting of answers from Edward and Jonny D, it indicates that water isn’t the biggest part of the correct answer.  Leaves do evaporate a lot of water independently of absorbing carbon from air, but I’d still have thought most of the tree’s mass was water, just as most of a human’s mass is water.  So, I give up.

  59. Brett says:

    Carbon Dioxide + Light

  60. Thales says:

    Carbon dioxide

  61. Grant says:

    The mass comes from carbon dioxide in the atmosphere.

  62. Alan De Smet says:

    The air.  High school biology: photosynthesis takes CO<sub>2</sub> and yields O<sub>2</sub>.  The plant holds on to the carbon.  While clearly the carbon isn’t all of the mass of the plant (water is another big part), it’s a major component.

    (This comment system need a preview feature so people can confirm their formatting works.)

  63. Chris in Atlanta says:

    Energy from the sun.

  64. Maurits says:

    A tree holds a certain amount of water which is absorbed by the roots and passes through the tree to the leaves.  That accounts for some of the mass.  When the tree dies, it dries out, and is still very massive.

    The growth of the cells is powered by

    * water from the ground and (to a lesser extent) the air

    * sunlight which powers photosynthesis

    * nutrients from the soil (mostly nitrogen from earthworm poo)

    * carbon dioxide from the air

    Sunlight has no mass, but adds energy.  The chemical reaction of photosynthesis transforms all these elements into oxygen (which the tree releases as waste,) complex carbohydrates and lipids that make up the new cell walls, and whatever else goes into making a tree.  The interior of the living cells is mostly water.

    The water is then replenished by the evaporation of the ocean, which is powered by sunlight.

    The soil is replenished by dust, blown from the Sahara and the occasional volcanic eruption.  Oh, and the death of other plants, but that’s a bit of a cyclic argument.

    The carbon dioxide is replenished by all those pesky air-breathing animals that are always running around.

  65. Cheong says:

    Soil + fertilizer -> The macro-nutrients is responsible for building the vital substances of a plant.

    Air -> Carbon dioxide change to carbohydrates by photosynthesis, and cellulose (a polysugar) is a major buildup material for stems.

    Water sure do contribute, but is relatively unimportant because most are lost by transpiration.

  66. B.Y. says:

    My guess is: air (carbon, nitrogen), water, and soil.

  67. artichoke says:

    carbon in the air

  68. Cheong says:

    EDIT: Water is relevent not important "for the growth of net mass". Of course the xylems and phloem and "vacuoles of plant cells" are mostly filled with water. My previous post if concren about the growth in "net mass" only.

  69. ericgu says:

    Trees are mostly cellulose, which is a carbohydrate.

    The carbon comes from the carbon dioxide in the air, and the hydrogen and oxygen comes from water. Oxygen is the by product.

    You also need nitrogen to make amino acids, and other trace minerals.

  70. Tom says:

    Surely most of it comes from the Carbon it absorbs from the air? I’d hazard a good 60% is that, perhaps more – then it’s water and the final tiny little bit is nutrients and suchlike from soil.

  71. Jody says:

    Mostly carbon from carbon dioxide in the air and then other nutrients from the soil.

  72. Jorge Coelho says:

    Those 999.999 kg of extra mass come from (where added to) the top (tips) of the tree – i.e.; trees do not grow from the ground up, but from the top up. Is that the answer you were looking for?

  73. benj says:

    Hmm… so it’s not water, not soil, not air.


  74. degert says:

    Air (or more specifically CO2)

  75. Lorenzo says:

    From the sun. Energy comes from the sun, mass = energy, ecc… Everything ultimately comes from nuclear fusion energy in the stars

  76. Paul Kavanagh says:

    The Sun ,via photosynthesis.

  77. Clueless says:

    Okay, let me embarass myself my being completely off.

    Almost all life consists mainly of hydro-carbons and water. All other stuff is probably insignificant. So where do plants get hydro-carbons from? Well, I know that plants produce oxygen from CO2 in the air (which is produced by animal life), which means that they get to keep the carbon, and produce oxygen. The water in the ground (H20), can also be used to produce oxygen, and keep the hydrogen. Carbon plus hydrogen should therefore suffice to build hydro-carbons.

