So where DOES the mass of a tree come from?

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

The answer is actually really simple: Carbon.  Photosynthesis is the act of converting CO2 from the air into O2 and a bit of H2O. 

It turns out that if you ask new Harvard graduates this question, the vast majority of them answer some variant of "It comes from the soil".  When people think of photosynthesis, they don't think about the carbon that's left behind.  They'll usually be puzzled as they answer, because in their hearts, they realize that "the soil" doesn't actually work as an answer, but they can't quite put all the pieces together.

If you ask 7th graders the same question right after they've finished their photosynthesis unit, they end up coming up with a variant of the same answer.  Or they say it comes from the water the plant absorbs. Soil and water have mass, and that seems to the determining factor in their answer.

You see, 7th graders don't seem to get the idea that air contains mass - it's just the stuff that's around them, it doesn't "weigh" anything.  Since they have always been exposed to the weight of air, it doesn't occur to them that it has any real properties at all.  If you show them a block of dry ice, and ask them what it is, they'll say "It's ice". If you ask them to weigh it, they get that part.  It's not until you follow that up and ask "So what's happening to the ice?" and they realize that the fog it's generating is disappearing into the air that they'll start figuring out what's happening - that the mass of the dry ice is being absorbed into the air.  The very smartest of the kids will then put the pieces together and realize that air DOES have mass.

Yesterday's "quiz" netted over 70 correct answers and about 15 incorrect answers, I've got to say that I'm pretty impressed. The reality is that since I only let the incorrect answers through, it biased the sample towards correctness (several people mentioned that they'd read the other comments and realized that their first thought was wrong).


Valorie had one more question: Could those of you who got the answer right, and who are under 30, and who were educated under the American education system post a comment below?


Comments (51)

  1. Anonymous says:

    "Could those of you who got the answer right, and who are under 30, and who were educated under the American education system post a comment below?"

    Sure thing. (18yr old)

  2. Anonymous says:

    A related question: when you fill a balloon with helium, does the mass increase or decrease?

  3. Anonymous says:

    I got it right and I’m 26 years old and went to US schools my whole life.

  4. Anonymous says:

    Well, I don’t see my comment from yesterday posted yet, but that means I got it at least partially right.  I learned in 9th grade biology, in 1994, that most of the mass comes from water.  Mr. Anderson was not completely correct it appears.

    Then I thought about it as I typed, and realized that we were burning trees at the camping trip I was on this weekend and producing carbon dioxide.  This means they need a lot of carbon, which is what makes them organic anyway.  That could only come from the air.

    I’d be interested to know what percentage of a tree is water and how much is carbon though.  Here’s how much of Soylent Green is water, although it depends on gender and fat/lean tissue ratios:

  5. Anonymous says:

    I was educated in the Cambridge, MA public schools and am under 30 (just barely)

  6. Anonymous says:

    I fit those criteria, even though I didn’t respond yesterday.

    I realized this fact after letting plants sit in a bowl of water and watching them grow, and realizing that the added matter must be coming from somewhere other than the soil.  And the fact that earth’s life is "carbon-based".

  7. Anonymous says:

    "A related question: when you fill a balloon with helium, does the mass increase or decrease?"

    The mass increases, but the density decreases. Eventually buoyancy causes it to rise.

  8. Anonymous says:

    Here’s a fun question:

    You’re in a boat in a lake. There’s a rock in the boat. You pick up the rock and drop it into the lake, and the rock sinks.

    1) (easy) How does the boat move relative to the water (up or down)?

    2) (moderate) How does the level of the water change relative to the shore (does it go up or down)?

    3) (hard) How does the boat move relative to the shore (does it go up or down)?

    For bonus points, in each case, how big is the change?

  9. Anonymous says:

    I got it right, I’m 26.  Educated in the US.

  10. Alan Page says:

    I’m 10 years beyond 30, but I remember doing an experiment in HS – or maybe it was JH where we proved that mass does *not* come from water (experiment involved starting with "dried" dirt, weighing a plant, watering the plant with exactly measured amounts of water and caring for the plant for a month or so, then extracting the plant, drying the dirt and weighing the plant again).

    I can’t remember if we took evaporation into account, but the overall growth of the plant was less than 1% of the watered amount – ymmv.

  11. Anonymous says:

    I didn’t bother replying, but had it right. Something else to think about: with all the talk about hybrid vehicles, ethanol, and other potential sources of fuel these days, i’d expect a more wide-spread understanding of the carbon cycle to emerge…

  12. Anonymous says:

    I was educated in the US and under 30, though I spent my high school years in a magnet school.

    "when you fill a balloon with helium, does the mass increase or decrease?"

