In meditating, we always retain in ourselves a little more carbon dioxide than we do in the normal process of waking consciousness. A little more carbon dioxide always remains behind in us. Thus we do not at once expel the full impetus of the carbonic acid, as we do in the everyday, bull-at-thegate kind of life. We keep a little of it back. We do not drive the carbon dioxide with its full momentum out into the surrounding spaces, where the nitrogen is all around us. We keep it back a little. (Steiner 2007:26)
How then does the plant itself live in the midst of this process? Down there below, the limestone-principle tries to get hold of it with tentacles and clutches, while up above the silica would tend to make it very fine, slender and fibrous — like the aquatic plants. But in the midst — giving rise to our  actual plant forms — there is the carbon, which orders all these things. And as our astral body brings about an inner order between our Ego and our ether body, so does the nitrogen work in between, as the astral. (Steiner 2007:27-8)
STEINER, Rudolf. 2007. The Agriculture Course. (Trans.: George Adams) Shrewsbury: Wilding & Son Ltd.