Answering my own question here...
It appears that, in an average tree at least, some 90% of the bio mass is
derived from oxygen and carbon in the air. I would guess that if you
weighed a healthy plant, around 90% of that weight would be water, but after
removing the water, the remainder - the actual structural parts - would be
primarily derived from elements in the air.
I had no idea it was that much. That's fascinating! I'm going to start
fermenting my beer next to my plants. See what a little more CO2 does. :-)
Let's think about that from the perspective of the underlying chemistry.
When we talk about water we
describe a molecule of two hydrogen atoms and one oxygen atom. But these
words refer to atoms, not the
energy which bonds the atoms together. Wood is mainly a carbohydrate called
formula, C6 H10 O5). Compare the mass of these atoms: carbon is about 12
mass units, oxygen about 16,
and hydrogen about 1. Thus carbon and oxygen contribute 72 (6 x 12 for the
carbon) plus 80 (5 x 16 for
the oxygen) mass units while hydrogen contributes only 10 (10 x 1) units.
Thus carbon and oxygen make
up 152/162 of the cellulose molecule by mass. And experiments using tagged
markers have shown that
this oxygen comes from the carbon dioxide molecule not from the water
molecule. The carbon is not
entering the plant in minerals, or in the water, but rather from the air.
Carbon dioxide enters plant leaves as a gas. It is combined with hydrogen
from water to produce
carbohydrates. In this process of photosynthesis, oxygen gas is produced as
a by-product of the reaction.
If wood is mostly carbon and oxygen, then wood comes mainly from air! That
rather unexpected point
should get students attention: the mightiest redwood, or rainforest giant,
is made mostly of elements
derived from air. These first two Activities use something readily
available, a pencil, and something less
easy to obtain--dry ice--to show students how carbon behaves as it changes
from a solid to a gas.