    Its mass increases, but the balloon + helium becomes less dense as the helium is less dense than the original ballon. Once this density drops below the density of the surrounding air it lifts off. This is also why a deflating helium balloon drops back to earth — it’s becoming more dense as it looses the less dense helium.

  13. Anonymous says:

    "It turns out that if you ask new Harvard graduates this question, the vast majority of them answer some variant of "It comes from the soil".  When people think of photosynthesis, they don’t think about the carbon that’s left behind.  They’ll usually be puzzled as they answer, because in their hearts, they realize that "the soil" doesn’t actually work as an answer, but they can’t quite put all the pieces together."

    There’s another reason this wouldn’t work. Trees don’t grow as quickly or really at all during the winter months… why?… no leaves.

  14. Serge Wautier says:

    Larry, I’m confused!

    The question was: Where did the 999,999 grams of mass come from?

    Your answer: Carbon.

    The mass comes from carbon ? A tree is 100% carbon ?

    I have a question then: What’s the role of the roots (obviously besides preventing the tree from falling) ? How do they fit in the picture ?

    I realize I may have thoroughly under-estimated the importance of photo-synthesis but aren’t you over-estimating it ?

    In all cases, thanks for raising this interesting topic.

  15. Anonymous says:

    I have Google to thank for my correct answer yesterday rather than a specific educatorial moment.

  16. Anonymous says:

    There is big difference between two questions:

    Q1) Where the mass comes from in a tree

    Q2) What is mass of a tree made from ?

    This is pretty confusing point, because the answers to these questions are different in quality.

    A1) The answer is: mostly from air, less from soil

    A2) The answer is: mostly from water, less from carbohydrates, very little from anything else; (in fact I have found that 55-60% of the live wood is water)

    The Q1 is not very usual, so most people convert it implicitly into Q2, some combining both by answering air & water.

  17. Garry Trinder says:

    "Photosynthesis is the act of converting CO2 from the air into O2 and a bit of H2O"

    Well it’s not just CO2 but rather CO2 + Water that gets converted into carbohydrates + O2 + somewhat lesser water than what went in. A lot of the tree is cellulose which is complex carbohydrates.

    To answer Serge Wautier yes the roots are very important in bringing water for the photosynthesis. And yes it is simplistic to say that a tree is mostly carbon. A tree is mostly water and carbohydrates (a lot of it as cellulose which is a complex carbohydrate) and carbohydrates are made by photosynthesis using CO2 and Water. Of course then there are the trace minerals which are also important. You cannot simplistically say that its all from CO2. Water is just as important.

    For the record I just turned 30 (darn it!) and studied in one of the Indian education systems 🙂 CBSE for you those of you from India.

  18. Anonymous says:

    Thanks for all the feedback everyone. Needless to say you are reassuring me about the science education system. So many of the presentations are talking about how little science understanding US citizens have these days. It’s always a little hard to figure out just how inflated their numbers are. Yes, I know Larry’s blog gets read by really smart people, but this means that some people are getting it 🙂


  19. Anonymous says:

    Serge, the experiment you did was first done by a fellow (who’s name I can’t remember) in Victorian times. It was something of a seminal experiment at the time not just because of the result but because of the scientific care with which he eliminated all of the variables and was able to prove that the increased mass of the plant came almost entirely from the air.

    Until that time it was thought that air wasn’t really the same as ordinary matter (such as dirt, or wood). The experiment showed that the wood in the tree must have come from the air, hence the air was probably made of much the same stuff as the wood. Modern chemistry is founded on experiments like this one.

  20. Anonymous says:

    Well, I’m afraid that I don’t make the 30 requirements (I’m 35), but all of my education is in the U.S.


    >>"Photosynthesis is the act of converting CO2 from the air into O2 and a bit of H2O"

    Actually, the O2 that is released into the air comes from water.  Some scientists (I believe in the ’40s) performed an experiment where they provided the plant with H2O that contained a "heavy" isotope of Oxygen, and C02 that used the common isotope of oxygen.

    They collected the O2 that the plant aspirated, and they found that all of it was  "heavy" oxygen, so it had to have come from the water.  I thought that was kind of cool.

    I also thought it was kind of sad that all of those High School textbooks tell us that the oxygen given off from plants comes from the CO2

  21. Anonymous says:

    I’m 22 and from the state of Canada in the Americas, so I’m an American.

    I asked a plains grain farmer about fertilizer once (trying to get an answer to this for myself), and he said they didn’t use any. I assumed crop rotation accounted for some of it, irrigation must bring in some scant dissolved minerals, and that carbon dioxide and water must make up the bulk. Fascinating stuff.

