Trees are made from air and water. How much of the wood comes from air and how much comes from water? Let’s do the calculations.
Dry wood is mostly cellulose, chains of C6.H10.O5, molecular weight 6*12+10*1+5*16 = 162.
The production of cellulose, in overview, is 12*C.O2 (from air) + 10*H2.O (water) –> 2*C6.H10.O5 + 12*O2 (back to air). Each C.O2 has weight 44, each H2.O has weight 18, and each O2 has weight 32. Thus the formula, in terms of molecular weights, is 12*44 (carbon dioxide from air) + 10*18 (water) –> 2*162 (cellulose) + 12*32 (oxygen back to air).
Multiplied out, 528 grams of carbon dioxide from the air, plus 180 grams of water, makes 324 grams of cellulose and releases 384 grams of oxygen back to the air. The process uses 75 percent air and 25 percent water, by weight.
Incidentally, notice that the number of C.O2 gas molecules consumed in the formation of cellulose, equals the number of O2 gas molecules released. There is no change in air pressure as a result of tree growth. If there were an excess or deficit of gas molecules resulting from plant growth, we might expect some interesting breezes and weather effects near forests and fields. Every aspect this world fits together! I have a naturalist friend who says he can tell there is lots of oxygen in the area behind my house — which is surrounded by lush woods and growing plants. Maybe so. I used to be able to detect low oxygen in the stacks of a poorly ventilated library, where the many book pages were slowly oxidizing.
The above is not, however, the full story about the constituents of wood. Living trees are not dry wood. They contain about 50 percent by weight of water. So let’s revisit the calculation. Consider an amount of living tree, let’s say 200 grams. Half of it (100 grams) is 75 percent from air and 25 percent from water. The other half (100 grams) is 100 percent water. The total amount of living tree comes 37 percent from air and 63 percent from water.
There’s even more. Dry wood is not free of water. It contains about 12 percent water. So again consider an amount of dry wood, let’s say 200 grams. The majority of it, say 88 percent or 176 grams, comes 75 percent from air and 25 percent from water. The remaining 12 percent, 24 grams, is moisture trapped within the “dry” wood. Total water content of the 200 grams, as water or as cellulose partly derived from water, is 34 percent. The dry wood is two-thirds (66 percent) derived from air, and one-third water or derived from water.
Conclusion: Dry wood is mostly derived from air. Green wood is mostly either water or derived from water.
Here’s a nice tree experiment from about 350 years ago. It exemplifies scientific patience and careful observation. It also indicates how, despite careful and accurate observation, one can make a mistake by using the wrong model.
This experiment was performed by Jan (Johannes) Baptista van Helmont, and reported in 1648. Van Helmont wished to determine which of the four elements (earth, water, air, fire) was the predominant contributor to plants. More exactly, he believed that plants were mostly made from water, and wished to validate his hypothesis. Here is his report of his experiment, which took five years:
Van Helmont: “That all plants immediately and substantially stem from the element water alone I have learnt from the following experiment. I took an earthern vessel in which I placed two hundred pounds of earth dried in an oven, and watered with rain water. I planted in it the stem of a willow tree weighing five pounds. Five years later it had developed a tree weighing one hundred and sixty-nine pounds and and three ounces. Nothing but rain (or distilled water) had been added. The large vesel was placed in earth and covered by an iron lid with a tin-surface that was pierced with many holes. I have not weighed the leaves that came off in the four autumn seasons. Finally I dried the earth in the vessel again and found the same two hundred pounds of it diminished by about two ounces. Hence one hundred and sixty-four pounds of wood, bark and roots had come up from water alone.”
That quote is from William H. Brock’s book “The Chemical Tree: A History of Chemistry”, which has also been published under the alternative titles “The Fontana History of Chemistry” or “The Norton History of Chemistry”. It begins the introduction of Brock’s book and justifies his title “The Chemical Tree”. A well-written book, enjoyable reading.
Van Helmont’s mistake, is that his model did not include the invisible air, which it turns out contributes about three-quarters of the mass of the tree material. He also excludes sunlight (= energy = fire) from his contributing elements. And of course the earth is also necessary, for nutrients and trace materials, and for support and context for the tree’s life (growth medium). One might say, in the four-element categorization, that trees depend upon all four of the basic elements, and could not flourish were availability of any one of those elements blocked.
We make advances by relying upon our predecessors. Van Helmont was a creative and careful experimenter. But all experiments benefit from prior experiments and also from sincere critiques from others.
An article in Plant Science Bulletin by David Hershey has a very interesting discussion of Van Helmont’s experiment. There are predecessors, contemporaries and later scientists who made related experiements. Hershey draws some conclusions about the scientific process, and also about the teaching process. The best formatting of the article is at this URL:
http://helmont1.tripod.com/hersheypsb49-3.htm
but it is also available at Plant Science Bulletin, vol 49 no 3 (2003), at this URL:
http://botany.org/PlantScienceBulletin/psb-2003-49-3.php
The Hershey article also quotes the above paragraph of Van Helmont’s in a 1662 translation, rather archaic language but useful as there are subtle differences between the 1662 translation and the more recent translation. If I could read Latin, perhaps there would be other subtleties to discover. However, there are other avenues of investation to pursue, and Latin does not make my priority list. German though … very important for science, of the past century and contemporary work.
Let’s have a photo of Jan Baptista van Helmont:
There’s a story behind this portrait. It has been mis-identified as Robert Hooke (of whom no portrait exists). However, one can see by comparing with other images of Van Helmont, for example in one of his books, that this portrait is Van Helmont.
Articles from Wikipedia about wood drying (re moisture content), about cellulose, about Van Helmont:
https://en.wikipedia.org/wiki/Wood_drying
https://en.wikipedia.org/wiki/Cellulose
https://en.wikipedia.org/wiki/Jan_Baptist_van_Helmont
Best wishes,
Ken Roberts
22-Jun-2015
Logic, Astronomy, Science, and Ideas Too 