    Since hydrogen weighs little in comparison to carbon (16 times less?), I conclude that plants get the majority of their mass from the air (not oxygen, but "air") itself.

  78. Huw Lynes says:

    air and water, the wonders of photosynthesis

    Sunlight + H2O + CO2 =  Tree + Oxygen

    this also mostly works in reverse as well. If you set a tree on fire what you mostly end up with is H2O and CO2

  79. procyon says:

    Mostly carbon from air (carbondioxide) and water from soil.

    The equation (for photosynthesis, I didn’t balance it but the idea remains the same) was something like CO2 + H2O = glucose, IIRC

  80. Sam Jack says:

    I guess it comes from sunlight, via photosynthesis!

  81. MikeE says:

    I’ll bite — it’s the air.  Carbon, oxygen, nitrogen – mix with a little energy from the sun and a dash of water and you get a tree.  Root systems are basically there to provide water and some nutrient transport, but it’s not where the bulk of the mass comes from.

  82. Tim Meadowcroft says:

    Mostly from the air, and quite a bit of water from the ground.

    The wood is formed from carbon which it gets from CO2 in the air (sunshine providing the energy to extract the split the CO2), and water which it sucks up through the roots.

    If it got the carbon from the soil – there’d be a huge hole around the base.

    Trees are solidified air – simplified version…

  83. Derek says:

    Water + CO2

  84. From the carbon atoms in CO2.

  85. Mark Sowul says:

    Energy from the sun?

  86. The mass comes from the carbon atoms in CO2.

  87. bmm6o says:

    It seems everybody agrees that water is a big one.  I don’t think much of the mass comes from the soil, and while the tree does get energy from the sun, the mass of this energy is going to be miniscule (e=mc^2).  I think most of the rest is carbon from CO2.

  88. KiwiGreen says:

    Cellulose, which probably makes most of the mass of the tree is C, H and O. These come from CO2 (atmosphere) and H2O (soil/atmosphere).

    AFAIK, there are no nuclear reactions going on in the tree, so "energy from the sun" won’t be the correct answer πŸ™‚

  89. Josh Goldshlag says:

    Mostly from the Carbon and Oxygen in CO2, as well as Hydrogen from H2O. They also get Nitrogen from bacteria that fix nitogen in the soil.

  90. Orion Adrian says:

    Trees grow from absorbing carbon from dioxide in the air. This accounts for a couple things.

    1) The fact that they burn very well; carbon is a necessary ingredient in oraginic molecules

    2) The fact that they don’t release or excrete carbon. Therefore it has to still be there.

    They also absorb water from their roots and in some plants also their leaves. This is what gives the plant it’s springiness.

    They also absorb other elements from the roots (e.g. nitrogen).

    Some plants like the Venus Fly Trap absorb nutrients (i.e. mass) from insects that are trapped.

  91. Edward says:

    Photosynthesis doesn’t create matter from energy, it just converts one set of molecules into another set.

    6 CO2 + 12 H2O + light β†’ C6H12O6 + 6 O2 + 6 H2O

    The C02 comes from the air, the water from the ground,

    So most of the mass of the tree ends up being Carbon that came from the air?

  92. Yesterday, I asked where the mass comes from in a tree.

    The answer is actually really simple: Carbon.&amp;nbsp;…

  93. bmm6o says:

    Edward: Photosynthesis doesn’t create matter from energy

    It wouldn’t have to.  Conservation principles say that the matter on the right of your equation will be (immeasureably) more massive than the matter on the left.  So it’s not true that the tree gets *none* of its mass from the sun’s energy, but it amount is going to be less than your measurement error.

  94. Richard Rudek says:

    Ah, but I think a more interesting question is wether there is any benefit to using the CO2 output from the burning of fossil fuels, such as coal-fired power stations, should be siphoned off and piped into extremely large greenhouse like structures, to increase the growth rate of plants such as Sugarcane. The Sugarcane, in turn, being used to produce Alcohol/Methanol in place of, or as additives to Gasoline.

    It’s not quite a renewable energy methodology, but I suspect that it is a way of slowing down the rate of new CO2 production… πŸ™‚