  22. Anonymous says:

    I was about to answer when I realized I turned thirty a month ago 🙁 Nonetheless, I did get the question right and was educated in public schools in Wisconsin and California. Of course, my wife studies carbon uptake and sequestration in photosynthetic marine grasses, so I’d be in trouble if I didn’t get it right!

  23. Anonymous says:

    well i am way over 30 but i had it right.  what clued me in is that, on my current diet, i weigh myself at night before bed and then right when i wake up.  i consistently lose 2-3 pounds overnight.  after looking around under the bedsheets for missing parts, i figured out it must be the co2 that i am venting all night.  so if i can lose a couple pounds a night, i am pretty sure a tree can gain a lot of mass from sucking in co2.  

  24. Anonymous says:

    Rob Leitman,

    if you fill an empty balloon with helium the mass increases as the balloon has helium added to its inside. Adding the gas increases the mass. However the density of the helium-filled balloon is less than the air surrounding it ending up that it wants to go up into the air.

  25. Anonymous says:

    Wednesday, August 02, 2006 1:14 PM by Rob Leitman

    > A related question: when you fill a balloon with helium, does

    > the mass increase or decrease?

    First we have to figure out the question, since the mass of the balloon doesn’t change (rubber or whatever it’s made of) and the mass of the helium doesn’t change.  But if we compare the mass of an empty balloon (in a vacuum chamber) to the mass of a filled balloon including its contents, then of course the filled one is more massive.

    As for air not weighing anything, I learned that it weighed somewhere around 1 newton per square metre.  Though that was in a foreign country more than 30 years ago, sorry.

    I seem to remember that even dehydrated fruits which can be bought in plastic packages are still more than 50% water.  Though fruits aren’t the same as trunks and other structural elements of a tree.  I’d still like to see some percentage stats on the actual water vs. cellulose components.

  26. Anonymous says:

    Another interesting thing is that most of the O2 produced by photosynthesis comes from the water, not the fixation of CO2.

  27. Anonymous says:

    32+1week, public school (montesorri preschool & 4th grade only) and public bs in cs

    my guess was water trace minerals and co2 from photosynthesis but i didn’t realize of course the o2 was released until later

  28. Anonymous says:

    I’m 20 and educated in California (high school, and now UC Irvine).  I must admit that my first thought was "isn’t it the soil?", but then I realized that organic matter contains carbon which couldn’t be scavenged from minerals in the ground.

  29. Anonymous says:

    Actually, the answer is not that suprising if you think about it:

    Charcoal is made by heating wood in the absence of oxygen, thus removing the water (that not forming other chemical compounds). And it’s size/weight does not lost that much after the heating process.

    For a record I’m 26 yrs from HK.

  30. Anonymous says:

    As for the rock in the boat I got asked that while going for university places (I am under 30 – but I was educated in the UK.)

    1 – Of course the boat goes up – it’s displacing less water now.

    2 – Assuming the lake isn’t connected to any other water mass then the water level will actually go down, as the rock is no longer displacing it’s WEIGHT in water but it’s VOLUME. (And most rocks are denser than water.)

    3 – I actually have to think about that. But the boat would still go up in relation to the land.

  31. Anonymous says:

    I got it! I got it!

    It comes from dead poets.

    For example, the poet that Knuth quoted:

     "I think that I shall never see

      a poem lovely as a tree."

    Knuth’s logs were published on dead trees,

    so it all balances out.

    New question:

    When you delete root from a tree,

    why do you still find bugs in it?

  32. Anonymous says:

    So Photosynthesis is:

       CO2 (from air) + H2O (from ground through roots) –> C*H* (carbohydrates) + H2O (in the tree) + O2 (to the air)

    Right? If so, then some of the mass comes from the CO2 in the air, and some from the water in the soil.

    BTW, I’m 30 (non-American), and I did not get this. Shame on me.

    As for the validity of the survey, I think it is kind of tilted because it includes:

    1. Only readers of this weblog, which should be more technically-inclined than the general population

    2. Only people who responded. I suppose people who are certain of their answer will have a higher tendency to post.

  33. Anonymous says:

    I’m 28 and I was educated in US schools, I’ve always been geeked about science though so I read a lot on my own.

  34. Anonymous says:

    I got the correct answer, was educated in American public schools, but I’m over 30. I think I first did photosynthesis in 4th grade, around 1975.

  35. Anonymous says:

    I’m 24, and I was educated in America.    

  36. Anonymous says:

    so what has more mass 1 cubic foot of dry air or moist air?

    surprising but understandable it is dry air…. molecular weight of an air molecule is more than a water molecule……so as water displaces the air molecules in humid air its mass per unit of volume decreases..

    i realized this after the latest .NET app im writing for a fan manufacturer,


  37. Anonymous says:

    So, larry,

    what conclusion is your wife coming to from the answers above? I am interested to know.

  38. Anonymous says:

    > so what has more mass 1 cubic foot of dry air or moist air?

    The later (added water, but no change in volume). However I’m more interested in the important questions: What is the air speed for a swallow?

    For the record, over 30, and educated in the UK, and got the right answer after a moments, thought: not much of the proportion of the mass of a tree (as compared to a human) is water, since much of the mass is "dead" wood giving structure rather than transporting water+minerals from the roots or air+carbohydrates etc. from the leaves.

  39. > 1) (easy) How does the boat move relative to the water (up or down)?

    > 2) (moderate) How does the level of the water change relative to the shore (does it go up or down)?

    > 3) (hard) How does the boat move relative to the shore (does it go up or down)?

    Well… let’s see…

    A floating object displaces its mass; an object submerged in water displaces its volume.

    Therefore the rock displaces more water in the boat than it does in the lake.


    1) The boat weighs less than the boat+rock.  It floats before and after.  It displaces less water, so it moves UP.

    2) The level of water relative to the shore goes DOWN.

    3) The lake goes down less than the boat goes up, so the boat goes UP relative to the shore.  No proof given until later, in the bonus round.

    > For bonus points, in each case, how big is the change?

    I will take MR to be the mass of the rock; VR to be the volume of the rock; and VWR to be the volume of an amount of water with the same mass as the rock.  VWR > VR.

    I will take MB to be the mass of the boat, not counting the rock; VWB to be the volume of an amount of water with the same mass as the boat.  VB > VWB.

    1) The boat displaces VWR less water, so it will go up by "that much."  The vertical change depends heavily on the shape of the boat.

    2) Before, the boat displaced VWB + VWR.  After, the boat displaced VWB and the rock displaced VR.  The net change is VWR – VR.

    3) The boat goes up VWR relative to the lake; the lake goes down VWR – VR relative to the ground.  Net result for the boat: UP by VWR – (VWR – VR) = VR relative to the ground.

  40. Anonymous says:

    Ah, Larry, a live tree is roughly 50% water by mass, so it seems unlikely to me that most of the mass comes from the air. Except in the sense that rain comes out of the air… more likely the dry weight is mostly carbon, but since you get the dry weight by killing and heating the tree, that’s not a good measure.

    If you’re searching, the key term is "dry weight", as used by every treekiller from firewood sellers to the US Congress.

  41. Anonymous says:

    I’m under 30, educated in the US and got it wrong. That said, I eschewed biology and chemistry in favor of physics while I was in school.

  42. Anonymous says:


    I related this to my wife who doesn’t take anything I say at face value, so I looked into it a bit more and I don’t think carbon is the correct answer.  Keep in mind that chemistry wasn’t my best subject many years ago and most of the brain cells that understood it have been reallocated.  None the less I think the mass of a tree is mostly Oxygen, not Carbon.

    I think trees are mostly cellulose.  Here’s a string of cellulose molecules:

    It looks like each cellulose molecule has 5 Oxygen atoms and 1 Carbon atom and some bits of Hydrogen.  Oxygen has a higher atomic weight than carbon so I think the mass of a tree is mostly Oxygen.  

    This makes sense because when I go camping and burn tree parts, what is left in the bottom of the fire pit is the carbon but the majority of the tree is gone.

  43. Anonymous says:

    Missed out on reading the original tree question but, if I could "branch" out a little here, the point about air having mass is fundamental to understanding weather as well. The amount of water air can hold changes at different temperatures.  As the air rises and cools off, it can hold less water. Until finally, it can’t hold it anymore and it rains.

  44. Anonymous says:


    I am an old (66years) professional Illinois Licensed electrical engineer with a masters degree in nuclear physics. The question is a simple one using simple reasoning and balanced equasions. If we look at trees we see that the surface of the ajacent land is flat (even) so how do you solve this equasion? The only answer is derived by answering a few questions. If the tree mass came from the soil there would be a trough by the trunk of the tree. If the tree mass came from the air (ONLY) there would be a hill by the trunk of the tree (the tree would be on a hill). Therefore the only answer is that the mass of the above ground and below ground comes both form the air and soil and must be pretty close to a balance combination of 50/50 percent (because the adjacent surface is pretty flat). You could say that the top of the tree (above ground) comes from the air and the below ground mass (root system) comes from the soil since supposedly the mass of the root system equals the mass of the above ground system (I have heard). Seriously though the answer 50/50 fits best. Probably most of the carbon comes from air CO2 and the minerals and neutriments come from the soil. Enough said since I am not a biologist.

    I hope this answers the questions for you young guys.

    Steve (

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